Immunoglobulin M (IgM) indirect enzyme-linked immunosorbent assay and the involvement of IgM-rheumatoid factor in the serodiagnosis of BHV-1 infection.

An IgM enzyme-linked immunosorbent assay (IgM-ELISA) for the detection of antibodies to bovine herpesvirus-1 (BHV-1) was developed. Its applicability was examined by serological studies in two calves experimentally infected with virulent BHV-1 over a period of 60 days. IgM antibodies were detected by ELISA on day 6 after infection, and there was an increase in IgG antibodies on day 9. Serum neutralizing (SN) antibodies were detected only on day 13, confirming the higher sensitivity of the ELISAs. A similar study of four calves treated with a commercial inactivated virus vaccine indicated no detectable IgM-ELISA response, and late SN and IgG-ELISA reactivity. Thus IgM-ELISA appears to be of value in assessing recent infection, whereas IgG-ELISA and SN cannot distinguish between infection and vaccination. The possible limitations imposed on the specific IgM-ELISA by the presence of IgM rheumatoid factor (IgM-RF) in bovine serum were examined. IgM-RF levels were determined in bovines of various ages. Elevated values of IgM-RF were found in the sera of older animals; their occurrence may lead to false IgM-positive diagnosis (16%) of BHV-1 infection. This was examined in 113 serum specimens from suspected cases of BHV-1 infection and in 32 bulls at an insemination center. Pretreatment of serum samples with an antibovine IgG serum eliminated false positivity of the IgM-ELISA. It is concluded that IgM-ELISA should be of particular value in the diagnosis of recent infection with BHV-1, mainly in calves.