Evaluation of an IgM-specific indirect enzyme-linked immunosorbent assay for serodiagnosis of bovine respiratory syncytial virus infection: influence of IgM rheumatoid factor on test results with field sera.

A commercially available indirect enzyme-linked immunosorbent assay for measuring bovine respiratory syncytial virus (BRSV)-specific IgG was adapted to measure virus-specific IgM. Using this assay, the development of rapid IgM responses in experimentally infected calves was observed 7-9 days postinfection, with peak absorbance values ranging from 1.698 to 2.873. When absorbance values were expressed as a percentage of a positive reference serum, a positive/negative threshold of 22% was determined by testing serum samples from 59 healthy 3-5-month-old calves. Acute and convalescent serum samples collected from 151 calves during 38 outbreaks of respiratory disease were tested, and 130 sera were positive. To determine the number of false-positive results due to the presence of IgM rheumatoid factor, a method for depleting serum IgG by pretreatment of sera with a suspension of protein-G-agarose was developed. All sera that initially tested IgM positive were retested following depletion of serum IgG. False-positive IgM reactions were detected in 23 sera (17.7%). Specific IgM responses were confirmed in 107 sera from 84 calves. Evidence of BRSV infection was detected in 34 of 38 outbreaks. In contrast, seroconversion was detected in 69 calves from 24 outbreaks, confirming the diagnostic potential of the IgM assay. Overall correlation between IgM and seroconversion results was 74.2%. Intra- and interassay reproducibility were 12.50% and 17.48%, respectively (mean coefficients of variation).