A Quick and Dirty Guide to Marek's Disease: Things every chicken keeper should know.

Marek's disease is a form of herpes virus that can infect domestic poultry and a few wild birds. In chickens, the virus can rapidly cause cancer...
By thunderclast · Jun 20, 2017 · ·
  1. thunderclast

    Hello fellow chicken folk! I'm writing this article after becoming intimately familiar with Marek's in one of my chickens. I've seen a lot of good information floating around this site, but I wanted to consolidate and summarize everything. My goal in this article is to quickly educate you about the key points of Marek's disease without overwhelming you with superfluous information.

    All information I'm providing here has been carefully researched and double checked with a poultry professor at a local university. Although I have made every effort to provide accurate information, some may still be inaccurate. If you have any corrections, please let me know.

    1. What is Marek's?

    Marek's disease is a form of herpes virus that can infect domestic poultry and a few wild birds. In chickens, the virus can rapidly cause cancer to grow all through the chicken, most commonly in the nerves, organs, and feather follicles. The virus is spread through chicken dander and is extremely contagious. If one of your chickens has Marek's, you can be certain that the rest have been exposed and are carriers. Wind and wild animals can carry Marek's for miles. Because the disease is so contagious, Marek's is considered ubiquitous worldwide. You should assume that every chicken is infected with Marek's unless it has been raised in a completely biosecure environment--nearly impossible for backyard chicken keepers.

    Marek's is NOT contagious to humans and a Marek's infected bird is perfectly safe to eat. In fact, you have probably eaten numerous Marek's chickens already. However, it's still probably not a good idea to eat a chicken that looks sick, since it's possible the chicken has another infection that could make you sick.

    2. What are the symptoms of Marek's?

    In many cases, Marek's causes no signs or symptoms. A bird with natural/genetic resistance can live a perfectly healthy life. This is why you must assume that every chicken has Marek's. There's simply no way to tell. However, in unvaccinated birds with no natural resistance, the virus can kill between 60%-100% of a flock.

    Keep in mind that Marek's symptoms will only begin to show up weeks or months after the bird is infected. Most commonly a hen will begin showing symptoms around the time when she begins laying, with roosters showing signs when they are slightly older. However, if a chicken's immune system becomes compromised, the symptoms may show up later or earlier. In most cases, a chicken who lives at least one year with no symptoms will never show symptoms, even though it is a carrier.

    The most common sign of Marek's is a chicken who appears to be paralyzed in one limb, usually one leg. The chicken will likely interact and behave completely normally, except that its leg doesn't work. If your chicken is showing this symptom, it's very likely that it has Marek's. Over time, the paralysis will worsen until the chicken dies of wasting, asphyxiation, or is killed by the rest of the flock. This can be heartbreaking to watch.

    Other common symptoms include:

    • Ocular degeneration. The iris may lose its color and turn blue, or become misshapen. The bird may become blind. This is probably the second most common form of the disease.
    • General neurological impairment. The chicken may stagger, act dizzy, or generally show signs of weakness or confusion.
    • Poor growth or frequent illness. The disease weakens the chicken's immune system and makes it more susceptible to all diseases.
    • Wasting away. The chicken may begin losing weight rapidly as its organs are consumed by tumors, and may become thin and weak.
    • Tumors on the skin and feather follicles.
    Note that it is very difficult to diagnose Marek's without a necropsy, since it can mimic other diseases. A blood test is usually not worthwhile, since most chickens will show positive. In my personal opinion, it's generally not worth the money of getting a firm diagnosis; I prefer to diagnose based on symptoms. Others would disagree with me and say that you should at least test the first chicken you lose to make sure it's Marek's and not some other disease that could be treated.

    3. Is there any way to treat or cure Marek's?

    Marek's cannot be treated or cured. Some keepers swear by "natural" and "homeopathic" remedies, but none of these have been confirmed by rigorous study and I won't devote any time to describing them here.

