Analysis of Antibody in Individual and Bulk Milk Samples is a Useful Tool for Investigation of Johne ’ s Disease

There is a need for improved cost-effectiveness when assessing a herd for the presence of Johne’s disease. The aim of this study was to determine whether this could be addressed by testing individual milk rather than serum samples, and screening targeted animals and bulk milk instead of an entire herd. The performance of an ELISA kit routinely used in our laboratory for quantifying serum levels of antibodies to Mycobacterium avium paratuberculosis (Map) was assessed for the analysis of milk samples. To this end, Map antibody titres were quantified in matched serum and milk samples taken from the same animal. Thereafter, bulk milk titres from herds infected with Map and from herds judged to be free of infection by sampling target animals within the herd were compared. There was 95% agreement between serum and milk ELISA results (r=0.84). Targeted sampling results agreed with the clinical history in all except one farm. There was a significant difference between the bulk milk titres from known infected herds and assumed negative herds (p=0.004). Analysis of serial dilutions of antibody positive milk samples in negative milk indicated that bulk milk sampling could be used to detect herds with a prevalence of antibody positive animals as low as 3% using the individual milk cut-off level. All bulk milk samples from negative herds had a percentage positive result of ≤10, suggesting that a lower positive cut-off level can be implemented for interpretation of bulk milk samples than for individual milk samples. Together, the results indicate that milk is a suitable medium for the quantification of Map antibodies by ELISA, targeted sampling has potential as a herd screening test and bulk milk sampling may be useful in monitoring herd infection level. Both of these latter techniques carry the limitation that they may be insufficiently sensitive to detect herds with a low prevalence of disease.