The Influence of Growth Rate on the Development of Marek’s Disease in Chickens

Abstract

The response of chickens to the JM strain of Marek’s disease virus was studied following directional selection for body growth rate. Comparisons were made for four pairs of selection lines, two pairs resulting from long term selection for growth rate and two pairs following a single generation of selection. In all cases, it was observed that the faster growing lines were more susceptible to the development of Marek’s disease than their corresponding lines exhibiting a slower growth rate. These observations suggest that selection for rapid growth rate would result in increased susceptibility, while reverse selection would result in decreased susceptibility. This correlated response can, however, be influenced by the residual genotype present. The results of IHA tests showed that the injected group had a significantly higher JM-antigen titer and incidence of Marek’s disease compared with the naturally exposed groups, indicating the parallel relationship between the virus level and the incidence of tumor formation. The accompanying higher titer of antibody did not provide protection against neoplasia. It appeared to be produced in response to the antigen level, since antigen and antibody titers were found to be positively correlated.