A simple immunoperoxidase plaque assay to detect and quantitate Marek's disease virus plaques

We report an immunoperoxidase-based staining technique that can be used to rapidly and accurately detect and quantitate Marek's disease virus (MDV) plaques. Monolayer cultures were fixed and incubated with a monoclonal antibody specific for MDV. After washing, a second antibody of horseradish peroxidase-conjugated goat anti-mouse IgG was applied, incubated for 1 hr, and washed with phosphate-buffered saline. After the cultures were incubated with diaminobenzidine, CoCl2, and H2O2, the plaques appeared as black spots and were easily seen and counted. Significantly more immunoperoxidase-stained serotype 1 MDV plaques could be counted at 4 days postinoculation than were seen in unstained cultures. With serotype 2 MDV-infected cells, the difference in plaque counts was less dramatic. Nevertheless, at 3 days postinoculation, significantly more stained serotype 2 plaques were seen than unstained plaques. Immunoperoxidase staining of turkey herpesvirus plaques did not increase the sensitivity of viewing plaques. Similar numbers of stained and unstained plaques were seen at 2 days postinoculation. We also demonstrated that we could count serotype-specific MDV plaques in a mixed infection that contained all three serotypes.