Partial inhibition by turkey herpesvirus of serotype 2 Marek's disease virus plaque formation and in vivo infectivity.

The increased use of serotype 2 Marek's disease virus (MDV) and serotype 3 turkey herpesvirus (HVT) as components of effective bivalent vaccines against Marek's disease (MD) prompted studies on the possible interactions of these two viruses in vitro and in vivo. The replication of the SB-1 strain of MDV was compared with replication of the FC126/2 strain of HVT in chickens and cell cultures infected with one or both viruses. Replication of MDV was reduced in the presence of HVT in both in vitro and in vivo systems. MDV plaque counts in dually infected chicken embryo fibroblast cultures inoculated with tissue-culture-propagated viruses were reduced by up to 91%; however, no inhibition was noted when inocula consisted of virus-infected buffy-coat cells. Plaque formation by MDV in chicken embryo fibroblast cultures was inhibited by virus-free conditioned medium from HVT-infected cultures. This conditioned medium also inhibited growth of vesicular stomatitis virus in a standard interferon assay. In chickens inoculated with both MDV and HVT, MDV viremia titers were lower and the dose required to infect 50% of susceptible chickens was increased 13-fold compared with chickens inoculated with MDV alone. In spite of these findings, there was no evidence that high concentrations of HVT interfered with either the ability of MDV to induce protective synergism in vivo or the protective efficacy of bivalent vaccines. No reciprocal inhibitory effects of MDV on the replication of HVT in vivo or in vitro were noted.