How long will a typical bacterial strain keep in a -80°C freezer?

How long will a typical bacterial strain keep in a -80°C freezer?

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I know that a -80°C freezer is the recommended means of long-term cell-line storage, and that cells will typically not last long in a -20°C freezer. But how long will a typical bacterial strain (e.g., E. coli DH5-alpha) actually stay good in a -80?

Searching around via Google, I'm finding a wide range of answers, anywhere from only 3 months to indefinitely, so I'd like to try to get a better grounded way of estimating longevity.

The reason you haven't found a definitive answer is that it depends on several factors. The first factor that comes into play if we consider only ultra-low freezers (- 80 °C) is the organism you're freezing. Thermofisher gives an approximate time of bacterial cultures viability between 1 to 10 years :

$$egin{array}{|l|c|c|} ext{Condition} & ext{Temp (°C)} & ext{Time (approx.)} hline ext{Super-cooled freezer} & ext{-80} & ext{1 - 10 years} end{array}$$

For ATCC, the shelf life is even shorter :

-80°C is sufficient for most bacteria and fungi for short-term storage (5 years or less)

However, in Table 1 from She and Petti (2015), we can see that the shelf life - for an ultralow-temp freezing storage (-70 to -196) - varies greatly both between the organism groups and inside each group : up to 1 year for the Streptococci group and up to 3 decades for the gram-negative bacteria for example.

$$egin{array}{|l|l|c|} ext{Organism group} & ext{Cryopreservative} & ext{Storage duration (yr)} hline ext{Streptococci} & ext{Skim milk} & ext{0.2 - 1} ext{Mycobacteria} & ext{Skim milk} & ext{3 - 5} ext{Spore-forming bacteria} & ext{Skim milk, glycerol} & ext{2 - 30} ext{Other Gram-positive bacteria} & ext{Skim milk, sucrose, glycerol} & ext{1 - 30} ext{Gram-negative bacteria} & ext{Sucrose, lactose, glycerol} & ext{2 - 30} ext{Filamentous fungi} & ext{Glycerol, DMSO} & ext{2 - 30} ext{Yeasts} & ext{Glycerol, DMSO, skim milk} & ext{2 - 30} ext{Protozoa} & ext{Blood, nutrient medium with DMSO or glycerol} & ext{} ext{Viruses} & ext{SPGA} & ext{1 - 30} end{array}$$

How the stocks are made and used are also important factors :

  • Among other things, ATCC recommends a gradual freeze and a quick thaw :

In addition to using a cryoprotectant, the rate of cellular dehydration during the freezing process can be managed by using a -1°C/minute cooling rate

Thawing should be rapid at 37°C

  • ThermoFisher recommends a high cell density as cell death will occur no matter what :

Also, the greater the cell density, the better the recovery is after thawing the cells. For most bacteria, a density of 107 cells/mL will result in adequate recovery if all conditions are properly maintained.

Cell death during storage is inevitable but should be minimized as much as possible

  • The number of freeze/thaw cycles is critical and thawing even partial should be avoided :

Try not to freeze/thaw your glycerol stock too many times. Placing the glycerol stock on dry ice while streaking onto LB agar will prevent it from thawing completely and will improve the shelf life. from

Coming back to the initial question, based on the information above and some of the feedback in this discussion stocks of the typical E. coli DH5-&alpha& stored at - 80 °C would probably last up to 2 or 3 decades maybe more but depending how you make them and use them it can be much less. To go further, depending on your workflow you may want to consider having working and stock vials to limit the number or freeze thaw cycles or using freeze-dried stocks for longer term storage.

Edit : One thing to keep in mind too is that frozen stocks shelf life is not all about cell viability upon thawing but also about "cell integrity". As mentioned by She and Petti (2015), van Griethuysen et al. have observed the loss of the mecA genes in 2 years old Staphylococcus aureus isolates frozen stocks.

The mecA gene was lost in 36 (14.4%) of 250 methicillin-resistant Staphylococcus aureus isolates after 2 years of storage at −80°C with the Microbank system (Pro-lab Diagnostics, Austin, Tex.). Further analysis of 35 of these isolates confirmed loss of the mecA gene in 32 isolates. This finding has important implications for the management of strain collections. I personally experience a similar issue with a routinely used strain that lost its adhesive phenotype so we had to use the backup stock.

References :

  • She, Rosemary C., and Cathy A. Petti. "Procedures for the Storage of Microorganisms." Manual of clinical microbiology (2015): 161-168.
  • Van Griethuysen, Arjanne, et al. "Loss of the mecA gene during storage of methicillin-resistant Staphylococcus aureus strains." Journal of clinical microbiology 43.3 (2005): 1361-1365.

Richard Lenski's lab at Michigan State university has been doing long-term evolutionary research on E. coli and maintaining a "frozen fossil record" of their experimental populations every 500 generations since 1988. They still periodically release research papers comparing the physiology of evolved and ancestral populations. For example, this recent paper looks at populations collected at 2000, 10000, and 50000 generations. According to this timeline, they used populations frozen over 3 decades ago.

This doesn't guarantee that your specific organism will last that long, but it does show that freezer stocks can remain viable for longer than some research labs will even exist. Most of their protocols are on the website. I've had lunch with Dr. Lenski, and he's a pretty approachable guy. I'm sure if you emailed him with specific questions that aren't covered on the website, he'd try to find an answer for you.


Changes in Cell Size and Shape During 50,000 Generations of Experimental Evolution with Escherichia coli. Nkrumah A. Grant, Ali Abdel Magid, Joshua Franklin, Yann Dufour, Richard E. Lenski, Journal of Bacteriology Mar 2021, JB.00469-20; DOI: 10.1128/JB.00469-20

Lenski Lab homepage:

Do Probiotics Need to Be Refrigerated?

Are there certain brands which don’t need to be refrigerated – or do all probiotics, regardless of labeling, need to be refrigerated?

Enviromedica » Learn » Do Probiotics Need to Be Refrigerated?

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Duration: 7 Minutes (2,149 Words)

Key Message: Are there certain brands which don’t need to be refrigerated – or do all probiotics, regardless of labeling, need to be refrigerated?

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Most people are aware that refrigeration increases the shelf life of probiotics. But it’s hard to sort out whether this is true for all probiotics. Are there certain brands which don’t need to be refrigerated – or do all probiotics, regardless of labeling, need to be refrigerated?

Researchers analyzed the contents of 20 store-bought brands and found that 20% of the products contained no viable probiotic bacteria 1 at all.

If you are as confused as most about when or when not to refrigerate, and why refrigeration is crucial for some products, read on. We’ll help you sort out the truth in regard to probiotic stability.

What destroys or makes Probiotics go bad or die?

In the recent past, studies show that the contents of some store-bought probiotics brands contained no viable probiotic bacteria at all.

At same time, tests by found that out of 19 probiotics for people, 5 contained only 16% to 56% of the listed amounts of organisms. Levels of organisms in probiotics for pets were so low as to question their usefulness.

So what destroy the good bacteria in probiotic supplements?

Prescript-Assist outlines some factors that determine the loss of freshness and viable cultures over time of a typical LAB-based probiotic.

1. Heat during Storage

Heat destroys probiotic bacteria. Low temperature is critical to keep the viability of probiotic bacteria.

  • At 70˚Farenheit, viability decreases 10-15% per month. Above 80˚Farenheit, the loss is more than doubled.
  • Maintaining storage temperatures below 36 -39˚Farenheit increases probiotic survival, specifically L. acidophilus and B. bifidum, however large scale losses must still be anticipated.

As Kaila Kailasapathy, a researcher with the University of Western Sydney in Australia, in Immunology & Cell Biology notes, in every probiotic culture, “the cell count at the end of incubation must be sufficiently high to allow up to 90% mortality of probiotic bacteria during storage – and yet still leave their number above the desired minimum of 106 CFU/mL viable cells,”

2. Non-Symbiotic Bacteria

Studies have revealed that L. acidophilus and B. bifidobacteria do not survive in several U.S. yogurt products, due to the presence of another bacteria, L. delbrueckii ssp. bulgaricus, which in effect kills the “good bacteria.”This lack of cooperation between some species is yet another factor reducing the viability of many probiotic products.

3. Humidity

Humidity and moisture are a huge problem for probiotic products. Moisture activates the bacteria and essentially starts the process of degradation, since the activation is intended to occur after ingestion. To combat this issue Some manufacturer’s products are sealed at low humidity as they are manufactured, which results in a foil-packed probiotic product. However, this only works as long the sealing is maintained. Once the product is opened, it is exposed to moisture from the air and the probiotic bacteria begin to deteriorate.
A few companies have turned to microencapsulation which still comes with some challenges. Yet in an effort to find solution some manufacturers have come up with SBO probiotics such as Bacillus subtilis which are said to have naturally adapted to be resilient against both humidity and temperature through a protective organic shield which is not shed until the bacteria reaches its final destination

4. Damaging Effects of Manufacturing Process

As mentioned earlier, 0ne of the major concerns in probiotic viabilty is heat. Too much contact with heat will kill the delicate bacteria, whether the exposure is during manufacture or storage. According to some manufacturers, microencapsulation is one technique that attempts to improve the viability while in the process.

However, industry experts have voiced skepticism, stating that common species like L. acidophilus, L. casei, B. bifidum will always require refrigeration, regardless of these protective measures. In fact, the microencapsulation process itself is known to destroy probiotic bacteria. The concern is that these novel technologies are simply not sufficient to overcome the inherent vulnerability of non-refrigerated acidophilus and Lactobacillus probiotics, and that more resistant species may be the only viable solution.

5. Packaging

Variations in packaging can also contribute how long the live cultures will survive. Resistant packaging such as microencapsulation can help to maintain the viability.

The scripts for our mathematical model and for the analysis of microfluidics time traces have been deposited in IST DataRep and are publicly available at

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Molecular Bio Test #2

4. Take some device (plunger of felt or filter paper) that will pick up some of the bacteria in the colony.

5. Touch the plunger on a brand new agar plate & leave bacteria. This time, that agar DOES NOT CONTAIN tetracycline it contains ampicillin.

6. Any bacterial cell that does not contain foreign DNA, has an ampicillin resistance gene

7. The missing colonies represent the ones that have had their ampicillin resistance gene split in two.

2. Spread those bacteria on an agar plate & mixed in the agar is some tetracycline

3. Any bacteria that's carrying the plasmid will grow on the agar

6. Any bacterial cell that does not contain foreign DNA, has an ampicillin resistance gene

7. The missing colonies represent the ones that have had their ampicillin resistance gene split in two.

2. This included (what was removed) the tetracycline resistance gene. The ampicillin resistance gene was retained (left in pUC plasmid).

