Factors associated with variation in bulk tank milk Mycoplasma bovis antibody-ELISA results in dairy herds

Abstract

The relevance and limitations for using measurements of antibodies against Mycoplasma bovis in bulk tank milk (BTM) as a potentially cost-effective diagnostic tool for herd classification has not been evaluated before. Assuming that an increasing or high seroprevalence is a result of on-going or recent spread of M. bovis in a dairy herd, we tested the hypothesis that increasing prevalence of antibody-positive cows and young stock are associated with increasing BTM antibody ELISA values against M. bovis in Danish dairy herds with different courses of M. bovis infection. Furthermore, we tested whether herd size was associated with variations in the BTM responses. Thirty-nine Danish dairy herds selected to represent 4 different herd-level infection groups [8 control herds, 14 acute outbreak herds, 7 herds with previous outbreaks, and 10 herds with elevated BTM ELISA-values directed against M. bovis (>64% optical density measurement)] were visited 4 to 5 times, approximately 3mo apart. At each visit, 65 young stock were blood sampled. At the milk recording date closest to the herd visit date, 50 milk recording samples from individual lactating cows were randomly selected. In addition, a BTM sample was collected as a representative sample directly from the bulk tank by the dairies’ milk truck drivers as part of the mandatory milk quality-control scheme. Blood and milk samples were tested for antibodies against M. bovis with a commercially available ELISA test (Bio-X BIO K 302, Bio-X Diagnostics, Rochefort, Belgium). A linear mixed effects model was used to analyze the effects of the prevalence of antibody-positive lactating cows and young stock and herd size on the BTM M. bovis ELISA results. Herd was included as a random effect to account for clustering of BTM samples originating from the same herd. Increasing prevalence of antibody-positive lactating cows was the only variable associated with increasing M. bovis BTM ELISA optical density measurement. In contrast, the prevalence of antibody-positive young stock did not correlate with the BTM optical density measurement. In conclusion, some M. bovis associated herd infections are detectable by BTM ELISA-testing, but limitations exist and further investigations of the effect of different clinical disease expressions in the herds are warranted.