Protection Provided by CV1988 Vaccine against Two Virulent Australian Marek's Disease Virus Isolates

An increase in virulence of Marek's disease virus (MDV) over time has led to failure of control of Marek's Disease (MD) in some countries in the world. We report the results of a replicated 2 x 3 factorial vaccine protection experiment using 236 ISA Brown layer chicks in 12 isolators. The effects tested were vaccination with Rispens CVI988 (Rispens) @ 4000pfu or sham vaccination at hatch and challenge with MDV (MPF57, FT158 @ 500pfu or sham) on day 5. We compared the protection provided against a vMDV (MPF57) with a vv MDV (FT158) up to 56 days post challenge (dpc) and investigated the best early predictors of MD outcome. Peripheral blood leucocytes (PBL), feathers and isolator dust were collected weeny to 28 dpc and at 14 dpc five birds from each isolator were sacrificed for tissues. Samples were tested for pathogenic MDV or Rispens by specific real-time quantitative PCR. Necropsy for gross MD lesions was conducted on all chickens. Mortality and incidence of MD lesions were significantly higher in chickens challenged with FT158 than MPF57 and both were reduced but not eliminated by vaccination. Mean protective index (PI) did not differ between the two challenge viruses (61% and 66% respectively). The load of MDV and/or Rispens virus in several sample types between 7 and 21 dpc were sigNficantly associated with MD incidence at 56 dpc. We conclude that Rispens provides adequate but incomplete protection against early challenge with two Australian MDV isolates, that protective index was not affected by the virulence of challenge vires and that the level of MDV virus in several sample types is a good predictor of subsequent MD incidence.