Effects of Rispens CVI988 vaccination followed by challenge with Marek's disease viruses of differing virulence on the replication kinetics and shedding of the vaccine and challenge viruses

•We examined the kinetics of Rispens CVI988 vaccine with two Marek's disease virus pathotypes of differing virulence.•Viral load was positively associated with virulence, particularly in PBL, consistent with previous findings.•Vaccination with Rispens reduced the pathogenic viral load with evidence that the inhibitory effect of vaccination was greatest for the less virulent MDV.•Challenge with MDV 5 days after vaccination with Rispens can negatively influence the viral load of Rispens in the weeks following challenge.

Vaccination with “imperfect” vaccines that prevent disease but not infection is strongly implicated in the observed increased virulence of Marek's disease virus (MDV) over the past six decades. The current “gold standard” vaccine, Rispens CVI988 (Rispens), has maintained efficacy despite use for five decades, raising the question of whether it too favours higher virulence MDVs. To investigate this, we studied the kinetics of Rispens CVI988 (Rispens) and two MDV strains of different virulence in 236 commercial ISA Brown chickens vaccinated with Rispens at hatch and challenged with vMDV isolate MPF57 or vvMDV isolate FT158 on day five. Each treatment was replicated in two isolators and from 7 to 56 days post infection (dpi) peripheral blood leucocytes (PBL), feather and dust samples were collected and subjected to differential quantitative PCR (qPCR). Rispens vaccination significantly reduced challenge MDV viral load in a sample-dependant manner with evidence of a differentially greater inhibitory effect on the less virulent MDV. Similarly, challenge with the more virulent MDV reduced the Rispens viral load in PBL. Rispens virus load displayed a distinctive pattern of viral load that was similar in PBL and feathers, but different in dust. The clear effects of vaccination and challenge evident in PBL and feather samples were less clearly reflected in dust samples. The data are consistent with the Rispens vaccine reducing replication of lesser virulent MDVs to a greater extent like the HVT vaccine. Likely reasons for the persistent efficacy of Rispens vaccine are discussed.