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,
- Latent 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.
• Weight loss
• Labored breathing
• Starvation and death due to an inability to reach feed and water and to trampling by penmates.
• Gray eye color
• Misshapen iris
• Weight loss
• 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