Studies on Marek's disease. I. Experimental transmission.

A strain of infectious agent, HPRS-B14, was isolated from a case of Marek's disease and maintained through 40 serial passages. This agent produced a disease characterized by neural involvement and, in some affected chickens, lymphoid tumors principally involving the ovary. This disease conformed to the description of the classical form of Marek's disease. This disease is contagious, spreading to chickens by direct and indirect contact with infected chickens. Females were more susceptible to the disease than males. Treatment with female and male sex hormones had no significant effect on the incidence of Marek's disease. Lines of chicken also differed in their susceptibility to disease produced by the HPRS-B14 agent, and it is concluded that these differences are due to the genetic constitution of the chicken. Chickens of a susceptible line inoculated with HPRS-B14 at 50 days of age were more resistant to the development of Marek's disease than chicks of the same line inoculated at 1 day of age. Blood and other organs and tissues of chickens, clinically affected with Marek's disease produced by HPRS-B14, were infective. Infectivity was associated with cell-containing fractions of blood and kidney tissue. It is concluded that infectivity of blood and other organs is due to an infectious agent, but we have insufficient evidence at present to classify the HPRS-B14 agent of Marek's disease.