Written by: Dr. Jacquie Jacob, University of Kentucky

Marek’s disease, also referred to as visceral leukosis, fowl paralysis, and range paralysis is caused by a virus. Although there have been reports of infections in pheasants, quail, game fowl, and turkeys, the virus most commonly affects chickens between the ages of 12 to 25 weeks. (Note that although Marek’s disease is very similar to lymphoid leukosis, lymphoid leukosis typically starts at 16 weeks of age.) This disease Is spread through the inhalation of feather dander contaminated with the virus. The virus may also be spread by excretions from infected chickens.

Marek’s disease is a type of avian cancer. Tumors in nerves cause lameness and paralysis. Tumors can occur in the eyes and cause irregularly shaped pupils and blindness. Tumors of the liver, kidney, spleen, gonads, pancreas, proventriculus, lungs, muscles, and skin cause incoordination, unthriftiness, paleness, weak labored breathing, and enlarged feather follicles. In the later stages of an infection, birds become emaciated and have pale, scaly combs and greenish diarrhea.


Clinical signs of Marek’s Disease include the following:

  • Inability to use the raise the wings
  • Partial paralysis
  • Blindness
  • Ataxia (lack of muscle control)
  • Emaciation


There are currently no treatments for Marek’s disease.


Vaccinations are usually available in hatcheries. All newly hatched birds should be vaccinated for Marek’s disease. Following a good biosecurity program helps reduce the likelihood that a flock will become infected.


Marek’s disease. Tina Savage, University of New Hampshire, and Michael Darre, University of Connecticut.

Marek’s Disease in Poultry. Merck Veterinary Manual.