Marek’s disease is a viral tumor-causing disease of chickens and is caused by 6 different herpes viruses that primarily affect young birds. There are 4 different forms of Marek’s:
Cutaneous (skin form)
Visceral (internal-organ form)
Neural (nerve form)
Ocular (eye form)
There are four phases of infection:
Degenerative changes caused by early productive-restrictive virus infection,
Another phase of cytolytic infection associated with permanent immunosuppression
Nonproductive infected lymphoid cells that may or may not progress to lymphoma formation, a “proliferative” phase.
The route of infection is inhalation. The virus then replicates in the lungs (in non-lymphoid cells). An acute phase of the disease can be seen within 72-96 hours where the lymphoid system, primarily bursa and thymus, undergoes cytolytic changes. Infected birds normally recover from the acute phase of the infection after 6-7 days and become latent. Infected lymphocytes carry the virus throughout the body, causing cell-associated viremia. Eventually, virus will be shed in the environment via feather debris and dander after the secondary cytolytic infection occurs in the feather follicle epithelium (~2 weeks post infection)
The signs and symptoms of Marek’s Disease vary depending on the form of disease present:
Cutaneous form: Enlarged reddened feather follicles and white bumps on the skin that form brown crusty scabs.
Visceral Form: Tumors on internal organs including heart, ovary, liver and lung.
Neural form: Characterized by one, all, or none of the following symptoms -
Progressive paralysis, usually of the leg or wing, a typical leg-paralysis victim will have one leg extended forward and one leg extended back. A swelling of the sciatic nerve is the cause.
Starvation and death due to an inability to reach feed and water and to trampling by penmates.
Gray eye color
Morbidity (number affected) in unvaccinated flocks can reach 60 percent. Vaccinated flocks fare better with less than 5 percent affected. Mortality is high in affected birds reaching nearly 100 percent over a 10-week period.
Increased feed cost
Less weight gain
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