Marek’s Disease – Six Things You Should Know

Marek’s Disease – Six Things You Should Know

Marek’s Disease is as confusing as it is frightening and devastating. Because it manifests a variety of different symptoms and many of those same symptoms occur in other diseases, many flock keepers aren’t aware that Marek’s is lurking in their flock.  And many flock keepers don’t realize that no matter how stringent they are in their husbandry practices and no matter how meticulously clean their coop, their flock still has a high potential for Marek’s.  If you’re concerned about the potential Marek’s in your coop, I suggest you take this easy two-step Marek’s detection test right now.  1—Go to your coop.  2—Open the door and look.  Do you see chickens?  Then your potential for Marek’s is high.  This, of course, is an exaggeration to make a point, but only a slight exaggeration. 

I wonder how the Hungarian vet, Dr. József Marek, would have felt back in the early 20th century had he known that in the future his name would strike fear into the hearts of poultry-keepers everywhere?  Flock keepers have good reason to be afraid of Dr. Marek’s disease.  It kills and maims chickens and it’s lurking practically everywhere.  If you’re like most backyard flock keepers, you’ve probably heard a lot about Marek’s and some of it may seem contradictory and confusing.  And if you’re like me, you probably have a mixed bag of birds—the chickens you bought from a hatchery may have received their Marek’s vaccine.  Those that came from a swap meet, the neighbor down the road, or from eggs you hatched probably haven’t.

Since I’ve had any number of chickens for more than a few years, I’ve seen a variety of different diseases in my flock.  Some of my hens have become debilitated or have died with symptoms similar to Marek’s symptoms – but because those symptoms are consistent with a number of other diseases, I don’t know, based on those nonspecific symptoms, if I’ve had Marek’s in my flock.  I recently had three of my hens tested for Marek’s, but while waiting for results, I brought home this year’s baby chicks – my six little Silkies.  These babies came from a breeder who does not vaccinate for Marek’s.  My overriding question became, “Should I vaccinate these little guys?”  And my immediate follow-up question was, “HOW do I vaccinate them?”  Other questions filtered through my brain one after the other: “What’s the real Marek’s risk to my flock?  What strategies can I exercise to prevent it?  And just how serious is it, anyway?”  I decided it was time to do some research.  The paragraphs that follow contain some of the information I was able to dig up.  I’m sharing it here with the hope that it may be useful to you.  Bear in mind that I’m not a Marek’s expert or a vet.  I’m merely a chicken owner sharing information.  Also, keep in mind that this is just a short thumbnail of a very complex disease.

1 - Marek’s Can Devastate Your Flock

The severity of a Marek’s outbreak in your flock can vary depending on a number of factors.  Susceptibility can vary from breed to breed and even line to line.  I’ve seen several reports that Silkies and Seabrights are especially vulnerable.  Young chickens are more vulnerable – chicks can become infected the moment they hatch.  The disease is most commonly seen in youngsters 12 – 24 weeks old, and some forms of Marek’s can cause a mortality rate of 70% in unvaccinated birds of that age.

Chickens over one-year-of-age can become infected carriers and never show symptoms.  Marek’s vaccine reduces symptoms, but vaccinated chickens can still acquire the virus and be carriers.  So, if you’re certain that you’ve lost chickens to Marek’s, you can assume that your entire flock, even those hens that appear healthy, are carrying the virus

2 - Marek’s Is a Viral Disease/Marek’s Is a Form of Cancer

Wait, what?  Is Marek’s a virus or a cancer?  Exactly.  It is both. Marek’s is caused by a herpes virus.  You’ve probably heard of herpes viruses in connection with genital herpes in humans.  That’s one of many diseases the various herpes viruses can cause.  Herpes viruses are also responsible for chicken pox, shingles, and mono.  Certain herpes viruses can cause a variety of cancers in humans, and that’s exactly what the Marek’s herpes virus does in chickens.

It happens like this:  A young chicken inhales some Marek’s virus riding on a speck of dust and the virus infects the cells in her lungs and then starts reproducing.  After about a week, the virus has incorporated itself into the cells of the chicken’s lymph tissue.  By the second week, the chicken is actively shedding viruses in her feather debris and dander.  Two to three weeks after infection, the chicken’s white blood cells carry virus from the original site of infection to other tissues throughout her body.  Three to four weeks after infection, viruses have taken up residence in her organs, nerves, eyes, skin, and feather follicles and have begun to form tumors.  It is only then that the chicken starts to show symptoms.  Death usually follows in a couple of weeks although some chickens do survive—but may be permanently affected by their bout with the disease, and without a doubt will continue to shed virus.

