Predicting Positive ELISA Results in Dairy Herds with a Preferred Status in a Paratuberculosis Control Program

SIMPLE SUMMARY: In many paratuberculosis control programs, test-negative herds are assigned a preferred herd status. This may induce the false belief that these herds are free of paratuberculosis. Hence, farmers may refrain from control measures that could prevent the spread of any undetected infections. The aim of the present study was to develop a predictive model to alert farmers with test-negative herds and a preferred status in the Dutch paratuberculosis control program if they are at an increased risk of positive ELISA results in a subsequent 30-month period. On the basis of the results of this study, we conclude that discrimination of herds with high (52%) and low (17%) risks of positive ELISA results is feasible. This might help farmers with the highest risks of future positive results to make better informed decisions regarding the need to take additional control measures to prevent the spread of any undetected Map infections. ABSTRACT: Dairy herds participating in the Dutch milk quality assurance program for paratuberculosis are assigned a herd status on the basis of herd examinations by ELISA of individual serum or milk samples, followed by an optional confirmatory fecal PCR. Test-negative herds are assigned Status A; the surveillance of these herds consists of biennial herd examinations. Farmers falsely believing that their Status A herds are Map-free may inadvertently refrain from preventive measures. Therefore, we aimed to develop a predictive model to alert Status A farmers at increased risk of future positive ELISA results. Using data of 8566 dairy herds with Status A in January 2016, two logistic regression models were built, with the probabilities of ≥1 or ≥2 positive samples from January 2017–June 2019 as dependent variables, and province, soil type, herd size, proportion of cattle born elsewhere, time since previous positive ELISA results, and the 95th percentile of the S/P ratios in 2015–2016, as explanatory variables. As internal validation, both models were applied to predict positive ELISA results from January 2019–June 2021, in 8026 herds with Status A in January 2019. The model predicting ≥1 positive sample had an area under the receiver operating characteristics curve of 0.76 (95% CI: 0.75, 0.77). At a cut-off predicted probability π(c) = 0.40, 25% of Status A herds would be alerted with positive and negative predictive values of 0.52 and 0.83, respectively. The model predicting ≥2 positive samples had lower positive, but higher negative, predictive values. This study indicates that discrimination of Status A herds with high and low risks of future positive ELISA results is feasible. This might stimulate farmers with the highest risks to take additional measures to control any undetected Map infections.