Problems in the interpretation of diagnostic tests due to cross-reactions between orbiviruses and broad serological responses in animals.

Tests presently used for the diagnosis of infections by bluetongue virus (BTV) or related orbiviruses are based on the use of 2 types of serological reactions. Those that are considered group-reactive tests are the agar gel diffusion precipitin (AGDP), complement-fixation (CF) and fluorescent antibody tests and those that are considered type-specific are a wide variety of virus neutralization tests (50% and 80% plaque reduction, plaque inhibition and microtiter neutralization) and cross-protection tests. These tests suffer from problems of standardization between laboratories and of specificity. Group-reactive tests (AGDP and CF) for the BTV serogroup also detect cross-reactions with viruses in the epizootic hemorrhagic disease virus (EHDV), Eubenangee (EUB) and Palyam (PAL) serogroups, with the EHDV cross-reactions being of particular concern. Further, multiple infections of cattle with PAL serogroup members can produce antibodies which will react to BTV and EHDV serogroup antigens in serological tests. Multiple infections of animals with related viruses can produce antibodies which will cross-react with orbiviruses in type-specific, virus neutralization tests to a virus which the animal has not previously been exposed. These observations stress the need to evaluate the tests at present being used, to assess the risks of cross-reactions between related orbiviruses and to develop new tests of defined specificity.