If the chicken is really healthy, not stressed, and has good genetics, or if she has been vaccinated, she may not show any symptoms.  But more than likely this seemingly healthy, vaccinated chicken will become infected.  The virus works its way into the chicken’s cells and while it doesn’t make her sick, it can turn her cells into little virus factories that continuously churn out Marek’s virus that will infect other chickens and cause disease in chickens that have less robust immune systems, or are less healthy, or unvaccinated.  If the carrier chicken becomes weakened due to some other disease or even old age, the virus can begin to produce symptoms and cause active Marek’s Disease years after the chicken was first infected.

3 - Marek’s Symptoms Can be Very Different from Chicken to Chicken

Because tumors can form in a number of different tissues, organs, or nerves, Marek’s can manifest with a variety of symptoms, with one or more of these manifestations occurring in any individual chicken.  The wide range of symptoms makes it difficult for the uninitiated flock keeper to realize that his hens are all suffering from the same virus.  And even the savvy chicken owner can’t always be entirely sure that his chickens are dying from Marek’s or something else.  Disease presentations can include:

  • Immune system suppression. With a weakened immune system, chickens can become more susceptible to other diseases such as coccidiosis that may have been in the background all along. It would be very easy, in this instance, to assume that the chicken had to succumbed to a particularly nasty case of coccidiosis and have no suspicion about Marek’s.

  • Cutaneous Marek’s: Skin involvement can range from skin nodules to nonhealing lesions or bleeding feather follicles.

  • Encephalitis: The viral tumors form in young chickens’ brains, resulting in rapid paralysis and death.

  • “Classic” Marek’s Disease: This form of the disease results from viral tumor formation in peripheral nerves and is usually seen in young chickens between six and twenty-four weeks old. It presents as partial paralysis—often the bird will sit with one leg to the front and one leg to the back, or one wing may droop and drag. “Torticollis” or twisted neck occurs when the nerves controlling the neck are affected. If the vagal nerve is affected, the crop may stay permanently dilated, or the bird will have problems breathing.

  • Ocular Marek’s: When tumors form in the eye or on the optic nerve, the pupil in one or both eyes can become tiny and not enlarge in response to light. Or the iris may become gray and misshapen. The chicken becomes functionally blind.

  • “Acute” or Visceral Marek’s: Tumors form in the lungs, liver, kidneys, or ovary. This can be the most devastating form of the disease, and can reach a mortality rate of 70% in young, unvaccinated chickens.

  • Sudden Death: Sometimes the only apparent symptoms are loss of appetite and weight or diarrhea. And sometimes there are no obvious symptoms at all, and your apparently healthy chicken just suddenly and spontaneously dies.

I've written a previous post on  blind Bonnie the Cream Legbar.  She has a gray opacity in her right eye. Her left eye is a pinpoint that never dilates. Marek's? Perhaps.

I've written a previous post on blind Bonnie the Cream Legbar. She has a gray opacity in her right eye. Her left eye is a pinpoint that never dilates. Marek's? Perhaps.


4 - You Can’t Cure Marek’s but You Can Take Steps Prevent It

If Marek’s strikes your flock there’s very little you can do except suffer as your sick chickens suffer, and end their suffering when it becomes severe.  There is no cure.

Your best strategy is to prevent Marek’s from infecting your flock in the first place.  While there are three prevention strategies, they are all, unfortunately, sometimes hard to follow.

Genetics:  There have been reports that certain breeds, like Silkies, are more susceptible to Marek’s and other breeds, like Fayoumis, are more resistant.  These reports are, as far as I know, anecdotal.  Lacking any firm statistics on Marek’s resistance by breed, depending on certain breeds to be more resistant to the disease is probably not the best strategy.

