Marek's disease is a herpes virus-induced neoplastic disease of chickens characterized by tumor formations in nerve, organ, muscle and epithelial tissue with pleomorphic lymphoid cells.
Affects chickens 2 to 16 weeks of age, stresses from other disease increase severity of MD. Herpes virus is cell associated and shed in skin scales and feather dander.
Birds remain viremic for life but infected carriers may or may not be clinically ill. vaccination protects against tumor formation but not against MD infection.
Occurrence is world wide wherever poultry is produced. Transmission is primarily by air within the poultry house, in feather dander, chicken house dust, feces and saliva. Infected birds carry virus in blood for life and are a source of infection to susceptible birds. Transmission by egg is of no significance.
Clinical signs "gray eye" caused by tumors in the pupils and blindness, tumors of the liver, kidneys spleen, gonads, pancreas, lungs, muscles and skin. Birds develop tumors, emaciation and death. Diagnosis is based on history of no vaccination, presence of typical tumor pattern and affected birds.
There is no treatment. Prevention is by vaccination at the hatchery, but vaccination only prevents tumor formation, not MD infection. All Marek's Disease vaccine must include HVT.