3. The cloning sites in pUC are clustered into a small area of the plasmid called the MCS: MULTIPLE CLONING SITE

- This is accomplished by cutting both the plasmid and the insert DNA with two different restriction enzymes. Then we cut source DNA with same two restriction enzymes. Now the insert DNA can't flip around. (Also prevents closing of the plasmid)

(2) take some nitrocellulose filters or nylon filters- barely put it on top of the wad of bacteria

(3) Bacterial cells will stick to filter & viral DNA that's coming from those plaques

(4) remove the filter. Usually place the filter on top of a UV transilluminator (strong UV will cross-link the DNA onto the surface of the filter- makes DNA stick to filter)

(5) Dentature the DNA on the filter- typically by soaking filter in an alkaline buffer (going to cause dsDNA on filter to denature)

(6) incubate the filter with radiolabeled probe molecule (the probe is a short sequence of DNA, sometimes cRNA, that is complementary to the DNA that you tried to insert into the phage

(7) Let filter sit in presence of labeled probe- probe will find complementary DNA & DNA will bind to it.

Like "classic" or "typical" asthma, no one really knows what causes cough-variant asthma. However, coughing may start after people are exposed to allergens, or when they are breathing in cold air. Coughing may also follow an upper respiratory infection. For example, sinusitis with asthma is common.

In addition, a cough that begins after a person has begun taking beta-blockers is likely to be cough-variant asthma. Beta-blockers are drugs used to treat high blood pressure, heart disease, heart failure, migraines, palpitations, and other conditions. Beta-blockers are also found in eye drops to treat glaucoma and other eye problems. These eye drops can bring on asthma symptoms, including cough. Aspirin-sensitivity is another cause of coughing with asthma.


1 Correlating lag times of individual cells with winner index values is not possible in our experiments, due to insufficient objective resolution.

Published by the Royal Society under the terms of the Creative Commons Attribution License, which permits unrestricted use, provided the original author and source are credited.


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Bacterial extracellular electron transfer (EET) is envisioned for use in applied biotechnologies, necessitating electrochemical characterization of natural and engineered electroactive biofilms under conditions similar to the target application, including small-scale biosensing or biosynthesis platforms, which is often distinct from standard 100 mL-scale stirred-batch bioelectrochemical test platforms used in the laboratory. Here, we adapted an eight chamber, nanoliter volume (500 nL) electrochemical flow cell to grow biofilms of both natural (Biocathode MCL community, Marinobacter atlanticus, and Shewanella oneidensis MR1) or genetically modified (S. oneidensis ΔMtr and S. oneidensis ΔMtr + pLB2) electroactive bacteria on electrodes held at a constant potential. Maximum current density achieved by unmodified strains was similar between the nano- and milliliter-scale reactors. However, S. oneidensis biofilms engineered to activate EET upon exposure to 2,4-diacetylphloroglucinol (DAPG) produced current at wild-type levels in the stirred-batch reactor, but not in the nanoliter flow cell. We hypothesize this was due to differences in mass transport of DAPG, naturally-produced soluble redox mediators, and oxygen between the two reactor types. Results presented here demonstrate, for the first time, nanoliter scale chronoamperometry and cyclic voltammetry of a range of electroactive bacteria in a three-electrode reactor system towards development of miniaturized, and potentially high throughput, bioelectrochemical platforms.

Estimating Community Dissimilarities (Beta-Diversity)

�ta-diversity,” as coined by Whittaker (1960), is “The extent of change of community composition, or degree of community differentiation, in relation to a complex gradient of environment, or a pattern of environments.” In other words, beta-diversity is the degree to which two samples are different. This is a rather different issue than within-sample richness and evenness, and can be measured in many different ways.

The choice of beta-diversity metric can have important consequences to subsequent analyses, such as clustering and ordination. This is partially due to the interplay between distance metrics and normalization techniques, which can widen or reduce the apparent distance between samples (Figure 3).

A true distance metric is one that is always positive, in which the distance between a point and itself is 0, the distance between A and B is identical to the distance between B and A and the sum of the distance between A and B and between B and C is no greater than the distance between A and C. This last assumption is the one that often fails for other dissimilarity measures. The appropriate metric for a study might depend on the size of the effect of interest and on the depth of sampling.

The most widely known true distance metric is the euclidean:

Where S1 and S2 are two samples and S1i, S2i are the abundance of OTU i in samples S1 and S2, respectively.

However, the euclidean distance requires very large effect sizes for statistical significance (Kuczynski et al., 2010) and doesn't perform well in datasets with many zeroes. A more appropriate metric is thus Jensen-Shannon's, a symmetric version of the Kullback-Leibler divergence. In Kullback-Leibler, the distance between S1, S2 is:

Thus, Kullback-Leibler is not applicable for 0-rich datasets. However, since Jensen-Shannon's compares samples S1 and S2 to their average, the problem of 0's disappears:

This formulation also automatically satisfies the other requirements for a distance metric.

In microbial ecology it is also common to use correlation coefficients, such as Pearson's product moment (Figure 4A):

Figure 4. Visual intuition to selected community dissimilarity metrics. In each panel, the same set of OTU (blue dots) is represented in a scatter plot from two highly correlated samples. Pearson's and Spearman's correlations can be intuitively thought as the degree to which the scatter deviates from a straight diagonal line, except Pearson is based on the numeric values of distances (A) and Spearman on their ranks (B). Bray-Curtis dissimilarity is displayed in (C).

To minimize the influence of noise, other researchers prefer Spearman's rank correlation, which is identical to Pearson's except that instead of the measured values, their ranks are used (Figure 4B). Finally, Bray-Curtis dissimilarity, while not very sensitive, is appropriate for 0-inflated datasets (Figure 4C):

An alternative to OTU-based distances is to use phylogenetic distances. While these approaches also require several non-trivial choices, such as the underlying phylogenetic tree and the placement of OTU in it, evolutionary distances are often more biologically meaningful, not least because phylogenetic relatedness is often associated to trait conservation (Martiny et al., 2015). As is the case for OTU-based metrics, using a quantitative or qualitative approach to community comparison can lead to very different results (Lozupone et al., 2007). This can be ameliorated through an appropriate weighting procedure, such as generalized Unifrac (Chen et al., 2012). Recent work by Schmidt and colleagues does a thorough review of commonly used distance metrics and proposes new taxonomic and phylogenetic distances based on co-occurrence networks (Schmidt et al., 2016).

Different approaches to community dissimilarity, such as OTU-based vs. phylogenetic, may highlight different aspects of the community and its functioning. It can therefore be useful to combine these different analyses to gain deeper insight into the system under study.

The Great Big Giant Marek's Disease FAQ

The Great Big Giant Marek's Disease FAQ
"Marek’s disease is one of the most ubiquitous avian infections it is identified in chicken flocks worldwide." - Marek’s Disease in Poultry by Jon Dunn.
Every flock, except those maintained under strict pathogen-free environments, has the potential to be infected. It is an incredibly common disease!

Note from the author: As this article deals with a viral disease, new information is always forthcoming. As such, this is a work in progress and I will come back in and edit to add (or remove) information as I learn more, in order to keep this information as up to date and factual as possible

This article is long! If you are just looking for specific information, I have broken it up into groups. Look for the chapter headers in ALL CAPS to help you find what you are looking for, faster!

This article is written for the average home chicken keeper. It is not aimed at larger production style chicken keeping, nor is it intended to be a scientific paper. It is not intended to be a compilation of every bit of Marek's information out there, but enough to get you started in case you need to research deeper. When I started researching this disease, I found there was a lot of information out there, but it was scattered to the seven winds and a lot of it was hard for the layman to understand. When in doubt, seek the professional advice of a licensed veterinarian, poultry examiner, or other qualified person in regards to this disease.

If you have concerns about Marek's in your bird or flock, or need help, see if any of the following FAQs can help you. Please be sure to check out the “resources” list at the bottom.

Disclaimer: I am not a veterinarian or an expert in any of the following. This information is a constantly evolving work of art that has been gleaned from months of research. Above all, you must never take this as medical or veterinary advice and when in doubt please contact an actual expert (see resources). My sources include: Avian veterinarians, published veterinary papers, diagnostics labs, personal research and the anecdotal experiences of others with this disease. While this article is written in my own words, my knowledge has amassed from all of these sources.

It is important to be open to new breaking research and findings about this disease. Like many human retroviruses, things can change, and what was once a rule is now the exception. Use common sense, don't be afraid to research, and never be afraid to ask questions. This disease is estimated to cost the commercial poultry industry in excess of $1 billion dollars annually, so new research is constantly being funded, and new vaccines are under trial.

What is Marek's disease?

Very simply, Marek's disease is a viral disease of poultry, primarily chickens. It can come in several "forms" and symptoms can include, but are not limited to: Partial or total paralysis of the legs, sometimes wings, and even neck visceral lymphoma (cancerous tumors in the body), blindness, tumors or growths on the skin, general listlessness, wasting away, or poor health, to name a few. Some chickens get only one symptom, and some get more than one. Some chickens get symptoms, while some don't show any signs. It is a very common disease, as it is spread via chicken dander (dust) and is easily spread on new chickens, the wind, your clothes and shoes, and even wild birds can transport it. It does not always show up in quarantine. It can be fatal, and there is no cure. Some chickens do survive it, and many build resistance to it. If you suspect you have Marek's disease in your flock, do not panic, but please do educate yourself. This is a somewhat complicated disease and the very best thing you can do for yourself and your chickens is to learn about it and take an educated approach to managing Marek's. I will discuss this disease in great detail below, so please continue reading.



Scroll down to see the different chapters! They are in a large red font.

To Start: A few Facts about Marek's Disease. Most of these are re-hashed in the various questions below, but it will get you started on this complicated journey:

  • Marek's disease is a virus. Specifically, it is a type of alpha herpesvirus, and more specifically it is a DNA virus.
  • Marek's cannot be transmitted vertically. This means that a mother hen cannot pass Marek's on through the egg to her chick. The chick would have to be exposed to the virus after hatching to “catch” Marek's.
  • There is NO CURE for Marek's at the time of writing this.
  • Chickens exposed to Marek's disease must be considered carriers for life, even if they were previously vaccinated and/or never develop symptoms.
  • Current research indicates that Marek's is dominantly spread via chicken dander (chicken “dust”) and is contracted by inhalation.
  • Marek's is highly contagious among chickens
  • Marek's is generally a disease of younger chickens, BUT chickens of any age may contract Marek's.
  • Marek's can be contracted in a chicken by:

1) Exposure to other chicken(s) that have Marek's disease, even if they show no symptoms

2) Exposure to an environment that has had Marek's disease shed upon it

What are the symptoms of Marek's?

Symptoms in animals are more accurately called 'signs', as an animal cannot literally describe symptoms to us. However, to keep things simple and easier to understand, I shall use the word "symptoms" in this article.

There are several “forms” or ways that Marek's can present itself in chickens. These symptoms may overlap, or not. They may also present in ways not listed below if the chicken is experiencing more than one illness at once. Marek's can be frustrating to diagnose.

Symptoms can include but are not limited to the following.