Biosecurity:  Commercial chicken operations with hundreds of thousands of chickens at one location rarely let outsiders onto the premises and those who absolutely need to enter the facility are often garbed in Tyvek suits and boots and nitrile gloves.  They take biosecurity very seriously.  Backyard chicken folks take biosecurity not so seriously.  Neighbors meander in and out—some of those neighbors have chickens of their own.  When the chicken owners go on vacation, the chicken sitter comes, and that chicken sitter may be traveling in and out of several other coops on any given day.  There are coop tours where large groups of people wander from coop to coop.  Chicken folks go to the chicken barn at the fair and then come home and tend their flocks.  They go to swap meets and bring home chickens from strange flocks.  All of these activities can transmit disease.  Bear in mind that to transmit Marek’s you don’t need to go tramping through the chicken poop in a strange chicken yard—all you need is to carry a little bit of chicken dander home on your sleeve.  If you’re serious about using biosecurity to protect your flock from Marek’s, you need to enact strict rules and you need to enforce those rules:

  • Don’t visit other coops.

  • Don’t allow any unnecessary people near your chickens or in your coop.

  • If you must allow a caregiver or someone else in your coop, and they’ve been in contact with another coop or flock, make sure that they’ve showered and changed clothes before visiting your coop.

  • Do NOT attend coop tours, don't enter poultry barns at the fair, don't go to swap meets or anywhere else where there are live chickens.

  • Don’t introduce new adult chickens to your flock that come from other backyard flocks. Quarantining the new bird for any length of time will not protect your flock from the asymptomatic carrier.

  • Buy babies from hatcheries that vaccinate—preferable those that use a trivalent vaccine (more on vaccination coming up!)

  • If you buy babies from a breeder or hatchery that doesn’t vaccinate, vaccinate the babies yourself—within couple days of hatching, and preferably when they are less than a day old. After vaccination, maintain the babies in strict isolation for 3-5 weeks.

I’ll be the first to say that these rules are stringent.  I don’t know anybody that strictly follows every rule.  I know I don’t.  But I should.


You may be a little discouraged after having read what I had to say about genetics and biosecurity and perhaps you’re holding great hope for what I have to say about vaccination.  Well, here’s the good news:  Vaccine manufacturers and hatcheries will tell you that vaccines have an effectiveness of greater than 90%.  While that statistic is undoubtedly true since it’s based on hard evidence, you need to bear in mind that for the vaccine to work, it has to be administered correctly and at the right time.  Also, since the virus is capable of mutating to more resistant forms, the current vaccines will without a doubt become less effective as time goes on.  There are currently three different varieties or “serotypes” of vaccine—each is made from a different strain of the virus:

Serotype 3 vaccine is made from a strain of Marek’s virus that occurs in turkeys that does not cause disease in chickens.  It’s also referred to as Herpes Virus turkey (HVT).  It was developed in the 1970’s in Michigan and is the oldest of the vaccines, thus it’s the least effective serotype due to mutated resistance. 

Serotype 2 vaccine is made from a strain of virus that naturally occurs in chickens but doesn’t cause tumor formation or disease. 

Serotype 1 vaccines (there are several of them) are made from the disease-causing virus.  These vaccines are “attenuated”—they’ve been manipulated in a laboratory to weaken their ability to cause disease but they still cause immunity in the chicken that receives the vaccine. 

Bivalent and Trivalent Vaccines:  Serotype 1 vaccine is not very effective, but when given in a “bivalent” vaccine combined with Serotype 3 vaccine, the vaccine is more effective than either serotypes 1 or 3 given separately.  Most hatcheries, as standard practice, give the bivalent vaccine or a trivalent vaccine that contains all three serotypes. 

One final important fact that you need to know about Marek’s vaccines:  An effective Marek’s vaccine will prevent tumors from forming in a vaccinated chicken.  It will NOT (unlike most vaccines against other diseases) prevent an exposed chicken from becoming infected with Marek’s virus.  SO, if your vaccinated chicken becomes exposed to Marek’s you can assume that the chicken will not get sick.  But you can also assume that the chicken will become infected with Marek’s and become a carrier.  And that’s the big problem with Marek’s vaccine.  Since vaccinated chickens don’t get sick and die, but can carry the virus, there is a large pool of carrier chickens where the virus can live and mutate to new forms.

5 - There Are Problems with Home Vaccination

 OK, vaccines are a good thing—they save chickens’ lives.  If you buy vaccinated chicks your flock is probably safe from Marek’s.  But if you buy chicks from some other source and plan on vaccinating at home the situation becomes a little dicier.  Here is a short list of some of the problems you’ll have to consider: 

  • The only vaccine available to backyard chicken keepers is the serotype 3 vaccine. It’s an effective vaccine, but as I mentioned before, because it’s the oldest vaccine, it’s less effective due to mutated viral resistance.

