Our objective was to determine whether the cell-associated
herpesvirus vaccines used in chickens to control
Marek's disease tumors can augment development of lymphoid leukosis (LL) induced by exogenous avian leukosis virus (ALV). Various single or mixed
Marek's disease vaccines were inoculated at day 1, and ALV was injected at 1 to 10 days, with chickens of several experimental or commercial strains. Development of LL was monitored at 16 to 48 weeks in various experiments. In several strains of chickens we repeatedly found that the widely used serotype 3 turkey herpesvirus
vaccine did not augment LL in comparison with unvaccinated controls. However, LL development and incidence were prominently augmented in several chicken strains vaccinated with serotype 2
vaccines, used alone or as mixtures with other serotypes. In one chicken strain, augmentation was demonstrated after natural exposure to ALV or serotype 2
Marek's disease virus viremic shedder chickens. Augmentation of LL by virulent or attenuated
Marek's disease viruses of serotype 1 was intermediate in effect. Serotype 2
Marek's disease virus augmentation of LL was prominent in three laboratory lines and one commercial strain of White Leghorns, but it was not observed in an LL-resistant laboratory line or four commercial strains susceptible to ALV
infection. Chickens developed similar levels of
viremia and
neutralizing antibodies to ALV regardless of the presence of augmentation of LL, suggesting that the mechanism of enhanced LL did not result from differences in susceptibility or immune response to ALV. We postulate that the serotype 2 herpesviruses may augment LL through one of several possible influences on bursal cells that are subsequently transformed by exogenous ALV.