Some chickens will have multiple symptoms. Some chickens will have only ONE symptom, and some will have none (frustratingly enough):

  • Paralysis of one or both legs, and sometimes wings. This can include staggering, increasing loss of motor control in one or both legs, inability to stand or balance. This may present sometimes as one leg forward and one leg back (“the splits”) or simple paralysis of the legs. This is caused by lesions to the sciatic nerve, that controls the legs movement.
  • Going off of food or inability to “connect” with food when trying to eat. Sometimes mistaken for a sour crop or other crop problems, lesions on the vagus nerve can cause dilation of the crop and/or proventriculus. The inner lining of the gizzard may also be effected.
  • Difficulty breathing, darkening comb. Caused by several factors brachial nerve lesions can restrict respiration. Additionally, lymphomas are known to grow into the heart muscle, reducing the chicken's ability for proper respiration. The comb may become very dark red or purple in appearance and gasping or trouble breathing may occur. Marek's does not cause discharge, watering eyes or nose, or gurgling-- however these symptoms may be the result of a secondary viral infection by a different disease.
  • Lymphomas / Neoplasms (cancerous tumors) throughout the chicken. Though these symptoms can usually only be observed after death and a necropsy is conducted, lymphomas typically grow on the thymus (located in the neck, near the crop). The tumors then move on to grow on any of the following: gonads, spleen, liver, kidneys, lungs, heart, proventriculus, adrenals, muscles, and sometimes skin (enlarged feather follicles). These lymphomas are often aggressive, fast-growing, and are usually fatal as they cause organ failure.
  • Weight loss, “wasting”, depression. Inability to eat, connect with food, or digest food because internal tumors can cause rapid weight loss in birds. They may also “waste away” more slowly with no other obvious symptoms.
  • Loose, watery, and/or bright green stool. As the digestive system shuts down, or because the bird is not eating enough, the stools become increasingly loose and consisting of very little to no solids. This may be accompanied by green, bright green, or yellowish coloration.

Ocular Marek's (ocular lymphatosis):

  • Discoloration of the iris. The iris may turn grey or a pale blue color.
  • Deformity of the pupil. The pupil may change shape. A typical form is a “keyhole” shape but the pupil may also look as if it is “melting” or might become amoeba-like in shape.
  • Pupil with no reaction to light changes. The pupil in one or both eyes may become constricted in appearance and might not react to changes in light. This can create a 'tiny pupil' appearance, called Miosis.
  • Blindness.
  • Lesions or deformities at the feather follicles. This may be minor to severe and can range from large bumpy nodules to crusty looking lesions. They may be rounded or hard.
  • Immunosuppression. Birds infected with Marek's may have a periodic or lifelong suppression of the immune system. Marek's virus is known to cause impairment of T-lymphocytes which weakens the immune system. Secondary diseases or illnesses may then present as the chicken's ability to fight them off is reduced considerably. Marek's infected birds with immunosupression are known to be much more susceptible to coccidiosis and viral respiratory diseases. Immunosuppression may be transient (does not last) or it may be permanent. Some birds experience full immunosuppression (they do not have any immune system left) in which case a secondary disease is usually the ultimate cause of death.
  • Come and go symptoms” with no obvious resolution. Some birds may present some symptoms, appear to get well, and then become symptomatic again at a later point. Problems might seem to come and go without much reason (this can be very frustrating to diagnose).

Maybe. There is no way to 100% diagnose Marek's disease (see “Diagnosis” below). However, there is a lot of good guesswork you can do. It is important to look at the big picture and try to make a flock history. Sometimes it is useful to write everything down as you discover "clues". Some questions to ask yourself: Was this bird properly vaccinated? Has the bird been exposed to any possible Marek's disease sources (see “Control” below) in the last several months? What symptoms do I observe? Do the symptoms fit any other disease? How old is the bird? Have I lost any other birds with these symptoms or due to a "mystery illness"? Is Marek's common in my area (you can call your county or state poultry extension to find out)? Has this bird or my flock been stressed or sick in the last several months? Does anyone near me have chickens that might have had Marek's?

Are there any other diseases that look like Marek's?


  • V itamin Deficiencies can cause weakness or paralysis, for example, and those can be reversed with careful supplementation. If you are experiencing paralysis or weakened muscle control in your chicken(s), be sure to explore this possibility: Note that B vitamin deficiencies can especially look like Marek's disease, due to the interactions of B vitamins with a healthy nervous system. If you experience leg/foot problems, lameness, paralysis, etc. with toe-curing especially, be sure to investigate B2 deficiency: The body's need and ability to use vitamins to maintain general and immune health is complex, and can stem from simple problems such as old feed that has lost nutritional value over time (several vitamin complexes break down quickly with age!), to incomplete feed, to a deficiency in the bird's mother (which is passed onto the chick through the egg), to complex genetics that interfere with the uptake of vitamins. The good news is that vitamin deficiencies are sometimes correctable, and may save a bird's life.
  • Avian Lymphoid Leukosis is a disease that can cause Marek's like symptoms, though signs are usually only visible upon necropsy . This virus causes lymphomas, much like Marek's disease, throughout organ tissue. Most chickens with this virus will experience weakness, and will "waste away" over time, becoming more and more emaciated as the tumors spread. This viral disease is often thought to be the 'sister disease' to Marek's, as it is very similar in many ways. Unlike Marek's disease, it can be transmitted through the egg (vertically) from parent to chick. Of important note, there have been some signs that in individual chickens with a genetic predisposition, that Serotype-2 Marek's vaccine (only hatcheries have this vaccine) may cause this disease to more rapidly harm the infected chicken.
  • Heavy Metal Toxicity in chickens (and other fowl, such as ducks) can look very similar to Marek's paralysis. Lead toxicity seems to be the most common for birds like chickens, that unlike other pet birds such as parrots, generally do not chew on metal objects but may swallow small metallic objects whole. Lead shot, BBs, pellets, (etc) are often mistaken for stones and ingested to aid the bird in digestion. A single BB or piece of lead shot is enough to cause serious illness in a large fowl chicken, or even eating old lead paint flakes, or finding them in the soil is enough to harm a chicken-sized bird. Heavy metal toxicity is one of the leading medical problems that vets and wildlife rehabilitators see in ground dwelling birds such as chickens, ducks, and geese, so it can be somewhat common. Symptoms include neurological issues such as partial or total paralysis of one or both legs and sometimes the wings. With lead toxicity, lesions of the nervous system and elevated white blood counts can also mimic Marek's infection. X-rays and/or blood tests might be necessary to diagnose this problem. Treatment generally involves injections of a chelating agent such as Calsenate. Large metal objects may need to be surgically removed. This problem is very hard to diagnose without veterinary help. For more reading:
  • Botulism in fowl can also mimic the symptoms of Marek's, in that it often causes neurological distress and paralysis. Often birds with botulism will present leg weakness, and neck weakness or paralysis. This can come on quite suddenly or gradually, depending on how much of the Botulism toxin has been consumed by the bird. Botulism is caused by the consumption of the toxin, either from decaying material (usually decaying carcasses) or eating an abundance of invertebrates that have been infected with the Botulsim toxin (such as maggots that have been feeding on decaying material). Generally, if a bird survives more than 48 hours, it will recover, so if Botulism is suspected in birds with sudden paralysis, immediate treatment is necessary. For more reading on Botulism consult the following: and
  • Egg Binding is obviously only a concern in hens, but an egg bound hen will often squat, lay down, act lethargic, and seem as if she can not use her legs properly. She may waddle, or fall over easily, or use her wings for balance. Severe egg binding or internal laying and other reproductive problems can also cause a hen to "walk like a penguin". If you have a hen displaying ANY of these signs, it is important to check her for egg binding right away before trying any other treatments. Egg bound hens must be assisted quickly as they can die rapidly depending on where the egg is. A cloacal exam may be necessary. Though this seems rude, it may save her life. Using a glove and a lubricated finger, feel inside and up her vent to see if you can feel an egg. More information and treatment:
  • Hypocalcemia / Layer Fatigue is another problem that is only a concern in hens. A pullet or hen that becomes very suddenly paralyzed may be experiencing this condition. It arises from hypocalcemia (extreme lack of calcium) while her body is putting the shell on her egg during her egg laying cycle. Even hens that are being supplemented with calcium in their diets can experience this either due to lack of uptake in calcium (osteoporosis), simply not eating the supplied calcium, or from a lack of phosphorus or vitamin D3 which are both essential in the proper processing of calcium in her body. Death can occur rapidly. If this condition is suspect, offering a large and immediate dose of calcium can save the hen's life. Crushed calcium tablets (such as Tums), crushed oystershell or even limestone can be fed directly. This condition is more often seen in production breeds such as red sex-links, ISA browns, etc. More reading on this and long term care:
  • Parasites can cause otherwise healthy chickens to exhibit a variety of sickly symptoms that might be mistaken for Marek's. An abundance of external (mites, lice, ticks) and/or internal (various worms and protozoa) can cause lethargic behaviors, poor balance, anemia, off-colored combs and wattles, poor quality droppings, decreased appetite, and various other "generic" poorly symptoms. Internal parasites are NOT always visible in droppings and so a visual diagnosis is not an accurate way to determine if your bird has internal parasites. External parasites such as mites can be very tiny and may require a lot of examination to see. If you see these symptoms, consider using an appropriate treatment to take care of worms and/or external parasites in your flock. Birds can "get" worms, mites, and lice in all weather, and some individuals might have it worse than others. It is very important to treat properly if you suspect an active infestation preventative measures such as (Diatomaceous Earth, Garlic, Pumpkin Seeds, Wood Ash, Apple Cider Vinegar, etc) may help prevent parasites but can not hope to cure a bird that is acting ill from a large parasite load. Valbazen is generally regarded by many chicken keepers to be the best broad-spectrum wormer: For external parasites, attempt to find out what sort of "bug" you are dealing with, and administer treatment based on your findings and unique coop situation. There are many products that work well, but many insecticides can have negative health impacts (both on chickens and their keepers) if used improperly, so it is best to research and use the one you are most comfortable with. Always use such products as directed.
  • Ear / Inner Ear problems or Infection can cause very Marek's-like symptoms in afflicted birds. Much like in humans, inner ear function is important to the balance in chickens, and if the bird is experiencing an infection or growth in the ear, the bird may demonstrate any of the following: dizziness/vertigo, nausea, weakness, loss of appetite, balance issues (etc). It is easy to see why this might look like Marek's. A few keepers with this problem have reported the chicken doing flips, and even tumor-like swelling near the ear. It is important to do a careful exam of both ears. Look for swelling, discharge, clogging, or discoloration. The small, stiff bristle-like feathers covering the ear opening may need to be gently spread to look into the ear canal (a small flashlight can be of value here!). Deep inner ear infections may have very few outward signs. Chickens with ear problems sometimes shake their head repeatedly, or scratch often at the head/ear region. Treatment includes careful and gentle cleansing of the ear/removal of physical blockages, and a round (minimum 7-10 days) of appropriate antibiotics. It is best to consult with a veterinarian if possible regarding dosage and proper antibiotics.
  • Avian Encephalomyelitis is a disease that can cause paralysis in chickens. It can also affect turkeys, quail, pheasants and pigeons. Like Marek's, this disease can show up as birds losing coordination, leg paralysis and/or weakness, including sitting on the hocks, neck spasms, and tremors. This disease is mostly observed in chicks, under the age of three weeks. As chicks less than three weeks can not develop Marek's paralysis, be sure to investigate the possibility of Avian Encephalomyelitis if you experience these symptoms in young chicks. Read more: itis/overview_of_avian_encephalomyelitis.html
  • Viral tenosynovitis is a viral form of arthritis that is transmitted in chickens and turkeys. Transmission is generally via fecal material of infected birds. Infected birds experience lameness and hock inflammation, swelling of the tendon sheaths (the 'tubes' that the leg tendons are encased in), and general lack of mobility of the legs. It is most commonly seen in commercial meat birds and has been reported less commonly in commercial leghorns in the past. Photos with more info (warning, necropsy images):
  • Mycoplasma synoviae (MS) is another disease of poultry that might look like Marek's. This bacterial disease affects chickens and turkeys, but may also infect other commonly kept fowl. It is a relatively common disease, and easily transmitted. Like Marek's, symptoms include lameness/paralysis of the legs and reluctance to stand and walk, as well as blue/purple comb and or wattles due to respiratory distress. The hock (ankle) and wing joints may become swollen. Birds may also experience rales (roughness of breath, rattling, wheezing) and may have some respiratory discharge. nfection_in_poultry_infectious_synovitis.html
  • Mycoplasma gallisepticum (MG) is similar to MS, in that it is a mycoplasma infection, is very common in chickens, and is easily spread. It can cause paralysis and lameness in birds, similar to Marek's, but is generally accompanied by respiratory distress, sometimes severe in nature. Respiratory distress can include but is not limited to: rales (rasping, wheezing), coughing, sneezing, nasal discharge, bubbling or discharge around the eyes, expelling mucus, and overall difficulty breathing. lasma_gallisepticum_infection_in_poultry.html
  • Mold, Moldy Feed, Stale/Old Feed (Aspergillosis) is a serious problem, as mold in grain, especially corn, is known to grow aflatoxins. Most of the aflatoxin problems on corn are caused by Aspergillus flavus, and the most potent toxin produced by this mold is called aflatoxin B1. These toxins may have degenerative effect on the nervous system of birds, causing Aspergillosis, which appears similar to Marek's such as weakness, paralysis, or spasms, along with pale combs, weight loss, and lethargic behavior. Aspergillosis is often reported alongside with respiratory issues, but it is very important to note that it often presents without them as well! It is a common ailment that can be attributed to overall poor health in a bird. It is very important to investigate feed, both bagged and in the bird's environment, to make sure it is fresh and free of mold. Check manufacturing dates when possible. Stale and old feed may not be moldy in appearance, but due to age might have a greater chance to harbor aflatoxins. Molds may not be visible to the naked eye, so when in doubt, discard old feed and supply birds with fresh feed. If you find moldy feed or suspect moldy feed and see any changes in health in your flock, immediately discontinue access to affected feed! A single source of aflatoxins in grain can affect many birds depending on the source of the feed. Read more: More information, and treatment:
  • Injury to the body, and especially to the head, can cause paralysis-like symptoms that look similar to Marek's. It is important to carefully check for hidden injuries, gently palpate bones for breakage, and observe your bird carefully to determine if injury is a factor. Injuries may be internal and not visible via simple physical exam. Head injuries can cause paralysis or weakness, and loss of motor control. Breeds with vaulted skulls (such as Silkies) are especially prone to brain damage, and can sustain brain injury and swelling that can create physical disability.
  • Bumblefoot and other leg and foot issues can cause chickens to limp and favor their feet and legs. Carefully investigate for bumblefoot, and consider leg sprains and other injury when assessing your bird.
  • If you suspect ocular Marek's, be aware that there is a condition that may look like ocular Marek's, called Coloboma . Colomba is a physical irregularity in the iris, which usually looks like a black 'notch' or sometimes a 'drip'. The notch is caused by genetics or trauma to the eye. It is possibly hereditary and may show up in chicks too young to have Marek's symptoms. Not much is known about Colomba in chickens in humans, Colomba of the iris does not cause blindness, and so if your chicken seems blind in the irregular eye, it might not be Colomba.
  • Erysipelas is a bacterial disease, not common but it can affect all commonly kept fowl. Turkeys seem especially susceptible. Generally few signs appear, though birds may become weakened and may have seeming leg paralysis very shortly before death. Mortality is generally less than 15% in infected birds. Birds that succumb will generally die within 24 hours. It is very sudden. This disease is serious, and while uncommon, if you suspect it, you may wish to test it because it can infect other animals and humans (through open cuts).
  • Pasteurellosis is also a bacterial disease. It is also not overly common, and most often seen in turkeys, but a few of the symptoms can look like Marek's disease. Lameness in the legs, difficulty breathing, and twisted neck (wry neck) are often observed, as well as loose stools and swollen wattles.
  • Equine Encephalitis , or EE (WEE, EEE), can infect chickens as well as other commonly kept fowl, such as turkeys, ducks, pheasants, and other game birds. This disease causes paralysis, staggering, going off of feed, and spasms. Birds that survive this disease may become blind and have lasting paralysis and similar neurological signs. It does not cause the lymphomic tumors of Marek's disease. If you experience "Marek's Like" symptoms in non-chicken poultry, be sure to read about EE. Be aware that, as the name implies, this disease is contagious to horses and other mammals (including humans). It is spread mostly by mosquitoes. For this reason, it is closely monitored. You can check these maps to see if it has been reported near you, as this will help you potentially diagnose this problem. If you live in an area that is reporting active EE and you have poultry with these symptoms, it may be worth taking seriously. If you suspect EE in your poultry, consider contacting your county for testing as it may be free. "Sentinel" poultry is an important tool for charting the yearly spread of this virus.

