The evaluation of the utility of bulk tank tests for the surveillance of Johne's disease and the effect of storage time and temperature on Johne's milk ELISA results
The first objective of this study was to evaluate the utility of bulk tank tests to detect the presence of Mycobacterium avium subspecies paratuberculosis (MAP) antibody in dairy herds for the purpose of Johne’s disease surveillance. Individual cow milk samples were collected by CanWest Dairy Herd Improvement customer service representatives in herds across Ontario, Canada. These samples, along with bronopol preserved bulk tank samples were collected from herds participating in the Ontario Johne’s Education and Management Assistance Program (OJEMAP), a producer funded Johne’s control scheme. Overall, there were 309 farms tested, with herd size from 15 to 986 milking cows. The relative sensitivity and specificity of the bulk tank ELISA test when a positive herd was defined as 1 or more positive cows was 54.7% and 90.6%, respectively. The second objective was to determine the effect of milk storage temperature and duration on the Johne’s milk ELISA test result. When herd level factors were considered in a logistic model, average monthly protein (%) and the percent of positive milk contributed to the bulk tank by milk ELISA positive cows were found to be significantly (p<0.05) associated with the probability of a herd testing positive on the bulk tank Hyper ELISA protocol. Positive and negative MAP milk samples were stored for varying times and under different temperature conditions. In a mixed linear model, time was found to be significantly (<0.001) associated with the log transformed ELISA optical density. When the results were dichotomized into positive and negative by the cut-off of 0.10 and cross classified, the amount of misclassification was considered biologically negligible.