Husbandry practices, badger sett density and habitat composition as risk factors for transient and persistent bovine tuberculosis on UK cattle farms

Abstract

Bovine tuberculosis (bTB) is a persistent problem in cattle herds in Great Britain and Ireland. Farm management and cattle husbandry practices can influence the risk of transmission of bTB and hence the likelihood of bTB breakdown (≥1 reactor to the tuberculin skin test). Biological differences are expected in the transmission dynamics, and hence risk factors for bTB breakdown, on farms where infection persists in the herd compared to farms where infection is more sporadic or short-lived. Comparative case–control studies were performed to test farm management practices as potential risk factors for transient (under breakdown restrictions for ≤6 months) and persistent (under breakdown restrictions for >6 months) bTB breakdown over 5 years (1995–1999) on 179 and 171 UK cattle farms, respectively. Farms were characterised for badger sett density and farm habitat composition by ground survey, farmers were questioned retrospectively on management practices, and cases and controls were identified from national tuberculin test records. Controlling for routine tuberculin testing interval, log-transformed herd size, regional location, badger sett density and farm habitat complexity, multivariable logistic regression identified increased odds of both transient and persistent breakdown on farms that bought-in cows (odds ratio (OR) ≥ 4.9; 95% confidence interval (CI) ≥ 1.1;22.8). In addition, the purchase of >50 head of cattle (OR = 4.0, 95% CI = 1.0;16.0) and the storage of manure for ≥6 months (OR = 4.4; 95% CI = 1.3;15.4) were risk factors for transient breakdown, whereas the use of silage clamps (OR = 9.1; 95% CI = 2.0;40.8) increased the risk of persistent breakdown. Decreased odds of both transient and persistent breakdown were associated with higher stocking densities (>3 cattle/ha) (OR ≤ 0.2; 95% CI ≤ 0.1;0.9), and running mixed herd enterprises compared to only beef or dairy (OR = 0.1; 95% CI = 0.0;0.7) was an additional protective factor against persistent breakdown. In these analyses, the covariates log herd size and tuberculin testing interval were significant predictors of both transient and persistent breakdown, whereas active badger sett density and regional location only affected the risk of persistent breakdown. The collective results suggest that the probability of transient breakdown is most strongly influenced by the purchase of cattle over other management variables and covariates, whereas the probability of persistent breakdown appears to be mostly affected by management factors relating to type of herd enterprise and silage storage in addition to the relative density of badgers. Implications for bTB management are discussed.