Field Studies on the Rispens CV1988 Vaccine Virus in Layer and Unvaccinated Broiler Chickens

Abstract

Rispens CVI988 (Rispens) is a widely used serotype 1 Marek's disease virus (MDV) used as a Marek's Disease (MD) vaccine worldwide. In this study we used Rispens and wild type MDV specific real-time quantitatfge PCR (qPCR) tests to investigate a) vaccination success using qPCR of feather or dust in comparison to enzyme-linked immunosorbent assay (ELISA) b) whether vaccinated chickens are persistently infected, c) whether vaccinated chickens are coinfected with wild-type MDV and d) presence of Rispens in unvaccinated broiler flocks. Feather and dust samples for qPCR and serum samples to detect antibody using ELISA were collected from birds aged 1-91 weeks from three layer farms. DNA extracted from 100 broiler shed dust samples positive for MDV, were re-tested using the Rispens qPCR. Overall 66% of 498 feather samples from vaccinated layers were positive for Rispens. The mean Rispens viral load in feather peaked at 8 weeks. Of 120 feather samples from vaccinated layers tested for wild-type MDV, 8 were positive (6.7%). Of 42 layer dust samples, 93% were positive for Rispens. Viral load in dust peaked around 4 weeks. Anti-MDV antibody was highest at week 8. Antibody levels and virus in feather and dust persisted at variable levels to 91 weeks. Of 100 dust samples from unvaccinated broiler flocks, 7 were positive for Rispens. We conclude that vaccine take can be measured by Rispens-specific qPCR of feathers or dust from 3 weeks post vaccination and infection with Rispens is lifelong. Infection of vaccinated chickens with wild-type MDV was low and there was evidence of escape of Rispens vires to unvaccinated flocks.