Bovine tuberculosis in Northern Ireland: Risk factors associated with duration and recurrence of chronic herd breakdowns.

This study investigated 8058 bovine tuberculosis (bTB) confirmed breakdowns occurring in Northern Ireland during the period 2005-2010 inclusive. The methodology used two case-control studies; one determined the risk factors associated with long duration bTB breakdowns and the other with recurrent bTB breakdowns. The analyses were implemented using a generalized linear mixed model analysis with variables relating to repeated measures on herds, locality and year of breakdown included as random effects. The case definition for long duration breakdowns (n=679) was any confirmed bTB disclosure with duration greater than one year. The case definition for recurrent breakdowns (n=657) was any confirmed bTB disclosure with duration less than one year, followed by two or more bTB breakdowns within 2 years from the end of the initial bTB breakdown. In the multivariable model based on duration of bTB breakdowns, significant factors were local area bTB prevalence, number of associated cattle herds, total years restricted in the previous five years, total number of bTB reactors during the breakdown and the presence of a bTB lesion at routine slaughter (LRS). The number of bTB reactors at the disclosing test was also significant; with increased numbers associated to reduced odds of a long duration breakdown. In the second analysis based on recurrence of bTB breakdowns, high local area prevalence, movement intensity into the herd, total years restricted in the previous five years, herd size, total number of TB reactors during the restricted breakdown and presence of a LRS were all statistically significant. Crown