    4. Is there any way to prevent Marek's?

    Sort of! Other than extreme biosecurity measures, there's no way to prevent a chicken from becoming infected. However, vaccination can give your chicks a better chance of surviving when they do become infected--survival in adequately vaccinated and quarantined chickens is approximately 90-95% (although success of the vaccine has been decreasing in recent years as the virus mutates).

    Vaccination will NOT expose your existing flock to the disease. The vaccine that's typically used is actually a turkey form of the virus. It does not cause any symptoms in your vaccinated chickens, and will not cause the chickens to spread the virus. It just gives them a better chance of surviving if they are exposed. If a vaccinated bird is exposed, it will become a carrier, even if it shows no symptoms.

    Vaccination should be done in ovo (inside the egg) or during the chick's first day after hatching for the best protection. This will help build the chick's immunity before it is exposed to the disease. Vaccination starts a race between the chick's immune system and exposure to the virus. After vaccination, it takes anywhere from 10 days to 5 weeks for the chicken to build adequate immunity. In other words, it's important to quarantine your new chicks for no less than 10 days, but preferably 5 or 6 weeks after vaccination. This will allow them to build immunity before being exposed to the virus.

    During quarantine, it's important to have very good biosecurity. You must keep vaccinated chicks in a separate location that will not become contaminated with any chicken dander. Since chicken dander may stick to your hair, skin, and clothing, it is a good idea to take a shower and change clothing and shoes before interacting with the vaccinated chicks or entering the quarantine area.

    Now, there may be some good reasons not to vaccinate. For example, a meat bird will likely be butchered before it shows any symptoms of Marek's. So vaccination of meat birds might be overkill. Some people say that the vaccine may cause the chicks to grow slower and smaller, but I don't believe there are any scientific studies that confirm this. In my opinion, it's still worthwhile to have meat chicks vaccinated.

    Another reason not to vaccinate is if you are intentionally trying to breed a Merek's resistant flock of birds. Some breeders have reported doing this successfully. This is out of reach for most back yard chicken keepers, since roosters are not allowed in most urban areas.

    Finally, there is some evidence that the vaccine can cause the virus to mutate into a more virulent (deadly) form over time. I have no opinion on this, as I think the research is not yet clear.

    Given that Marek's is a horrible cancer and wasting disease that can easily kill most of a flock, I personally believe that vaccination is worthwhile in all but a few special cases. I will make sure all of my future birds are vaccinated.

    5. What should I do if one of my birds is showing signs of Marek's?

    First: DO NOT PANIC

    Second: Acknowledge that you will lose some birds to the disease, perhaps most or all of your birds if your flock is not vaccinated. In most cases, once symptoms show, the disease will only get worse and worse until the chicken starves to death or asphyxiates. In my opinion it is much better to put the chicken out of its misery while it still has a reasonable quality of life. Further, since chickens who are showing symptoms are likely shedding more of the virus, it's probably a good idea to cull sooner rather than later.

    Third: Decide how you will manage the disease going forward. A Marek's infection doesn't mean the end of your chicken keeping days. For backyard keepers, I recommend continuing business as usual, while keeping an eye out for sick birds and culling when appropriate. I do not recommend culling your whole flock and attempting to disinfect your yard and start over. This requires special virucides, and in most cases a backyard keeper will not be able to adequately disinfect their entire yard. Furthermore, any new flock will likely be contaminated anyway by neighboring chickens, wild birds, or even just the wind. So a mass culling and sterilization would likely not help anything.

    In summary: I believe the best approach to managing a Marek's infection in a backyard flock is to (1) cull sick birds as necessary, (2) vaccinate and properly quarantine all birds you acquire in the future, and (3) never to give or sell your birds without informing the buyer that they are a carrier for Marek's.

    As a final note, please be aware that you may spread the disease to other flocks on your clothing/shoes/hair. Please take adequate precautions to prevent spreading the disease.

