Duration of vaccinal immunity against Marek's disease.

Specific vaccinal immunity against Marek's disease (MD) virus was measured by a short-term challenge test in which chickens vaccinated with turkey herpesvirus (HVT) and control chickens at various ages up to 40 weeks were inoculated with virulent MD virus and evaluated 2 weeks later for microscopic lesions and MD viremia. Vaccinal immunity induced by high (2,900 PFU) and low (29 PFU) doses of HVT persisted without detectable reduction through the 40th week in line 151 chickens, thus indicating that resistance of older vaccinated chickens to MD may not be based solely on general age resistance (immunocompetence) or on subsequent exposure to field MD virus. The HVT viremia titers (number of buffy-coat cells inducing virus plaques in vitro) decreased with increasing age, but even chickens from which no HVT was isolated were usually immune to MD. The dose of vaccine appeared to influence, at least slightly, the magnitude of MD immunity and vaccine virus viremia.