How long after being infected will a chicken show symptoms?

A bird may never show symptoms. Generally, Classic Marek's (with paralysis and/or lymphomas) has an incubation period of 3-25 weeks. Meaning, the 'soonest' that a chicken might show visceral symptoms after being infected is about three weeks. but on the other hand it may not show symptoms for up to 25 weeks. Ocular and cutaneous Marek's may not show up for much longer! I had a hen that developed ocular Marek's years after being exposed. In general, most chicken keepers worry about visceral/classic Marek's more than ocular and cutaneous, as visceral/classic Marek's is almost always fatal.

  • Approximately 7 days after infection: Virus latency (meaning the virus has now stored a 'blueprint' of itself in the chicken's cells).
  • Approximately 10 days after infection, until death: Full replication of the virus is carried out and the chicken begins to "shed" the virus.
  • Approximately 7 days to 3 weeks after infection: Lymphocytes carrying latent Marek's virus travel through the body, to visceral organs and nerves. Meaning, the virus is spreading through the chicken's body.
  • Approximately 3-4 weeks after infection: In chickens that do not develop resistance, the lymphocites in the organs and nerves undergo a transformation and become gross lymphomas (cancerous tumors). Major nerves develop lesions. It is only at this point that symptoms appear and, sadly, death often follows shortly after (most chickens die within 1-2 weeks of showing major symptoms of visceral Marek's).

Infected chickens generally do not present with paralysis and tumor growth after one year of age (though some individuals still can, it is more uncommon). Infected birds may present any of the other symptoms at any age after 4 weeks, however. There are no hard and fast “rules” as to when a bird may become symptomatic.

As it can be very hard to pinpoint the time of exposure/infection, this can be very frustrating.

Are older chickens less likely to get Marek's?

There is a common misconception that once a chicken makes it to a certain age, it is “safe” from Marek's. This is not so. It is less likely to be fatal to older chickens (less likely to cause cancerous lymphomas), but birds can be infected at any age and can develop symptoms at any age after 4 weeks old.

Can other bird (species) contract Marek's? Are my _____ birds safe?

To date, Chicken Marek's disease virus (MDV) is NOT known to be zoonotic (contagious) to other commonly kept fowl or other bird species, except rarely in quail, and in some cases enclosed commercial breeding of turkeys in Europe (said turkeys had been housed in closed quarters close to infected broiler chickens). Ducks, pheasants, guinea fowl, doves, pigeons, other waterfowl (swans, geese), peafowl, and ratities (emus, etc) are NOT known to contract Marek's. Turkeys can contract a similar disease (MDV-3, Turkey Herpesvirus) but this presents differently and is not contracted from chickens with Marek's (MDV). Passerines (songbirds) and parrots are also not known to contract Marek's. Be warned that there are indeed avian diseases that can spread across unrelated species, but so far, Marek's does not seem to be one of them.

How does the virus work?

Marek's is a type of DNA alpha herpesvirus. Without getting too technical, it is inhaled by the chicken on carrier dander/dust. At this point the chicken is infected. What happens next matters on a number of factors, such as the overall health of the bird, immune system response, genetics, and stress level.

In a chicken that has built resistance (either via vaccination or natural resistance) and has an overall strong immune system, the virus will be overcome by the immune system and the chicken may not develop any symptoms. In this case, the virus will sneakily make copies of its gene and inserts it into an RNA strand in the cells of the host. This is called latency and in this way a chicken will carry the virus for life, and may shed the virus. If, later in life, the chicken becomes immunosuppressed, the Marek's virus can “wake back up” and become active in the system again, thus showing symptoms months or even years after initial infection.

In a chicken whose immune system cannot fight the virus off (or into dormancy), the chicken will display symptoms. When there is an active infection taking place in a chicken, the virus inserts itself into the bird's cells, and tricks the cells into manufacturing many, many copies of the virus. In this way the virus can overwhelm the host chicken, in addition to 'shedding' large amounts of virus into the chicken's environment.

What are strains? What are serotypes?

There are (currently) three described serotypes of marek's. Serotype refers to the species of virus, and as such there are: MDV-1, which is chicken Marek's herpesvirus, MDV-2 which is attenuated Marek's and does not cause disease, and MDV-3 which is the turkey herpesvirus/Marek's. All three serotypes are closely related, but can have much different effect in a chicken's body. Only MDV-1 causes the symptoms that we see, above. The other two are considered 'benign' when a chicken becomes infected with them.

When testing blood or tissue samples for Marek's, testing is able to determine which serotype your chicken has. This can be important to know, so that a true Marek's diagnosis can be confirmed. vs. a "false positive" from having one of the other serotypes from vaccination.

A "strain" refers to a mutated or changed virus within the same species. As such there are MDV-1 strains, which represents all virulent Marek's disease virus strains and is further divided into pathotypes, designated as mild (m), virulent (v), very virulent (vv), and very virulent plus (vv+).

This information may be helpful when trying to understand the results from diagnostic testing, or when discussing with your vet or poultry expert. Be aware that if having PCR (blood) testing done for Marek's diagnosis, that the Rispen's vaccination may give false positives:

How does the vaccine work?

The most important thing to know about chicken Marek's vaccines is that they are not a cure, they are not an immunity, and they are non-sterilizing. In short, this means that giving a chicken a vaccination gives the chicken's immune system exposure to a related virus (usually HVT MDV-3, HVT stands for HerpesVirus of Turkeys). This related virus cannot cause symptoms in a chicken, but gives the immune system something to target and build a resistance. The hope here is that if the chicken is ever exposed to Marek's virus later in life, that the immune system will react appropriately to fight off progression of the virus into lymphomas, and the chicken will not develop the symptoms.

