Cattle-associated risk factors for human tuberculosis in rural livestock-keeping communities, Uganda

Tuberculosis (TB) is a leading infectious cause of human death worldwide. In cattle, TB infection results in productivity losses, trade barriers and zoonotic transmission via milk, meat or direct contact. We conducted a cross-sectional study in rural communities in Uganda between 2014 and 2016 to evaluate the association between tuberculosis skin test (TST) positivity in humans and cattle-associated risk factors: household herd positivity and raw milk consumption. Human and cattle TSTs were performed in communities followed by a survey of household practices. TST data were available on 493 humans, 184 (37.3%) with positive results, and 1,441 cattle, 50 (3.5%) with positive results. We fit separate log binomial generalized estimating equation models for the herd positivity-human TST positivity association, stratified on sex; and for the raw milk consumption-human TST positivity association, stratified on frequency of milk consumption. Having at least one TST-positive bovid in the household's herd was significantly associated with lower risk of TB among men (prevalence ratio [PR] 0.66, 95% CI 0.49, 0.87) but was not significantly associated with TB among women (PR 1.21, 95% CI 0.76, 1.95). This apparent protective effect was contrary to our a priori hypothesis of higher exposure effect among men, the primary caretakers of cattle. This finding may be the result of residual confounding by socioeconomic status; wealthier individuals may be less likely to be TB positive, but more likely to have TST-positive herds by virtue of larger herd sizes, ability to purchase new and possibly infected stock, and propensity to keep more TB-susceptible European breeds. For raw milk consumption, effect estimates were close to one and not statistically significant. Thus, in settings where bovine TB prevalence is low, such as Uganda, cattle-associated zoonotic transmission may be rare, and cattle-associated risk factors may not be important drivers of human TB burden.