Evaluating evidence of association of bovine tuberculosis in cattle and badgers.
Abstract
Bovine tuberculosis (TB) occurs in cattle and badgers in the UK and control efforts are undertaken to reduce the spread of the disease. This study evaluates relationships predicted by nine epidemiological two-host models of disease spread generated by various combinations of density-dependent, frequency-dependent and environmental pathogen transmission. The relationships of interest are between measures of TB in cattle and in badgers from 10 sites which were randomly selected to be proactive badger culling sites in the UK Randomized Badger Culling Trial. The data are from the initial badger cull only. There was most support (Akaike weight=0.562, R2=0.869) for models that predicted a positive linear relationship between density of infectious cattle per square kilometre and the density index of infectious badgers. There was less support (Akaike weight=0.060) for a model that predicted a positive linear relationship between density of infectious cattle per square kilometre and the proportion of badgers infectious with Mycobacterium bovis. A correction to reduce effects of badger carcase storage and an examination of effects of the 2001 foot-and-mouth disease epidemic had little impact on estimated relationships. Synthesis and applications. The results provide support for two-host disease models of TB in cattle and wildlife such as badgers, although the form of disease transmission cannot be identified clearly by these analyses. The implication of the results is that the best-fitting models predict that, in the absence of intervention-related changes in badger behaviour, a reduction in density of infectious badgers should reduce the density of infectious cattle. However, analysis of bait-marking data collected following experimental badger culls indicated that culling badgers profoundly alters their spatial organization as well as their population density, potentially influencing contact rates. Effective vaccination of badgers, were it to become available, would be expected to reduce the density of infectious badgers without directly affecting their behaviour.