Effective vaccination in chickens will not ever prevent the chicken from becoming infected with Marek's virus. In fact, it can be assumed that any chicken exposed to the virus will become infected. Instead of preventing infection, the vaccine builds resistance within the immune system which in turn will prevent lymphoid tumors from forming. As these tumors (neoplasms) are the most fatal part of the disease, effective vaccination can save the chicken's life.

Confused? In a nutshell: Vaccinations for Marek's virus are not like most of our proven human vaccines. Vaccination for Marek's only gives the chicken an immune "warning" but does NOT prevent infection.

Should I vaccinate for Marek's?

This is a personal choice. The best way to make this choice is to determine the risks for exposure to Marek's, evaluate your goals with your poultry, and understand the disease. If you are not sure if you want to vaccinate, you are in the right place. Keep reading and make the best informed decision that you can!

How effective is the Vaccine?

This is a very complicated question with a complicated answer. There are many variables at play, including: Which vaccine(s) were administered, if they were administered correctly, the breed of the chicken, the genetics of the chicken, the natural resistance vs. vaccinated resistance in the chicken, how the chickens are kept, overall health, what "strain" of Marek's the chickens are exposed to, etc.

All things being equal, you will see many hatcheries and vaccination manufacturers claim 90% or better effectiveness. This has been true in the past. Today, however, we are seeing mutated strains of Marek's that are challenging the older vaccines. The most commonly used vaccine (derived from MDV-3, which is Turkey Herpesvirus/ HTV) is also unfortunately the 'oldest' and has been used so widely on so many chickens (mostly commercial chicken keeping) that mutated strains can now challenge it and overcome. Some research suggests that the virus is mutating faster than previously thought, to remain virulent in spite of vaccinations. It is also important to remember that the 90% figure comes from proper vaccinations of chickens kept in commercial farms, most of which in the USA employ long indoor housing, with chickens in great densities, that often practice "all in - all out" keeping, and can properly deep clean all indoor surfaces. "All in - all out" is about as you might guess: complete rotation of new flocks with sterilization in-between. Our backyard chicken keeping methods are quite a bit different! We have much less control over environmental factors, and often (and to the woe of some) we swap chickens at meets, sales, state fairs, retired battery hens, craigslist, etc etc. We rarely bring in a new flock all at once, and then cull (kill) them all and sterilize their facilities (impossible to do outdoors) each time! Under our backyard flock conditions, we are unintentionally creating a perfect situation for the virus to challenge the vaccine.

These conditions and the existence of new strains (for more details, read Disease Details below) are causing the common vaccine to be 'challenged'. So, in any given flock, more than 10% of vaccinated birds that are exposed to a more virulent strain might succumb to Marek's. It's not necessarily common that this happens, but it has to be mentioned because it's a lot more than the 10% of yesteryear. Some flock keepers have experienced more than 50% loss to vvMDV-1 (very virulent Marek's).

When should I vaccinate?

Marek's vaccinations must be administered to chicks that are less than 36 hours out of the egg. Within 24 hours is recommended. This is to give the undeveloped immune system the proper amount of time to build resistance to the harmless MDV-3.

Most hatcheries will vaccinate day-old chicks for you. The cost of vaccination is typically less than .30 at the time of writing. It is even less at some hatcheries.

You can vaccinate your hatched chicks at home within this 36-hour period, but all proper vaccination procedures must be followed in order to guarantee an effective vaccination. Always follow all pharmaceutical guidelines offered by the manufacturer of the vaccine. Home vaccination can be very difficult to do properly, due to the cold storage requirements of the vaccine and short working time.

Chickens that were not vaccinated correctly by 36-hours-old should never be considered “properly vaccinated” and should NOT be sold or traded under the pretenses that they were vaccinated for Marek's.

There is also some research that suggests that a follow-up vaccination at the age of two to three weeks old may increase the chances of immunity. Find out more:

How do I vaccinate? Is it easy? Can I store it? (etc)

Unfortunately this is one of the hurdles for private breeders and keepers of chickens. The only vaccine available to the public to purchase currently is the Sereotype 3 HVT vaccine*, which has some very specific storage, shipping, and use necessities. This vaccine must be stored/shipped cold (via liquid nitrogen) in order to be effective, and must be used entirely within one hour of mixing the vaccine and dilutent. Due to the expense of shipping and storage cold, and timing with eggs hatching, this is often diffucult for private individuals to make work. It is not impossible, but it is not quite a simple as buying a vaccine and administering it. The vaccine as discussed here is administered subcutaneously, usually by wetting the feather fluff at the back of the chick's neck with isopropyl alcohol, pinching the skin in this location to give room to work, and administering the vaccine using a sterile 18 to 20 gauge needle. This FAQ is not intended to give exact directions on vaccination, and should be done at your own risk especially if you are not experienced in administering subcutatneous vaccinations to live animals. Here is a complete breakdown on administering Sereotyoe 3 HVT vaccine.

*If other vaccines become available to private breeders/individuals since the time of writing this article, please let me know and I will add that information at that time.

How long does it take for the vaccine to work?

After vaccination at hatch (before 36 hours of age!), it is important to isolate (quarantine) chicks from exposure to Marek's disease for a minimum of 3 weeks, for the maximum benefit and best chance for the vaccine to develop resistance within the immune system. Some chicken keepers prefer to go longer periods of time, while others do not. Exposure to other chickens (or their dander, which is easily carried on your clothes and hair) before the 3 weeks is over is not recommended. Exposure before 3 weeks should be avoided at all costs if you suspect or know your flock already has Marek's disease and are carriers. Exposing vaccinated chicks before three weeks may compromise the effectiveness of the vaccine.

My chicks were not vaccinated. Can I vaccinate after the 36 hour period?

Here is where we enter into some theory, and you must not take any of this is rock solid fact. It's hardly close, and based only on the speculations of chicken owners and their real life experiences with this disease. We're entering the realm of untested science with this question, and it's very very important to remain skeptical!

Vaccinating chickens that have not been previously (properly) vaccinated cannot hurt healthy birds.

It may or may not increase their chances of building resistance if they are later exposed to MDV. No one knows for sure. Probably not, but no one knows.

If you vaccinate chickens older than 36 hours, you should NEVER consider them properly vaccinated, but at the same time it might give them a better chance than no vaccine at all (again, this is unproven). The younger the better, of course.

If you have chickens that have already been exposed to Marek's virus, there is little chance and no findings to suggest that vaccination will help them at this point. It will not cause a healthy bird any harm but it is probably a waste of time. If it makes you feel better, you can try, but do not expect anything from it.

My chicken has Marek's symptoms right now! Should I vaccinate?

I can find NO sound reason to vaccinate an actively symptomatic bird. In fact, many manufacturers of the vaccine warn against doing so. Marek's vaccine is not a cure, it is not a medicine, and it is not even an immunity. It makes little sense to introduce another virus into a bird's system when the immune system is already fighting against MDV-1 (chicken Marek's virus). Read about the 'timeline' of infection below to understand why vaccination after symptoms show is probably useless and possibly even dangerous. Do so at your own risk.

Aren't vaccinated chickens carriers for life?

This is a misconception and a misunderstanding of how the virus and disease work. The MDV-3 HTV (turkey herpesvirus) vaccine itself can NOT cause the chicken to shed live chicken Marek's disease (MDV-1). It simply does not contain any chicken Marek's disease virus, and thus a chicken can not spread or shed Marek's simply by virtue of being vaccinated. What happens, and what has caused this myth, is that a vaccinated chicken that is later exposed to an outside source of Marek's disease can then become infected, not develop symptoms (because of proper and effective vaccination) and yet will shed the virus, thus becoming a silent carrier. Studies suggest that birds that are exposed to Marek's disease become carriers regardless of vaccination, and that there is no real difference in the amount of virus shed in vaccinated vs. unvaccinated individuals. Meaning, vaccinated birds will still shed the same amount of virus once exposed to it. Reference:

The other two types of vaccines (Rispens and SB1, aka MDV-2) are not known to cause symptoms in chickens as they are a different serotype. These two vaccines are only administered by a few hatcheries and cannot be obtained for home vaccination (at this time).

Can the vaccine cause the chicken to get Marek's?

No, it cannot cause a chicken to get the Marek's disease that makes them sick. The commonly administered (MDV-3 HTV) vaccine alone can not possibly cause chickens to become infected with MDV-1, which is the type of Marek's disease chickens get that makes them symptomatic. See above answer for more detail.

If I do not vaccinate and my flock is exposed to Marek's virus, will they all die?

Probably not. There are many, may variables at play that make an individual bird susceptible to the virus or not. Except in vvMDV+ infections (mutated forms, very virulent) approximately 60% of any unvaccinated flock might succumb. This number might be far less if the flock members have been bred for immunity, and have natural resistance the the virus. or might be far more in the flock members are particularly weak against it.

Is it true that Marek's vaccine makes Marek's virus worse?

Possibly, but this is a complex issue. Marek's vaccine is a "leaky" vaccine. This means that a chicken that has been properly vaccinated for Marek's can be exposed to the virus, develop immunity, never develop symptoms, but still SHED the virus. Shedding the virus is like a dog shedding fur during periods of virus production within the chicken's body, the chicken will actively shed the virus (usually through dander/dust/feather dust) and so other chickens can still catch the virus from that chicken. If all chickens are vaccinated, normally losses are few or none, but the real problem comes when some chickens are vaccinated, some are not (chicks in commercial operations are vaccinated rapidly with machine assistance and some are missed/improperly vaccinated), and then they become exposed. It is thought that when this occurs in commercial chicken operations, where huge numbers of chickens are housed together in very close proximity, this can cause the virus to mutate more rapidly into more virulent strains. Again, it is vital to remember that almost all scientific research done on this virus is done in commercial operations, because those operations are the ones that can afford to fund studies. How this applies to small, backyard, and pet flocks can vary a great deal. This is a recent article that explains some of the findings of how Marek's vaccine has lead to possibly worse strains of the virus in commercial poultry operations. This article sounds pretty scary, but read it alongside with all of the other information we have here, and be thoughtful about your specific flock, goals, and husbandry. Also note this quote at the end of the article:

"However, in the end, Read said, leakiness isn’t a strike against these vaccines, but more motivation to conduct surveillance of their effects after they exit clinical trials and enter the broader population. Take Marek’s disease for example.
'Even if this evolution happens, you don’t want to be an unvaccinated chicken,' Read said. 'Food chain security and everything rests on vaccines. They are the most successful and cheapest public health interventions that we’ve ever had. We just need to consider the evolutionary consequences of these ones with leaky transmission.'"

Why does the vaccine fail?

