Characterization of a new serotype of Marek's disease herpesvirus.

The HPRS-24 strain of Marek's disease herpesvirus was selected for closer study from a group of virus isolates that appeared apathogenic under standard test conditons. Protection studies revealed an immunological relationship between this virus strain and acute Marek's disease herpesvirus. In chicken kidney cell cultures, the HPRS-24 strain caused small and slowly developing plaques, and the proportion of infected cells was less than with other strains. In chicken embryo fibroblast cultures, this virus multiplied rapidly, yielding a comparatively high proportion of infected cells. Electron microscopic studies revealed that infected fibroblasts contained more enveloped virus particles than those infected with other strains of Marek's disease herpesvirus. Infectious cell-free virus was extracted from cultured fibroblasts with titres high enough for use in neutralization studies. Cross-neutralization, immunofluorescence and precipitin tests served for serological comparison of the HPRS-24 strain with turkey herpesvirus and representatives of acute and classical Marek's disease herpesvirus. Antibody titres were 4-10 times higher against the homologous than against the heterologous virus strains. Qualitative differences between precipitating "A" antigens were characterized by spur line patterns of precipitation bands. These results suggest that the group of Marek's disease and turkey herpesviruses consists of at least three serological types. One of them is represented by the HPRS-24 strain of apathogenic Marek's disease virus. The other two types comprise pathogenic strains of Marek's disease virus and their attenuated variants, and turkey herpesvirus.