Effect of Widely Divergent Vaccination-Challenge Intervals on Protection Provided by Rispens CV1988 Vaccine Against Very Virulent Marek's Disease Virus Challenge in Isa Brown Chickens

Rispens CVI988 is a highly effective serotype 1 Marek's disease virus vaccine currently used to vaccinate layer and breeder chickens in Australia and worldwide. We tested the effects of Rispens CVI988 on the level of protection provided to commercial ISA Brown layers following challenge with very virulent Marek's disease virus (vvMDV) isolate 02LAR at various intervals before and after vaccination. Protective index was measured for vaccination challenge intervals of 0, 5, 10, -5 and -10 days with the negative values indicating challenge prior to vaccination. All treatments were tested in duplicate with 25 ISA Brown chickens per isolator and appropriate controls. Chickens were challenged by injection with 400 pfu of MDV and/or vaccinated with 3200 pfu of the Rispens vaccine virus subcutaneously, and the presence of gross MD tumours was assessed up to 56 days post challenge (dpc). MDV challenge in unvaccinated chickens resulted in tumours in 52% of chickens. Rispens vaccine provided no significant protection when the birds were challenged prior to vaccination, i.e. at VCI -5 (-4%) and -10 (21%). Rispens CVI988 provided 85% and 100% protection at VCI 5 and 10 respectively, Interestingly it provided significant protection (60%) when challenge and vaccination occurred simultaneously.