Is there an association between levels of bovine tuberculosis in cattle herds and badgers?

Wildlife diseases can have undesirable effects on wildlife, on livestock and people. Bovine tuberculosis (TB) is such a disease. This study derives and then evaluates relationships between the proportion of cattle herds with newly detected TB infection in a year and data on badgers, in parts of Britain. The relationships are examined using data from 10 sites which were randomly selected to be proactive culling sites in the UK Randomized Badger Culling Trial. The badger data are from the initial cull only and the cattle incidence data pre-date the initial badger cull. The analysis of the proportion of cattle herds with newly detected TB infection in a year, showed strong support for the model including significant frequency-dependent transmission between cattle herds and significant badger-to-herd transmission proportional to the proportion of M. bovis-infected badgers. Based on the model best fitting all the data, 3.4% of herds (95% CI: 0 – 6.7%) would be expected to have TB infection newly detected (i.e. to experience a TB herd breakdown) in a year, in the absence of transmission from badgers. Thus, the null hypothesis that at equilibrium herd-to-herd transmission is not sufficient to sustain TB in the cattle population, in the absence of transmission from badgers cannot be rejected (p=0.18). Omitting data from three sites in which badger carcase storage may have affected data quality; the estimate dropped to 1.3% of herds (95% CI: 0 – 6.5%) with p=0.76. The results demonstrate close positive relationships between bovine TB in cattle herds and badgers infectious with M. bovis. The results indicate that TB in cattle herds could be substantially reduced, possibly even eliminated, in the absence of transmission from badgers to cattle. The results are based on observational data and a small data set to provide weaker inference than from a large experimental study.