Recurrence of bovine tuberculosis breakdowns in Great Britain: risk factors and prediction

Bovine tuberculosis (bTB) is an important economic disease worldwide with implications for both animal and human health. In Great Britain the number of herds that test positive for bTB, termed "breakdowns", has increased over the last two decades. Despite more intensive testing during a breakdown, around 23% of breakdowns recur within 12 months of the previous breakdown ending, and around 38% within 24 months. These "recurrent" breakdowns may be important for onward transmission of infection. Detailed case-control data were analysed to identify factors associated with recurrence within 12 months. The model predicted 83% of all recurrent breakdowns, with a positive predictive value (PPV) of 44%. A further model, restricted to data currently available nationally, was not sufficient to predict recurrence reliably; at a sensitivity of 72-76%, the PPV was 33-36%, when validated on independent data. Factors identified to be associated with recurrence are consistent with previous studies; namely, the number of reactors, a recent bTB history in the herd and a lack of association with the confirmation status of the initial breakdown. These variables are indicative of a higher level of infection or residual infection, and could be useful in the future development of predictive models for bTB recurrence.