There are many reasons, but some of the most common are:

  1. The chicken's immune system is challenged by (encounters) a very virulent strain of Marek's virus (vvMDV+). These are mutated forms of Marek's that have become especially nasty, and beyond the scope of the vaccinated chicken's immune system's ability to handle.
  2. Improper vaccination. Many things can go wrong with vaccination, such as not administering the vaccine within the 36 hour period of life, not administering enough vaccine or in the wrong location, or administering damaged vaccine (old, or torn vaccine).
  3. The chicken was immunosuppressed in other ways-- this is a fancy way of saying that the chicken was already sick or had been sick in the past, and the immune system was busy fighting another disease. Marek's is very opportunistic.
  4. Exposure to Marek's disease too soon after vaccination. Ideally, chicks should be isolated entirely from the disease for a minimum of three weeks after hatching and vaccination, to give their immune systems a chance to build resistance.
  5. Due to genetics and many other variables, some breeds seem much more susceptible to Marek's than others. Particularly vulnerable breeds include: Silkies, Polish, chicken breeds that have not had exposure before such as new imports, (others?). Note that this information is anecdotal and it is possible that some specific lines of a chicken breed are more susceptible than others.

There are currently three commonly used vaccinations for use in chickens against Marek's disease. They are derived from the different “related” viruses that will not cause a chicken to become infected but instead develop resistance. The first is HTV (MDV-3) which is a type of turkey herpesvirus. This is the vaccine discussed in this article, because at this time this is the only Marek's vaccine available to the consumer. The other two types are known as Rispens and SB1 (MDV-2), which are attenuated chicken Marek's virus (this is a fancy way of saying that they are benign).

Some hatcheries use a combination of all three vaccines (HTV + Rispens + SB1) to give the chickens the best possible resistance to very virulent strains of Marek's. Most hatcheries and all home vaccinations use HTV (MDV-3).

Be aware that if you are having PCR/Blood testing done for Marek's disease, and the Rispen's vaccination was used, that it may give a false positive:

Should I only consider hatcheries that use all three vaccines?

If you currently have a flock of chickens that are carrying a very virulent strain of Marek's and normal vaccination with HTV /MDV-3 dos not seem effective, then it might very well be worth getting chicks that have had the three-part vaccination. If you do not have Marek's on your property and are simply trying to prevent the disease, it may be less important to you. This is a personal decision based on your wishes, your flock's health, and experiences.

Which hatcheries use which vaccines?

If it is important to you as to which vaccines are administered, it is important to contact your hatchery of choice and ask them directly. This may change from year to year and season to season depending on availability and administrative decisions. Any hatchery worth your time should be willing to tell you which vaccine(s) they administer.

I got my chicks/chickens at a Feed Store such as Tractor Supply, or private breeder. Were they vaccinated?

You will need to contact the individual responsible for ordering the chicks, or the breeder, to find out. It is important to note that sometimes feed store employees are not aware if the chicks have been vaccinated or not and may have misinformation. While it is true that some feed stores will special order vaccinated chicks for you if you request them, in general, it is the practice of most chain feed stores to NOT vaccinate against Marek's. You may need to do some detective work to find out. Unfortunately, it has come to light that some customers are mislead (usually out of ignorance, not malice) on this issue, and may be told that chicks were vaccinated when in truth they were not. When in doubt, you can usually assume that they were not.

Very few private breeders vaccinate, but it is worth asking. Some do, and some may vaccinate for you upon request and for an additional fee.

Where can I get Marek's vaccine for my chickens?

Different retailers come and go, but for now you can try the following (I am not affiliated with them in any way, shop wisely):

I'm worried my chicken might have Marek's disease. How can I tell for sure?

There are a few ways to figure out if you have Marek's in your bird(s). There are more ways that listed here, but some of the most common ones are:

  1. Self diagnosis based on symptoms. Least accurate method, but is better than nothing. You can make some well educated guesses (see the question above, under the “Symptoms” header). These are still guesses though, as there are many diseases that can 'look' like Marek's.
  2. Necropsy (home). This can only be done on a dead bird. If the chicken showed signs of classical Marek's, sometimes tumors can be found on the internal organs as described above. Be sure to check out the resources links below to find photos of what to look for. While tumors are a good indicator of probably Marek's, be advised that Avian Lymphoid Leukosis Disease can also cause tumors that are visually identical.
  3. Necropsy (as preformed by an experienced avian examiner). This can only be done on a dead bird. This may be done at a state poultry lab or at an experienced avian veterinarian's office. Some states offer free or reduced cost necropsy of poultry. Contact your local extension office or state lab to find out about these services and any fees associated. Birds to be checked must NOT be frozen, but stored under refrigeration, and sent for diagnosis as soon as possible (within two days if possible). These necropsy reports are generally a good indicator of the presence of Classical Marek's but it is very important to note that even under skilled medical examination, not all Marek's infected birds will have symptoms. A necropsy is not a 100% for sure diagnosis. It is an educated guess.
  4. DNA blood test/ PCR testing. This is done via blood draw on a live bird or samples taken from a deceased bird. The blood or samples can then be shipped (as a medical sample) to a diagnostics lab for full testing for DNA markers of Marek's virus. This is especially useful in cases where necropsy is inconclusive or tumors are similar to Avian Lymphoid Leukosis Disease. See references for links to labs that preform this service. There is often an extra fee. See previous chapter for an explanation of the difference between serotypes and strains. PCR testing is a bit complex. Please continue reading on to learn more about PCR.

If my chicken's gross necropsy came back as “probable Marek's”, what does this mean?

A gross necropsy usually involves examining the bird inside and out for visual clues as to the cause of death. Common signs of Marek's disease include tumor growth in key organs, as well as possible neural lesions (sometimes visible under microscope), as well as swelling or enlargement of the sciatic or vagus nerves, and irregularities of the digestive system. Unfortunately, there are other diseases that are visually similar to Marek's, even upon examination. Avian Lymphoid Leukosis looks very similar to Marek's when lymphoid tumors occur. Other diseases may also appear similar, especially if they cause neoplasms (tumors). That said, Marek's is a very common disease, and often it genuinely is Marek's.

My chicken had a PCR test run, how accurate is it?

Some labs will do PCR testing during a necropsy if the bird in question was a Marek's suspect, and some will not unless it is requested. PCR testing can also be run on blood samples from live birds. You may need to ask ahead of time to determine what testing will be done. PCR stands for Polymerase Chain Reaction, and in short is a way of taking a sample and amplifying copies of select DNA in that sample. In this way, scientists "look" for copies of the DNA belonging to the Marek's virus. There are two main types of PCR testing, and this is important to know- I will explain why shortly. There is conventional PCR testing, and "Real time" PCR testing (this is referred to as qPCR). A conventional PCR test reads the DNA data at the end of the sequencing, whereas a Real-time qPCR test reads the sequencing data in real time using a laser. It is currently thought that qPCR (real time) is much more selective and sensitive to finding specific DNA in a sample.

Whew. Are you still with me? This is complex.

PCR testing in general is quite accurate, but only within a sample. Meaning, if the sample tested contains enough Marek's virus DNA, the PCR test is very likely to isolate and amplify it, therefore giving a "positive test" for Marek's. But, if a sample does not contain Marek's virus or does not contain very much of it, the PCR test may fail to amplify the DNA and therefore read as "Negative".

The problem then arises. Marek's virus is wonderful at becoming latent in an otherwise healthy chicken. Put very simply, think of this as the virus being in remission. It is hiding out in some of the bird's cells, but not causing an active infection. It can cause an active infection later on (especially if the chicken experiences a weakened immune system) or not at all. But the chicken can still "shed" and spread the virus. So it is important to know if a bird currently has Marek's (in live birds) or if a bird had the disease (in a deceased bird being considered for testing).

In birds experiencing latency (no active symptoms), PCR testing may not be sensitive enough to detect the virus, and may give a "negative" result, especially if the only sample submitted is a blood sample. When testing for Marek's in a bird that has been exposed but is not showing symptoms, qPCR is recommended because it is more sensitive, but it is not a guarantee.

Even in birds experiencing active symptoms, PCR run on blood samples alone may not find enough of the virus DNA to read as a positive result.

PCR is most reliable when it is a real-time test (qPCR) and it is run on sampled tissues where neoplasms are either found or are known to grow, such as: gonads, spleen, liver, kidneys, lungs, heart, proventriculus, adrenals, thymus. Unfortunately, such samples need to be taken from a bird that has already expired.

In conclusion, PCR testing is a very useful and important diagnostic tool, but must be considered only one part of the puzzle piece and should be considered in combination with other findings.

Where can I get a PCR test run? Who will do the "real time" qPCR testing?

Please see the reference links below. UC Davis is known to use only real-time qPCR testing. Read more:

How much does all this testing cost?

This varies greatly. Here in the USA, some state extensions will do free or reduced cost gross necropsy (bird autopsy) exams. Some charge fees between $50-200. Avian veterinarians also may do a necropsy on you bird for a fee. If you are a chicken keeper, it might be worth your time to make a few phone calls or emails to find out what services are available via your state, local university (look for a poultry department or farm extension), or vet. PCR sequencing is only done regularly for home owners at a few labs, and as such might be more expensive. If your state or local/state university offers testing, you can inquire as it might be reduced cost or free. If your state does not offer these services, you can expect to pay anywhere from $50-$300 for DNA PCR sequencing as it is a time consuming and specialized test. If you live outside of the USA, you will need to learn where testing might be done and ask for a quote. Note that if you need to ship your deceased chicken overnight to a testing facility, you must do so properly and just cold-packing and overnight shipping can easily be over $50-70 depending on your location.

So, if my chicken's necropsy came back with no signs of Marek's, is my flock “clean”?

Maybe. It is important to note that each bird can present symptoms differently, and not all of the symptoms can be seen during a necropsy. This is frustrating for many owners. You are not alone! This is a complex guessing game. Use every clue that your bird gave you to put the puzzle together as best you can.

I'm confused. So can I find out 100% for sure if my flock has Marek's or not?

You are not the only one! I am currently researching this very thing and hope to bring the best answers that I can to this FAQ. This is a very complex science and I am just a simple enthusiast. As best I understand, no SINGLE test can 100% confirm or deny Marek's disease in an individual chicken. It is when viewed as a whole, that tests, symptoms, flock history, and other clues can give us the best answers.

I have Marek's in my flock! Do I cull them?

This is a bit controversial, and ultimately it is a personal decision. Many backyard chicken keepers choose not to cull infected birds or flocks.

Simply, if you confirm that one or more of your chickens has been infected with Marek's disease, you must assume that the virus has been shed by that bird (or birds). Any place that bird has been, any part of the yard, coop, house, etc. probably has some of the virus. The virus can live for months or years outside of the chicken's body, and has the chance to infect any new chicken that comes in contact with it.

For most chicken keepers, this means that by the time you realize you have Marek's, it is already everywhere that all of your chickens have been. It is easily carried on the wind, by rodents, on your shoes, clothes, and hair. Even if you have separated coops and flocks, if you walk back and forth between them you have spread the virus. Essentially, what I am saying is, unless you take drastic daily biosecurity measures, you probably have Marek's disease everywhere in the places you keep chickens.

So, culling would really not provide any benefit. All of the exposed chickens are probably infected. It is possible for infected birds to survive the disease.

