Marek's Disease in Chickens

In an economic loss, Marek's disease is a most important malady of chickens. It often causes severe death loss in pullet flocks and has been a major cause of condemnations at the broiler processing plant. Generally, chickens under 16 weeks of age are most often affected.


Caused by a herpesvirus, the disease is often characterized by abnormal cell growth in the peripheral nerves and central nervous system. Hence, the common name for one form of Marek's: fowl paralysis. In addition to the nerves, however, the disease also may cause lesions on visceral organs and other tissues, including feather follicles of the skin. The most prominent lesions may be tumors on the liver, kidneys, testes, ova, spleen and lungs. In such cases, nerve swelling may not be involved.

How Marek's Disease is Spread

Chicken "dander" from feather follicles spreads the disease. The virus also is excreted in the saliva, and the virus probably enters the body through the respiratory system. Transmission via the egg is not significant.


Some chickens die without any clinical signs of Marek's disease. Most of the affected birds will have some degree of paralysis, although chickens with the acute form may not show this condition. Those with paralysis may die because they are unable to reach feed and water. The first indication of infection is a variation in the growth rate and degree of feathering.


Swelling of the peripheral nerves, particularly of the nerves of the leg and wing, is often noticeable. The visceral organs may contain tumors ranging from microscopic size to fairly large. Such tumor lesions may be confused with those of lymphoid leukosis without a qualified laboratory diagnosis.


Tumor formation from Marek's disease can be prevented through vaccination. Salsbury MD-Vac, a vaccine of chick-embryo tissue culture, is recommended. Vaccination at one day of age usually protects birds through their lifetime. There is no treatment for Marek's disease.