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 fo r measuring bovine respiratory syncytial virus (BRSV)-specific IgG was adapted to measure virus-specific IgM. Using this assay, the developm ent of rapid IgM responses in experimentally infected calves was obser ved 7-9 days postinfection, with peak absorbance values ranging from 1 .698 to 2.873. When absorbance values were expressed as a percentage o f a positive reference serum, a positive/negative threshold of 22% was determined by testing serum samples from 59 healthy 3-5-month-old cal ves. Acute and convalescent serum samples collected from 151 calves du ring 38 outbreaks of respiratory disease were tested, and 130 sera wer e positive. To determine the number of false-positive results due to t he presence of IgM rheumatoid factor, a method for depleting serum IgG by pretreatment of sera with a suspension of protein-G-agarose was de veloped. All sera that initially tested IgM positive were retested fol lowing depletion of serum IgG. False-positive IgM reactions were detec ted in 23 sera (17.7%). Specific IgM responses were confirmed in 107 s era from 84 calves. Evidence of BRSV infection was detected in 34 of 3 8 outbreaks. In contrast, seroconversion was detected in 69 calves fro m 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%, respecti vely (mean coefficients of variation).