Protection Provided by Rispens CV1988 Vaccine Against Very Virulent Marek's Disease Virus Challenge in Isa Brown Chickens

Abstract

As part of a wider study into kinetics of vaccinal and wild type serotype Marek's disease virus (MDV) serotype 1 viruses in the same host, we tested the protection offered by the Rispens CVI vaccine to MDV challenge in Isa Brown chickens. Apart from the vaccine used, the protocol largely followed that of the Avian Diseases and Oncology Laboratory (ADOL) of the USDA. In a 2x3 factorial isolator experiment with two replicates we tested the effects of Rispens vaccination (vaccinated, unvaccinated) and MDV challenge (MPF57, FT158 at 500pfu/chicken, unchallenged) in 236 female Isa Brown chicks. Vaccination was at hatch, challenge at day 5 and the experiment duration was until 56 days post challenge. The very virulent isolate FT158 induced a significantly higher level of mortality (33%) and incidence of MD (65%) than the virulent isolate MPF57 (10% and 39%). Rispens vaccinated challenged chickens had lower mortality (13%) and MD incidence (28%) than unvaccinated chickens (30% and 76%). The protective index provided by Rispens against FT158 (61%) did not differ significantly from that provided against MPF57 (66%). These results demonstrate that current Australian isolates of MDV can induce significant MD and mortality in Rispens vaccinated chickens at a vaccination challenge interval of 5 days. However the protective index is higher than reported for HVT and bivalent vaccine in previous experiments using the same virus (MPF57) and protocol in Isa Brown chickens.