Only one of my chickens has symptoms of Marek's! Should I cull it?

See the above answer. If that chicken does indeed have Marek's, and has been around your other birds or even been to the places that they go, it is too late. It has already spread. Culling the bird will not prevent the spread of the virus unless it has been in isolation for months. If you have a bird that is obviously suffering, consider that sometimes the kindest option is to humanely cull (end its life). Only you can decide at what point this should happen.

Should I isolate any chickens that are showing symptoms of Marek's disease?

In general, this is a good practice no matter what the disease. It's a good idea because even if it is too late to prevent the spread of the virus to the rest of your flock, it is documented that chickens that are presenting active symptoms also shed a lot more of the virus than those that do not show symptoms.

It will also help to monitor their health, weight, food and water intake, and administer any care you need to give to the sick birds.

In addition, if it is not actually Marek's but another disease, isolation might help prevent its spread!

That said, chickens are highly social animals. Some keepers find that their birds fall into depression and might stop eating and become more lethargic when separated from flock members. It is still very important to isolate any bird showing signs of an unknown ailment, as a precaution to keep it from spreading, but if the risk of spreading any illness is low, do what you feel is best for your individual bird.

My chicken(s) are showing symptoms. What should I do now?

First, isolation of the sick bird(s) would be a good idea, if for no other reason else than to observe them. See previous question on if isolation is the best choice for your situation or not.

Give them an area that is a comfortable temperature, free of stress, and away from dangers or disturbances. If the bird is presenting with paralysis, extra support might be necessary. A laundry basket, small cage, or even rolled up towels/rags on either side of the bird can help prevent it from falling over due to paralysis. Many people try to build a sling for chickens that have leg problems such as paralysis, with mixed results. Here is a good Backyard Chickens thread that has photos of various slings people have made. You can search for forum and Google for more ideas. This can be made with common objects. Chickens should not spend all of their time in a sling, but it can help them if there is a chance they will regain motor control of their leg(s).

Make sure the bird has ready access to clean water and nutritious food.

If you have a scale, it is extremely beneficial to monitor the bird's weight twice a day (once a day minimum). This can be used to determine if there is weight loss, gain, or stability.

Observe the bird for behaviors and symptoms. Observe if the bird is eating, drinking, or defecating.

Check the bird's crop. The crop should pass food over time. Chickens normally roost with at least some food in their crop to digest overnight. Many chickens go to bed with a full crop. An empty crop at "bedtime" can be a red flag.

If you feel there is hope for your chicken, and it will not eat, you can consider tube-feeding a chicken that won't eat. Tube feeding puts food directly into the crop and bypasses the beak. This has mixed results sometimes getting food helps a chicken get over the edge and the bird recovers, but other times the chicken is "doomed" from the start (such as if they have visceral lymphomas/cancer). Tube feed at your own risk. Here is an excellent Backyard Chickens thread on how to tube feed properly. Never force feed or offer liquids into a chicken's beak unless you know exactly what you are doing, as it is very easy to aspirate (choke) a bird, which can cause pneumonia or death.

If the bird has symptoms matching another disease besides Marek's, consider trying to treat the alternative disease (as there is no cure for Marek's, but other diseases might be treatable!).

Seek medical advice from your local avian veterinarian or state poultry department. If this is unavailable, it may be worthwhile to describe the situation in as much detail as possible to chicken-keeping peers, such as the Backyard Chicken Forum, to seek aid.

There is no cure for Marek's. You can provide supportive care and sometimes chickens will recover (but will never be cured), and sometimes they do not. Continue reading below for holistic therapy ideas.

Are there any medications I can give specifically for Marek's?

There are no cures for Marek's disease. Any medications administered in relation to Marek's are generally given to help with secondary problems, or are holistic/home remedy in nature. Some holistic medicines have claims to help ease symptoms, though it is important to understand the difference between the medicines treating actual Marek's symptoms and treating what appears to be Marek's but is possibly a different disease or problem. Even a bird that recovers from symptoms (with or without the aid of holistic remedy) is never truly cured, but is in remission, and the virus can still be spread and the chicken may still have a relapse, depending on the form that Marek's presents with.

  • St. Johns Wort, specifically Hypericum
  • Turmeric + Black Pepper, specifically Curcumin with Bioperene (thought to be a blood cleanser)
  • Vitamin B ("Super B") Complex, Nutritional Yeast (check label to be sure of high vitamin B-12/Thiamin percentages!)
  • Cold-pressed Coconut oil (medium-chain fats help vitamin absorption in general),
  • Cranberry, specifically high quality whole berry (can be chopped, mashed, or cooked into a sauce) or no-sugar sauces/juices in a pinch
  • (others? if you know of one that has helped, please let me know).

It is currently unknown how cranberry works, however some claim success with its use, and fortunately feeding cranberry in moderate amounts has no known negative side effects. I have read that it is similar in composition to wild berries that wild Jungle fowl will eat when they experience disease. I don't know if this is true or not! In short- it might not help, but it won't hurt.

B-vitamin complex is scientifically known to aid in nervous system repair in mammals, and though it is hardly a cure for a bird with Marek's, it may help if the bird is able to survive neurological Marek's. Nerve damage can take months to repair. Lymphomas, once they form and spread in the chicken's body, are nearly always fatal regardless of therapy. Some birds may experience nerve damage without lymphomas, so it can not hurt to try different therapies if you are sure you are experiencing Marek's in your bird. If you are curious about dosage and how to administer holistic/home remedies, I am no expert, so I please encourage you to research each item on its own. There are many threads here on Backyard Chickens that you can search, as well as Google.

It is understood that these approaches are used to help manage symptoms at best, and are never a "cure".

My meat chicken died, and had Marek's. Can we still eat it?

Yes. Marek's disease offers no threat to humans or even our other pets that might eat chicken (such as cats/dogs). Often, symptomatic birds are very thin and may not be worth processing, but if they are, there is no risk to you or your family. Use safe preparation, handling, and cooking methods, as always. People unwittingly consume latent carriers (with no symptoms) all the time. There is no harm.

One of my chickens just died, and I want to test it! How do I do this?

First, immediately store your deceased chicken under refrigeration, ideally gently wrapped in plastic. DO NOT PUT BIRDS TO BE TESTED IN THE FREEZER. I know this can sound awful, but it is the best way to preserve the bird and make sure the necropsy will be accurate. Take some time to mourn if you need, but try to find out where you need to take or ship your bird within 24 hours if possible. The longer you wait, the less accurate any findings will be (in general). Please see the "Diagnosis" section for help, and see the resources at the end of this article for links on how to find someone to help you. See the link "How to Send a bird for Necropsy" at the end of this article, if you need to ship your chicken to a testing facility.

My egg layers have Marek's. Can we still eat the eggs?

Yes. See above answer-- there is no risk from eating eggs from Marek's positive birds. If you are medicating your birds, always follow proper withdrawl times for medication before consuming their eggs.

My flock has confirmed Marek's. Can I ever bring in new birds?

Yes, but you must be careful. Even with proper vaccination it is important to remember that there is still anywhere from a 10-50% chance of loss, depending on virulence and resistance.

  • You can consider ordering day-old vaccinated chicks from a hatchery. Please see “Vaccinations” for more details.
  • You can consider hatching chicks yourself and vaccinating at home before they are 36 hours old. You must then practice very good bio-security and use every precaution not to expose the new chicks to the virus for a minimum of three weeks, to insure the best resistance. You might even consider re-vaccination. See “Vaccinations” for more details.
  • You can breed your own flock for resistance. This is a long-term process and can be heartbreaking if you are attached to your birds. See below question.
  • You can also bring in older birds (older than 36 hours) that have not been properly vaccinated, and hope for the best, but be aware you will probably experience losses.

Marek's virus is spread by several different vectors. The virus is an encapsulating virus, meaning it creates a protective shell around itself after it leaves a host. Because of this, it can survive outside of a host for a long time! Think of it as a tiny, awful seed that holds the virus inside, just waiting to be "planted" on a new chicken. It is important to remember that most commonly it rests in the dander (dust, feather particles, etc) of an infected chicken. Dander can be so fine that it's barely visible. Infected chickens may never show symptoms.

  • Other chickens in proximity to one another, either because they are a flock, or are housed (even temporarily) in the same area, room, property, or are neighbors.
  • Exposure to infected chickens seems to be the primary way Marek's is spread.
  • The wind can blow dander, sometimes for several miles.
  • Wild birds that come into contact with chicken dander (such as birds that forage where chickens do, or roost in barns with chickens, etc) can spread the virus. The wild birds can carry the virus on the outsides of their bodies, but are NOT known to become infected. They are just little transporters.
  • Rodents that track through chicken dander or litter can carry and spread the virus onto neighboring farms or properties the same way wild birds can.
  • Human keepers can spread the dander on shoes, clothes, and hair, the same way wild birds or rodents unknowingly spread the virus.
  • Any materials that are exposed to chicken dander, either directly or indirectly, can spread the dander.

Doesn't this mean that Marek's is EVERYWHERE?

Yes and no! Marek's IS common but it is not necessarily everywhere. There is hardly any “guarantee” that any given flock or chicken will become exposed to Marek's. I have heard some say that it is not a matter of “if” but “when”, which is a little misleading. Marek's is very common and is very easily spread but should not be considered “everywhere”. Despite it not being “everywhere”, it is still important to practice good bio-security and use caution!

Can't I cull all my chickens, clean everything, and start over?

Maybe, but it is risky, very impractical, and unlikely to totally eradicate the virus. It is important to remember that Marek's travels on chicken dander, which can blow everywhere on your property. It can live for months or even years in the environment. Even if you culled all your birds and managed to 100% sterilize your coop, the means by which to remove it from the soil on your entire property (and possibly neighbor's, if you do not have a large lot) is financially beyond the means of most backyard keepers. It is also likely you have tracked it into your home at some point. And, even if you managed to do all this, there is the chance it might come back in on the wind or a bird (etc) anyhow.

In the end it is a personal choice, but the measures to eradicate it from home, backyard, and farm type environments are extreme and it is hard to do. Most keepers opt to vaccinate new birds and/or breed for resistance.

What if I build a new coop on the other end of my property?

This is unlikely to change anything. As noted before, Marek's spreads very easily on a breeze or on your shoes, clothes, hair, when a chicken flaps, on the dog's feet when he runs across the yard. etc. It would not be impossible to imagine many ways for the virus to make it from one end of a property to another, even if the property is many acres in size. Marek's is known to spread for miles quite easily.

How long can the Marek's virus live without a host, such as in the soil or surroundings?

