Studies on acute Marek's disease. I. Characteristics of isolate GA in chickens.

During the last 5 years, condemnations for an acute form of leukosis in young chickens have increased rapidly in the United States (11). Sevoian (4) has designated this as a Type II form of lymphoid leukosis. Biggs (1), however, classifies it as an acute form of Marek's disease (MD), in contrast to the less explosive classical form, which appears in slightly older birds with a lower incidence of visceral lesions. Under field conditions MD affects chickens primarily between 6 and 20 weeks of age; however, Sevoian et al. (6,7,8,9) report that leukosis developed consistently within 2 to 3 weeks of inoculation of day-old chicks with the JM isolate in preparations of blood, plasma, or tissue suspension, and was also transmitted by air. Biggs et al. (2) showed that isolates HPRS 16, 18, and 19 can be transmitted with blood and tumor homogenate, with the latent period being 21 to 28 days. The present study was done to: 1) report on an MD isolate found in Georgia and discuss the pathogenic aspects of this agent; 2) ascertain the optimum age for obtaining plasma from birds experimentally infected with isolate GA; and 3) compare MD in birds injected with plasma or tumor suspension for a series of 9 passages at bi-weekly intervals.