  • It needs to be stored really cold—in liquid nitrogen—until it’s thawed for use. If you don’t have a local supplier (you probably don’t) you need to buy it from an on-line supplier and it needs to be shipped cold, overnight express. If it gets warm in shipment, it will not work.

  • It is only available in 1000 dose vials. How many chickens are in your backyard flock?

  • Once you’ve opened the vaccine and mixed it, it’s good for an hour*. So, there’s no possibility of using some and throwing the left-overs in the freezer for next year’s baby chicks.

  • The manufacturer’s instructions are to use it on chicks less than a day old. Some hatcheries actually vaccinate while the chick is still in the shell. If you wait until the chick is older to vaccinate, the vaccine won’t harm the chick, but it may be useless. Every second that ticks by after a chick hatches increases the window of opportunity for the chick to be exposed to Marek’s virus. If exposure occurs before vaccination, the vaccine will not work.

  • The actual vaccination involves administering 0.2 cc of vaccine into the neck of the chick. How are you at poking needles into tiny, fluffy chicks?

  • After giving your chicks the vaccine, you have to be meticulously careful that they aren’t exposed to other chickens or any source (chicken dander on your sleeve, for instance) of virus for at least two weeks, and preferably up to six weeks.

6 - You Can Vaccinate At Home!

 The birds in my flock have come from a variety of sources over a period of years.  Most came from two different commercial hatcheries and have been vaccinated.  But a few came from a local farm store, and I have no idea about their vaccination status.  Given the mixed vaccination status of my chickens, the fact that even vaccinated chickens can shed virus, and the unfortunate truth that my own biosecurity protocols have been far from flawless, I decided that in spite of all the obstacles to home vaccination, I would give it a try.

Preparing Marek's Vaccine at Home

Preparing Marek's Vaccine at Home


I checked around with various other flock keepers and local chicken forums to see if I could find a local source for Marek’s vaccine.  I could not.  So, then I turned to the trusty internet.  Valley Vet Supply an on-line animal pharmacy located in Maryville, Kansas, has serotype 3 vaccine made by Zoetis.  For $29.95 I could get a 1000 dose vial—the only size available.  The overnight shipping charge was $22.50.  Thus, the total order was $52.45.  After I placed the order, I found a local small animal vet who is willing to order vaccine for me with her weekly vaccine order in the future.  She doesn’t use this vaccine or treat chickens, but ordering the vaccine as part of her shipment will save me the cost of shipping.  I also ordered 10-pack of syringes from an online retailer for $8.99. 

I had the vaccine on hand when brought my babies home.  The vaccination procedure was simple.  I reconstituted the vaccine with the provided diluent, and preloaded six syringes with 0.2 cc of vaccine.  Then, one-by-one I swabbed the back of each chick’s neck with alcohol, pinched the skin on the back of their necks and injected the vaccine into the fold of skin.  Afterwards, since the vaccine contains live virus, I boiled the remaining vaccine, the vial, and the syringes.  I disposed of the syringes at my local sharps disposal site.

One of the baby Silkies gets her Marek's shot.

One of the baby Silkies gets her Marek's shot.


If you do the math, vaccinating my six baby Silkies cost me $10.24 per chick.  Was it worth the expense?  What would you be willing to pay to save the life of your pet chicken?

One final thing:  This post, as I already said, is a thumbnail.  A great source for more information about Marek’s Disease is a huge, comprehensive, and well-maintained Marek’s FAQ on the BackYard Chickens forum.  That compendium of information is aimed at backyard flock keepers and was compiled by backyard flock keeper Jennifer Miller aka "Nambroth" after she lost her “sweet and wonderful Cochin rooster,” Trousers, to the disease.  While she claims not to be a professional or expert, she is perhaps the most well-informed amateur on the planet after five years of compiling and maintaining this collection of information.

*In a previous version of this post, I indicated that the vaccine, once reconstituted, was good for two hours. In fact, the package insert with the Zoetis vaccine I used mandates that the vaccine be used within an hour of being reconstituted. I have changed the text in this article to correct the error. And of course in all cases, with any vaccine, you should follow the instructions provided with the vaccine.


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