Oncogenesis of Marek's Disease1

and is characterized by lymphoid infiltration in the periph eral nerves and development of lymphoid tumors in the visceral organs, muscle, and skin. The lymphoma appears to be multifocal and affects the ovary, testes, kidney, liver, lung, and heart. The incidence of lymphoma in the ovary seems to be higher than in the other organs. The disease is highly contagious and affects chickens of all ages with pre dilection for younger chickens. Existing evidence (34) is against the vertical transmission of the disease; most chickens are infected with the virus shortly after hatching. Among several neoplastic diseases of man and animals with possible herpesvirus etiology, MD is the only naturally occurring lymphoma with a proven etiology (1, 8, 22, 37). The virus is ubiquitous in nature and can be isolated from chickens raised under natural conditions. However, only a low percentage of infected chickens develop the lymphoma. Chicks hatched from a previously infected hen have ma ternal antibodies to the virus (2, 7). The level of this anti body declines and becomes undetectable by 3 weeks of age. However, shortly after infection the level of acquired anti body increases. Infection may induce the development of progressive lymphoma with death or, in some instances, regression. In most instances, the infection remains latent without the development of any signs of disease. The nature of the natural resistance to M D is not well understood. Experimental inbred lines of chickens have been developed that are highly resistant to the lymphoma. This resistance, however, is not to the infection with the virus but rather to the development of the lymphoma. Lymphoid cells are not the only type of cells that be come infected with the virus. Other types of cells and in