Marek’s Disease in Chickens

Marek’s disease is a common poultry illness caused by the herpes virus, which causes cancer. It’s harmless for turkeys but may be fatal to chickens.

Marek’s disease can affect many different types of poultry, although it is most prevalent in chickens between the ages of 4 weeks and 18 months.

Symptoms of Marek’s Disease

This viral infection mainly primarily infects the nerves of young and growing chickens, but can also affect aging chickens.

Symptoms of Marek’s disease may include:

  • Leg paralysis (with one leg stretched forward and the other stretched back)
  • Droppy wings
  • A floppy neck in young chicks
  • Skin tumors
  • Tumors on the sciatic nerve and internal organs
  • Ruffled feathers
  • A pale comb
  • Vision loss (looks like cataracts on their eyes)
  • Egg production stops in hens
  • Diarrhea
  • This could result in the chickens dying suddenly or slowly losing weight as a result of eating very little or nothing at all.

There are a lot of different symptoms of this disease and you want to prevent your chickens from being infected by this disease because it has a 60% mortality rate.

And while some chickens initially seem to recover from Marek’s Disease, it may also increase the susceptibility of chickens to other diseases.


The vaccination does not protect chickens from infection or from transmitting the virus, but it does protect them from paralysis.

Chicks must receive the vaccine prior to being exposed to the virus for it to be effective. As a result, the majority of hatcheries offer to immunize chicks within the first 24 hours after hatching or they can inject the vaccine into the egg on day 18 of its incubation.

Unfortunately, the vaccine will not work if it’s given to a chick that is more than a day old.


A chicken can spread Marek’s disease to other chickens through feather dander, skin, and other cells coming off birds. These infected cells can survive years and can also settle on feeders, drinkers, roosts, and in the ground.

These cells can even travel in the air to your neighbor’s chickens!

Whether or not they exhibit symptoms, chickens that have been infected with Marek’s disease can spread the virus for the rest of their lives.


Unfortunately, there’s no cure for Marek’s disease.

If a chicken survives the initial stage of Marek’s disease, the virus becomes dormant and doesn’t produce any symptoms. The chicken is now considered to be a carrier, meaning it doesn’t exhibit any symptoms of the disease but it’s still capable of transmitting the virus.

The virus then becomes active again weeks or even months later due to a stressor. (For more information on what may make a chicken stressed, check out my post: Chickens & Stress).

The Marek’s reactivation causes the chicken’s immune cells to develop into tumours known as lymphomas. The tumors can grow anywhere in the chicken’s body.

Almost all chickens with tumors die.

Conclusion: Preventing Marek’s Disease in Chickens

In backyard chickens, it is practically impossible to entirely eliminate Marek’s Disease. This is especially true if you add new chickens to your flock or your neighbors have chickens.

However, its effects can be significantly reduced with a combination of sanitation, keeping closed flocks (article), and vaccinations.