Differentiation between strains of Marek's disease virus and turkey herpesvirus by immunofluorescence assays

Two serological types of Marek's disease virus and a herpesvirus of turkeys have been differentiated by indirect immunofluorescence tests as (1) pathogenic strains of Marek's disease virus (MDV) and their attenuated variants: HPRS-16, HPRS-16/att, HPRS-B14, JM, JM/att, GA, VC and 'Oldenburg', a recent field isolate; (2) apathogenic strains HPRS-24 and HPRS-27 of MDV; (3) herpesvirus of turkeys strain FC126 and its HVT(A-) variant. Virus strains could not be distinguished on the basis of qualitative differences in immunofluorescent staining of intracellular virus-induced antigens. Results were similar whether chicken kidney, chicken embryo fibroblast or duck embryo fibroblast cell cultures were used. Fluorescence of virus-induced antigens was stronger with homologous than with heterologous antisera. Using the direct immunofluorescence technique Marek's disease virus and turkey herpesvirus infections could be distinguished. There were never any significant differences in the appearance and distribution of antigen in infected cells treated with homologous or heterologous antisera at dilutions of comparable activity using the indirect immunofluorescence technique. Antibody titres of antisera were 4 to 8-fold higher in the indirect immunofluorescence test against the homologous virus-induced antigens than against heterologous antigens. Cross-reactions between the 3 serological types could be prevented by absorption of antisera with the appropriate antigens. Cross-reactions could also be prevented by the appropriate dilution of antisera before use in the indirect immunofluorescence test.