Comparative effects of host and viral factors on early pathogenesis of Marek's disease.

A series of experiments on the early pathogenesis of Marek's disease was conducted according to a uniform scheme. In each experiment, there was a single variable-age, genetic strain, or virus strain. Virus assays from spleen, buffy coat, and bone marrow, and fluorescent antibody tests on spleen, bursa of Fabricius, and thymus were conducted on five birds per group daily from the 3rd through the 10th day postinoculation. From these data, it was apparent that the response could be divided into two periods: 4 to 6 days = early; 8 to 10 days = late. Serological tests showed all groups except the 1-day-old group to have neutralizing antibody by the end of the 10-day period. With few exceptions, none of the variables tested exerted any appreciable influence on the level of virus growth in spleen, bursa, or thymus during the early period. High levels of infection occurred in all birds during that period. Changes in infection pattern which occurred during the late period were significant and could be correlated with occurrence of Marek's disease in test samples of birds held until 7 weeks after infection. Infectivity levels dropped appreciably in the case of resistant N-line birds given JM virus, and, during the late period, infection levels were significantly higher in GA-infected birds than in those given viruses of lower virulence. Whereas the virus titers during the 8- to 10-day period usually reflected the eventual clinical pattern of Marek's disease, the levels of viral antigen (fluorescent antibody tests) were much less consistent. One further experiment conducted by the same uniform scheme demonstrated no significant effects on early pathogenesis or course of Marek's disease in birds given continuous oral medication with amino-ureido-sulfone.