Evaluation of Risk Factors Associated With Herds With an Increased Duration of Bovine Tuberculosis Breakdowns in Castilla y Leon, Spain (2010-2017)

Abstract

The persistence of bovine tuberculosis (bTB) in certain cattle herds is a major concern in countries pursuing disease eradication worldwide. The chronic nature of the disease, the lack of performance of diagnostic tools, and the presence of wildlife reservoirs may lead infected herds to require longer periods to achieve the officially tuberculosis-free (OTF) status. Here, we evaluated the impact of farm and breakdown characteristics on the probability of disease persistence in infected farms in Castilla y Leon, a bTB-endemic region of Spain, using survival and logistic regression models. Data from bTB breakdowns occurring in 3,550 bTB-positive herds detected in 2010-2017 were analyzed. A multivariable Cox proportional hazards model was fitted using time to recover OTF status as the response variable, and a multivariable logistic regression model using the chronic status (yes/no) for herds experiencing particularly long breakdowns as the outcome variable was also used. Both analyses revealed that county-level bTB herd prevalence, herd size, number of incoming animals in the previous 3 years, number of skin test reactors in the disclosing test, and number of days between the disclosing and follow-up tests were associated with increased breakdown duration. Production type was not consistently associated with chronic infection, suggesting that once infected, it is not a significant predictor of outbreak duration beyond the initial stages of the breakdown. Province-level location and number of animals that are bacteriology-positive also affected significantly the expected herd breakdown duration, but their effect became less significant over time. Risk factors identified in this study may help to identify herds more prone to suffer chronic bTB infection that may require additional control measures early on in a breakdown.

Keywords: Mycobacterium bovis; bovine tuberculosis; case-control study; cattle; chronic breakdowns; risk factors; survival analysis.

Figure 1

Diagram of bovine tuberculosis (bTB) breakdown definition. Plus and minus signs denote bTB-positive and bTB-negative tests, respectively.

Figure 2

Kaplan–Meier survival estimates of bovine tuberculosis (bTB) breakdown duration by production type. Crosses indicate censored observations. Number of censoring table shows the number of herds that did not clear the infection during the study period or were depopulated during the study period at regular time intervals.

Figure 3

Spatial distribution of case and control herds in Castilla y Leon included in the case-control study. The red and blue circles denote the location of case and control herds, respectively.