Marek's virus has been shown to be very long-lived outside of a host, as many herpesviruses are. It has been documented to live for 65 weeks (that's over a year and a half!) with no host, in an uncleaned environment. Research seems to indicate that Marek's virus lives the longest in DRY environments, such as inside coops, homes, anywhere dust can accumulate, and also in more arid climates and drought stricken areas. It seems to break down more quickly in wetter environments, such as in the soil and in moist climates where it is exposed to the elements. There are many factors at play that determine exactly how long the virus can live, so it is important to 'assume' that it will be there longer than you expect. While we can get things such as coops and non-porous surfaces clean with proper cleaners (see section as to what cleaners kills Marek's virus), it is impossible for most backyard chicken keepers to clean the soil and surrounding environment, and so it must be assumed that any place that an infected chicken has been also is laced with shed virus that can live for over a year and a half at a minimum. Though it has been documented to live for over a year and a half, there are those that think it may live for over 7 years in an uncleaned environment (such as indoors, or in a dusty coop, etc). There are other factors that may come into play with how long virus might live however it has been shown to survive even very hot summers and cold winters, so you cannot rely on your environment alone to negate it.

So it's nearly impossible to eradicate. Should I still clean things?

Yes! This is still a very good idea. In fact, other thank keeping the chickens overall in good health, it might be the most important thing you do. Reducing the amount of virus in your chicken's environment can only help to prevent spreading it further either on the wind, on yourself, via wild animals, etc. It is best to concentrate on areas where chicken dander accumulates, such as inside of coops, barns, etc. physically remove as much as possible (I like to use a shop vac with a drywall filter) and then saturate surfaces with a virucide to kill the virus. If you use an earthen floor in your coop, consider replacing the top several inches. If you cannot do this, research the safe use of lime in the soil. You cannot hope to completely eliminate 100% of the virus, but you can significantly reduce the amount. The side benefit to this is that by keeping a cleaner environment for your surviving chickens, they will have less stress and chance of becoming sick from a secondary illness. Anything you can do to make your Marek's positive chickens healthy and stress-free will give them a stronger immune system.

My chickens have been vaccinated, but I have or have had Marek's in my flock. Will they still shed the virus?

Yes. Studies have shown that it does not make a difference if birds are vaccinated or not-- once exposed and infected, even if they never show symptoms, they shed the virus. There is no considerable difference in the amount of virus shed, between vaccinated and unvaccinated individuals, once they have been exposed. All birds exposed to Marek's virus should be considered carriers, for life. Reference:

Bleach is not known to be effective against Herpesvirus (Marek's) on porous surfaces.

What kills Marek's virus?

You want a cleaner that has a known virucidal that kills Herpesvirus. Use as directed on the label as a virucide, and always use proper personal protection! These can be harmful if misused.

The following are listed to be killers of herpesvirus:

Activated Oxine Oxine.pdf - Note, it MUST be activated to kill Marek's!

Virkon S - Specifically mentions that it can kill Marek's, and on porous surfaces. Probably the best choice for serious situations.

Article: Disinfection of Marek's Disease Virus in Poultry Dust: Note that this is an older article and while still useful, it is probably best to use one of the products listed above for home use.

If you know of other viable methods of control, please contact me and I will add them.

How do I keep from spreading this virus to others?

It is very thoughtful to try to keep this virus from spreading. You may want to dedicate a pair of shoes as 'chicken only' shoes that you wear when you do daily chicken chores or walk out to the coop.

When you plan to go to a place that other chicken keepers go, such as the feed store, consider spraying your shoes with one of the virucides listed above.

Don't give away, sell, or show any chickens from a flock that has had Marek's virus exposure.

If other chicken keepers visit your property, offer them baggies for their shoes, and/or spray their shoes with a virucide before they leave. Do the same for your shoes when visiting other chicken keepers property.

Can I show a chicken that has been exposed to Marek's disease?

This is putting other chickens at the show at risk, and could be considered irresponsible and unfair. If you plan on showing chickens, it may be important to confirm if you have Marek's disease in your show birds or not. Remember that some other diseases look like Marek's. A blood test might be worth your time and money if showing chickens is very important to you.

Can I breed and sell birds that have been exposed to Marek's disease?

Again, this is putting other chickens and chicken keepers at risk. If you wish to breed and sell birds, it might be worth your time and money to get a blood test run on your breeding birds to be sure if they have Marek's (or not). While they cannot transmit the disease vertically (through the egg), any hatched chicks would become exposed upon hatching. It is possible to hatch eggs in an incubator even if the flock is Marek's positive, but EXTREMELY good bio-security measures would need to be taken and the chicks would need to be raised in nearly clean-room levels of bio-security. It would require a great deal of effort and dedication to keep the chicks from becoming accidentally exposed until the point of sale.

It may also be possible to sell hatching eggs, if you exercise extreme caution about accidental exposure of the eggs or packing materials to the virus. It would also be ethical to let potential buyers know of the risk.

Can I give one of my Marek's exposed chickens to someone else that has Marek's?

This would be risky. There are now several strains of Marek's, some of which have mutated. There is a chance that the strain that your birds have is different than the one in someone else's flock. If this were the case, it could infect both your bird and theirs and cause a new flush of symptomatic birds. Especially if any of the birds already have compromised immune systems! Do this only if the risks are acceptable. It might only complicate the problem.

How can I prevent Marek's disease in my unexposed flock?

First: Educate yourself. The fact that you have made it this far gives you a head start!

Now that you have the information, you can decide if you want to vaccinate, or not. See: Vaccinations, above. There are benefits to vaccinating day-old chicks, and possible benefits to vaccinating older, unexposed birds.

Then, do everything you can to practice good bio-security.

If you visit other areas with chickens, consider washing your shoes, clothes, and even your body as soon as you get home. Think critically about where other chicken keepers frequent, such as the feed store. It is not over-doing it to spray your shoes off after getting back from the feed store!

Use a lot of caution when buying, adopting, or getting new chickens from swaps, fairs, markets, Craig's list, other keepers, etc! Don't be afraid to ask questions about their flock, any past illness, any deaths in the last few years? Trust your gut if you see any red flags, and always quarantine new birds for a minimum of 30 days (60-90 is best but might not be practical for most keepers). Quarantine is not a guarantee (remember, a bird can be infected and never show signs!) but it is better than nothing. Understand the risks! Most people that bring home Marek's do so by bringing home new, already exposed birds. Backyard Chickens forums are full of stories of people bringing home new birds and watching in dismay as over the period of a few months, diseases seem to appear from nowhere.

Finally, do everything you can to keep your birds healthy, clean, and stress-free. Stress weakens the immune system, which invites trouble. By keeping things clean and removing dander periodically, you reduce the amount of disease vectors in their environment. And, a healthy chicken is going to have the best immune system! A good diet is core to long-term health.

If you have chickens. even if they have never been sick, now is the best time to take some time and make yourself a list of contacts. This will save you a lot of headache in case something happens. Having the information already written down can really help you cope with a chicken medical situation!

  • Do I have a local avian veterinarian? (not all vets know avian biology!) Do they see chickens? What is their phone number and location? What are their fees?
  • If I do not have a local avian vet, is there one that is not local that I can call and talk to for consultation? Some vets offer this while others demand to see patients in person. Some are understanding about home-treatment of livestock animals, and others are not. It is worthwhile to find out now, before you have a problem, if there is a vet that will at least talk with you on the phone.
  • What is my state's poultry department? Who is their poultry contact? What is their phone number? (this may be a county or state agricultural extension office, or it might be called something else-- type your state into google and "extension office" or "agricultural department", etc)
  • Does my state offer necropsy on dead birds for testing? What are the fees for this? Who do I contact if I need this service? Where do I send my bird?

They offer PCR (genetic/DNA) testing of blood samples for Marek's markers. Blood can be drawn from a live chicken. You may need the assistance of a veterinarian or a vet tech for a proper blood draw and/or for a sample vial. Call for up-to-date fees and any special instructions.

They have done extensive PCR testing for me via blood sample and tissue samples after necropsy. You will need to contact them to find out how to submit samples and what their fees are.

UC Davis, Veterinary Medicine (California) -

Offer necropsy services and qPCR testing. I am unclear if they will take out-of-state samples calling them directly for inquiries is your best bet! This is an invaluable service for California residents.

Contact your state's agricultural / extension offices to inquire if they preform testing. As PCR testing continues to improve with technology and up to date sciences, more and more labs offer these diagnostics. It's worth asking. If not, they should be able to point you in the right direction toward someone that does. This information is great to have on hand before you have any problems.

Poultry/Extension Labratories, by state:

List of State Veterinarians:

This may not be possible in some areas, and not all avian vets see chickens regularly, but if you have this resource, it can be invaluable! My own vet has helped me more than I can ever say.

Experts in Poultry / Avian Medicine

Dr. Jarra Jange at Cornell University of Veterinary Medicine (Ithaca, NY) -

Dr. Jange is very knowledgeable about poultry and specializes in Field investigation of common diseases of poultry.

Dr. Laura Wade, Board-Certified Avian Veterinarian (Buffalo, NY) -

Dr. Wade has quite a bit of experience in diagnosing and treating poultry in the past. In my personal experience, she went above and beyond to diagnose and test for Marek's disease in my bird.

List of veterinarians, as compiled by BYC users, that will see poultry (not necessarily "experts") -

Note that not all of these veterinarians can be considered "experts". It is always a good idea to ask questions and find out how experienced a vet is about poultry medicine. In a pinch, though, sometimes any vet is better than nothing. Use your best judgement. If a vet is experienced with Marek's disease, all the better.

If you know of any other experts in the field of poultry disease and/or medicine please contact me and I will be happy to add them!

These can aid you if you are preforming a necropsy at home. Warning: Graphic photos!

Take special note of the different tabs above the photos. Clinical signs are outward symptoms. Gross lesions are photos of what you can expect to find inside the bird. Normals are photos of what normal, healthy organs look like, for comparison.

This file contains a section on Marek's disease, as well as photographs of lymphomas.

Articles, papers, more reading

It is important to note that most scientific research conducted on Marek's disease has been conducted on large scale farming operations, typically of meat birds and production layers. Studies are very expensive and usually only commercial operations pay for them. Not on small backyard flocks, small flocks that are able to range in uncontrolled outdoor conditions, or pet chickens. Because of this, many such articles that are targeting commercial operations suggest culling all infected birds. This is because in industrial applications, the entire area where chickens live (indoors) can be treated with virucides and the virus can be completely killed off, which we cannot do in our yards. Some of the research is also out of date with the current types of vaccines used by some hatcheries and some of the mutated strains of Marek's. Use your best judgment when taking advice from this article or any other article you read. Check the dates on the articles. Don't feel alone-- many of us have been on the same frustrating and confusing journey that you are embarking upon!

Various Articles by the American Association of Avian Pathologists:

ARS article on new PCR testing for Marek's as well as the development of new vaccine (August 2014):

The best support community I have found is the Backyard Chickens Forum. There is a group of us “Marek's People” on there that are constantly researching, sharing information, and offering hugs through this difficult process. You can search us out or start a new thread here:

There are now more resources online than ever before. if you find yourself wanting to research more deeply, go for it.

If you find factual errors, inconsistencies, or other problems with the information in this article, please do not comment but instead email me ([email protected]) so that I can make the appropriate corrections. I often miss comments on this article. I am just one person and might make mistakes.

If you would like to make a scientifically sound addition that you feel would be helpful to other readers and chicken keepers, feel free to email me.