Marek’s disease is a viral disease that primarily affects chickens and is caused by Marek’s disease virus, a herpes virus. It is a significant concern for both commercial and backyard poultry due to the potential for severe production loss and mortality. The virus affects the nerves and can lead to the formation of tumors in the internal organs of infected birds. While chickens are the primary species affected, the disease can occasionally occur in other bird species.
Marek Disease Management in Chicken
Causes of Marek’s Disease
- Marek Disease is caused by a herpes virus that can survive for extended periods in poultry houses, droppings, litter, dust, feather follicles, and dander.
- The virus is contagious and can be transmitted by direct and indirect contact, mainly through the inhalation of infectious materials such as dust, dander, and epithelial cells of feather follicles, as well as through fomites, personnel, and beetles.
- Suppose the virus is introduced into a chicken flock. In that case, it spreads quickly from bird to bird, with infected chickens becoming carriers for long periods and acting as sources of infectious virus.
- The incidence of the disease varies on factors such as the strain and dose of the virus, age at exposure, maternal antibody, host genetics and gender, environmental stress, and concurrent diseases. Vaccination can reduce the shedding of the virus but not prevent it entirely.
Disease Cycle of Marek’s
- The early cytolytic phase: In this phase, the virus infects the epithelial cells of the feather follicles and starts replicating. The infected cells undergo lysis, releasing infectious virus particles. The virus then infects other epithelial cells, B and T lymphocytes, macrophages, and other immune system cells.
- The latent phase: In this phase, the virus infects B and T lymphocytes and establishes a lifelong latent infection. During this phase, the virus does not produce infectious particles and is not detected by the immune system.
- The late cytolytic and immunosuppressive phase: The virus reactivates and infects T lymphocytes, causing lymphomas to form in various organs such as the liver, spleen, and kidneys. The virus also produces immunosuppressive factors that impair the function of the immune system, making the bird more susceptible to other infections.
- The proliferative phase: In this phase, the virus causes the proliferation of T lymphocytes, resulting in the formation of lymphomas in various organs. The virus also causes other proliferative disorders, such as nerve tumors and tumors of the iris.
Symptoms of Marek’s Disease
Marek’s disease is a viral disease that affects chickens and other poultry. It appears in two distinct forms: acute and classical. Here are the symptoms of Marek’s disease:
- Lameness or paralysis: Lameness or paralysis around 12 weeks of age is a common symptom of Marek’s disease. Birds may have partial or complete paralysis of wings and legs, making them unable to stand. A transient paralysis syndrome may also cause a characteristic posture of one leg held forward and the other held backward as lesions progress.
- Sudden death: The acute form of the disease is manifested by sudden deaths due to the formation of lymphomas in the visceral organs.
- Depression: Birds may show depression and may be reluctant to move around.
- Anemia, anorexia, and emaciation: Birds may experience anemia, anorexia, and emaciation due to the effects of the virus on their immune system.
- Weight loss and diarrhea: Marek’s disease can cause weight loss and diarrhea in birds.
- Incoordination: Classical form of the disease involves lesions in the nerves of birds above 12 weeks, which commonly results in incoordination.
- Twisting of neck: Twisting of the neck (torticollis) may result from the involvement of cervical nerves and paralysis.
- Dilatation of crop: Paralysis of the vagus nerves may cause dilatation of the crop.
- Blindness: Unilateral or bilateral ocular involvement may cause blindness in some cases.