Mareks Disease in Chickens: Symptoms & Prevention

What is Mareks disease?

Mareks disease is a virus that is very common and highly contagious to other chickens. On rare occasions quail and domestic turkeys in close proximity to affected chickens can contract the disease. Wherever there are chickens, you are likely to encounter it in some form. It can survive in exposed environments – such as the soil of a chicken run – for up to 5 months, possibly even years. It’s important to clean and disinfect your coop before housing new chickens (see Why You Should Clean Out Your Coop Before Your Baby Chicks Arrive). Mareks Disease is spread via chicken dander (dust) so that means it can be spread by new chickens, on clothing, by wind, clothes, shoes, wild birds, or anything really that has feet or wings. It’s currently believed to be contracted by inhalation so when the dander from infected birds is disturbed by scratching, pecking, or fluttering, the uninfected chickens breathe it in and contract the disease.
Although Mareks can be fatal at times, there is no cure for the disease although many chickens build immunity to it if they survive the initial infection. Once exposed to Mareks, chickens should be assumed to be carriers of the virus even if they show no symptoms. The disease cannot be transmitted vertically (meaning that the disease cannot pass from parent to offspring – offspring must catch it after hatching to become sick). Humans cannot contract the disease so the meat and eggs from infected chickens is perfectly safe to eat. No worries there.

How do I know if my chickens have it?

Diagnosing Mareks disease can be extremely frustrating. The following symptoms may overlap and only one symptom may be visible or all of them. It can be tricky to pick out Mareks from other conditions such as vitamin deficiency or other viruses.

Here’s the Classical Symptoms

Symptoms often stem from the root cause of tumors which squeeze or affect the nervous system.

  • Partial or total paralysis of the legs, sometimes wings, and even neck: This may include staggering, losing control of both legs, splits, loss of balance, etc. This is caused by tumors on the sciatic nerve which controls leg movement. This symptom may also be caused by B vitamin deficiencies (See Curled Toes? Splayed Legs? Time to Learn About B12 Deficiency in Chicks & Ducks).
  • Not eating: Lesions (or tumors) on the vagus nerve cause dilation of the crop (the pouch where food is ground up) which affects eating & digestion. This symptom can also affect the gizzard lining.
  • Difficulty breathing and/or darkening comb: The presence of brachial nerve lesions can restrict respiration. Tumors can also grow into heart muscle which restricts blood flow and circulation. The comb, which has many blood vessels in it, will turn dark from lack of circulation.
  • Tumors: Internal tumors can only be diagnosed after death. Perform an autopsy on your culled or dead chicken and check for the presence of tumors. Tumors will grow on the thymus which is in neck near the crop then, as the disease progresses, tumors may appear on to gonads, spleen, liver, kidneys, heart, adrenals, muscles, or pretty much anywhere else inside the chicken.
  • Weight loss: Internal tumors may cause rapid weight loss since they restrict the digestive process. They may also slowly waste away without symptoms.
  • Loose, watery, and/or bright green poop: This is typically a result of the tumors which cause weight loss. The inability to digest food causes poop with little substance.
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Symptoms may also show in the eyes. This is known as Ocular (having to do with the eye) Mareks.


Normal chicken eye (Left) Chicken eye stricken with Ocular Mareks (Right)

  • Iris Discoloration: The iris of the eye will turn grey or pale blue in color.
  • Pupil Deformation: The pupil goes from round to keyhole or amoeba shaped.
  • Pupil Doesn’t React to Light: The pupil of the eye is meant to close and open to let in light or restrict light. If the chicken has Ocular Mareks the pupil will likely not react to light. Get a flashlight and go into a dark room and see if the iris reacts to stimulation from the light.
  • Blindness: Pretty self explanatory.

Symptoms may appear on the skin. This is known as Cutaneous (having to do with skin) Mareks.

  • Lesions or Deformities at the Feather Follicles: Rounded or hard lesions can appear on the skin. They can be from large bumpy nodules to crusty looking lesions and may be moderate or severe.

Mareks can cause immunosuppresion, or a condition where immune system is suppressed. The chicken may survive Mareks but because the immune system is suppressed the chicken may get sick from another disease, such as coccidiosis (read more about Coccidosis: Cures, Symptoms, & Prevention). This means that even though your chook will survive Mareks it may contract another disease and won’t be able to fight it off. This is why Mareks is so frustrating to diagnose. The symptoms may look like Cocci (and it may be Cocci) but the root cause may be Mareks. The best bet is to cull a chicken that is extremely sick and do an autopsy. Look of the tumors that indicate Mareks disease. You then know that the other chickens likely have it (although they may have developed a resistance to it).
Mareks disease symptoms may come and go so if the chicken appears sick, then heals up magically, then becomes sick again it may be a clue. It can be very frustrating diagnosing a chicken that randomly shows symptoms only to appear healthy the next day.
The incubation period for Mareks is 3-25 weeks. This means that after the chicken has inhaled the virus, it takes about 3 weeks before it could show symptoms. It may not show symptoms to up to 25 weeks. The chicken becomes contagious after 10 days of contracting Mareks.

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How do I prevent it?

Vaccinations will not prevent chickens from contracting the disease, it only increases their chance of survival and reduces their symptoms by limiting tumor growth. It also helps limit the horizontal (meaning from chicken to chicken) spread of the disease somewhat and does make the flock less likely to spread it to uninfected flocks, though you should still consider chickens exposed to Mareks contagious. We do offer Mareks vaccinations on nearly all our chickens.

You can try quarantine but even if you get new chickens and quarantine them for a week or so, they may carry the disease and not show any symptoms. So it’s nearly impossible to tell definitively if you are introducing Mareks to your flock. Also, since Mareks is trasmitted via dander, the disease may drift from a neighboring farm that has the disease. It’s extremely tough to prevent this disease. Chickens that are left free to roam outside from little on up are more likely to develop resistance to Mareks as they are exposed to small amounts of it at a time.

If you do vaccinate your chicks, it does take up to several week for the vaccine to take effect so it’s important to quarantine your new baby chicks from the established flock until they are at least eight weeks old or so. Remember to wash your hands and change clothes before handling the new chicks, especially if you were just in the coop outside. Dander can even be in your hair or on your shoes. It’s best to try to keep the chicks isolated from dirt or dander from the outside coop until they are older. You may let the chicks have supervised play time outside but keep them upwind of the coop of older chickens.

If I vaccinate chicks, do they become carriers of Mareks?

No – The vaccination isn’t actually a weak strain of Mareks. It’s a vaccine derived from Turkey Herpesvirus (MDV-3) so chickens will not become carriers of Mareks if they are vaccinated. Even if they are vaccinated but are exposed to Mareks, they should be considered carriers.

I hope that helps! This is by no means exhaustive and it shouldn’t be considered a scientific paper, but merely a tool to teach a you little about this common poultry disease. Customers frequently ask “What is Mareks Disease” when asked if they prefer to get their chicks vaccinated.

Yours in Poultry,


More resources:

Wikipedia article on Mareks Disease

Mareks Disease Article on

RELATED: Why Don’t We Vaccinate Ducks?

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