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Recent User Reviews

  1. ronott1
    "Good Article"
    4/5, 4 out of 5, reviewed Sep 7, 2018
    I would like to see footnotes to support statements in article

    Well written and a good resource for Mareks!
  2. k2panman
    "Excellent information - every newbie should read!"
    5/5, 5 out of 5, reviewed Jul 26, 2018
    Well, I never knew! In the many hundreds of BYC post and articles I have read on raising chickens, I’ve never come across information on Marek’s disease. This article is infor And to the point- a must read for every chicken owner.
  3. CCUK
    5/5, 5 out of 5, reviewed Jul 25, 2018
    Marek's is nasty. But it's not the end of the world. Just a different approach to how you manage your flock and tough decisions will have to be made.


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  1. AudieWarren
    There is so much information floating about concerning Mareks. I'm pretty sure my flock might have it.... Thanks local auction for having the breed I was looking.... Anyway I am thinking the kind I am seeing is either visceral or.... That which effects the skin... (having a brain fart and can't remember the name of it) I have culled a few hens at this point. Tomorrow morning I plan to do a few more. If they aren't healthy AND producing eggs, I don't have use for them. I know that sounds harsh, but that's the truth of it. I have livestock not pets. I love them and want the best, but I am not going to babysit sick birds. I have a hen with one cat eye. She doesn't lay anymore, but acts fine. She survived a major injury to her thigh too. She's one tough cookie. I have a runty rooster.. A have a pullet with a eye that is at half mast. She has a sister and they are both runty. The 2 I've culled had scabby skin on thier backs and were looking emaciated. Culling was the most humane thing to do. I guess I'm asking what's your opinion? Any feedback is welcome.
  2. DebiC215
    I had one 3 month old break his leg two weeks ago and had it splinted. He just went back in with the flock this week and is doing well (still splinted-bandage changed when needed)- Now I have one of my 3 month old silkies start dragging her leg. She hides and I was afraid they would run over her or she would not eat, so I put her in a sep coop where she can still see the others and she is eating and drinking well. I could not find any injuries - but assumed it was done the same time as the other chicken (a dog just walked around the coops and they went nuts-never seen a dog before - just mini horses). Could she have Mareks? I hope not as I have two runs with coops set up side by side and cannot move them - all the others seem fine. Can I vaccinate her now - would it hurt? I got her from a feed store and am not sure if she was vaccinated.
    1. thunderclast
      It sounds like it could be an injury, but based on your description (limping with no apparent injury, 3 mos old) it could very well be Marek. Keep an eye on her. If she start's getting better, it's not Marek's. If she keeps deteriorating, it probably is Marek's. Vaccinating at this point would not help; you just have to hope her immune system gets it under control. If not, culling may be the most humane option.
  3. msyfoopoo
    Thanks for writing this. Very clear.
      thunderclast likes this.
  4. Coop de Grass
    Is it possible for hatching eggs to contain the virus? I just purchased hatching eggs and after that read something on a FB page about people having problems with Marek's. They did not purchase eggs, they purchased birds.
    1. thunderclast
      Marek's cannot be passed to the chick inside the egg. The chick must come into contact with infected dander after hatching.
  5. Herdcutter
    Thank you for a direct description of Marek's. 3 yrs ago, I ordered my first chickens. I received 4 of them and they were/are the best birds. There were 2 very friendly, 1 so-so friendly and 1 fairly shy chick. Well, as they went thru the growing stages, I noticed 1 very friendly not coming out to free-range my backyard. Also, she started loosing weight. I chalked it up to her being the low hen in the house.

    We ended up making the decision to cull her when I realized she couldn't even walk. It was a slow progression.

    As I read this article, you have answered and educated me. I think she had Marek's. I paid for vaccinations. But was it enough? The other 3 hens are still alive and well.

    I will always vaccinate my animals.
      thunderclast likes this.
  6. Whittni
    You've forgotten to cite your research article sources and your interview with the professor. Typically, these are places at the bottom of the article.
      thunderclast likes this.
    1. thunderclast
      You are right! I regret not keeping a better research log so that I could cite my sources--and I understand that the article is less useful without those. I'll certainly try to do better about that in the future, although for this article I think it's too late to salvage.
      Herdcutter likes this.

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