Protection provided by Rispens CVI988 vaccine against Marek's disease virus isolates of different pathotypes and early prediction of vaccine take and MD outcome.

We tested the level of protection provided by the Rispens CVI988 (Rispens) vaccine against challenge with a virulent Marek's disease virus (MDV) pathotype (vMDV) and a very virulent pathotype (vvMDV) and the accuracy of a range of predictive measures of Marek's disease (MD) incidence and vaccine take. Commercial layer chicks (n = 236) were vaccinated (or not) with 4000 plaque-forming units (pfu) of Rispens vaccine at hatch and challenged (or not) with 500 pfu of each challenge virus five days post vaccination. The vvMDV pathotype FT158 induced higher MD incidence (65%) and mortality (33%) when compared with the vMDV pathotype MPF57 (39% and 8%, respectively). The protective index provided by the Rispens vaccine against FT158 (61%) did not differ significantly from that against MPF57 (66%). This provides additional evidence that protection provided by the Rispens vaccine is not influenced by pathotype determined in studies using vaccines of other Mardivirus species. The challenge viruses did not differ in MDV or Rispens viral load in spleen at 14 dpc (days post challenge) determined by specific quantitative polymerase chain reaction test. MDV load in peripheral blood leucocytes at 7 and 14 dpc, splenocytes at 14 dpc, feather cells at 14 and 21 dpc and isolator dust at 21 dpc were significant early indicators of subsequent MD incidence to 56 dpc. These are potentially useful as the sampling can be carried out well before the onset of MD and some measures are non-invasive. The Rispens viral load in both invasive and non-invasive samples was more useful as a measure of vaccine take.