Marek’s disease is a highly contagious viral condition that primarily affects chickens globally. It is caused by a type of herpesvirus, leading to the development of tumors in various parts of the bird’s body. This disease represents a form of avian cancer, impacting both commercial flocks and backyard poultry keepers. While it is a significant concern for poultry, Marek’s disease poses no known health risk to humans.
Symptoms and Forms of the Disease
Marek’s disease can manifest in several distinct forms. The neural form results in paralysis of the legs, often with one leg extended forward and the other backward, or paralysis of the wings and neck. Birds affected by this form may also exhibit labored breathing or a twisted neck.
The visceral form involves the growth of tumors on internal organs such as the liver, spleen, kidneys, heart, lungs, and ovaries. Affected chickens may show symptoms like weight loss, depression, and diarrhea. Sudden death can also occur without prior symptoms.
In the ocular form, the iris of the eye can become gray, and the pupil may develop an irregular, distorted shape. This condition often progresses to partial or complete blindness. The cutaneous form is characterized by enlarged feather follicles, which can become reddened and form small, raised bumps or tumors on the skin, sometimes leading to crusty scabs.
How Marek’s Disease Spreads
Marek’s disease is highly infectious, as the virus sheds primarily from the feather follicles of infected birds. This creates airborne, virus-laden feather dander. These particles can contaminate the surrounding environment, including dust, litter, and soil, remaining infectious for several months.
Inhalation of these airborne particles is the primary route for chicken infection. The virus is hardy outside the host and can also be carried on clothing, footwear, and equipment, facilitating its spread between coops or flocks. The virus is not transmitted through the egg.
Vaccination and Biosecurity
Vaccination is the most effective strategy for preventing Marek’s disease in chickens. The vaccine is administered to one-day-old chicks, either through injection or in ovo (while still in the egg). Chicks require 7 to 14 days to develop immunity after vaccination.
While vaccination prevents clinical signs like tumors and paralysis, it does not prevent infection or viral shedding. Vaccinated birds can still carry the virus and transmit it to unvaccinated flock members. Therefore, strict biosecurity measures remain important, even in vaccinated flocks.
Biosecurity practices include quarantining all new birds for at least 30 days before introducing them to an existing flock. Using dedicated footwear and clothing solely for the coop area prevents carrying viral particles. Minimizing contact between domestic chickens and wild birds reduces exposure to pathogens, including Marek’s disease.