Biological hurdles to the control of TB in cattle: A test of two hypotheses concerning wildlife to explain the failure of control

Abstract

Since the 1970s the control of bovine tuberculosis (bTB) in cattle, predominantly in the SW of England has proved continually recalcitrant; it is currently increasing at an annual rate of 18%. This deterioration has occurred despite a succession of government schemes involving killing badgers, Meles meles, with the intention of reducing transmission of bTB to cattle. Of various hypotheses proposed to explain this failure of control, some concern agricultural practice, but two concern wildlife. The latter are, first, that wild mammalian species other than badgers are important in the transmission of bTB, and second, that a perturbation effect amongst those badgers surviving a cull countermands the hoped-for reduction of bTB transmission to cattle or even increases it. We review our own studies of these two hypotheses in the context of other findings. We conclude that the other species hypothesis does not provide a general explanation for the failure of bTB control. We also conclude that the perturbation hypothesis is supported by the data and does provide one plausible mechanism to explain why culling badgers has not generally achieved control of bTB in cattle. We have reviewed the relevance of perturbation with respect to three key questions: (a) is there evidence of a perturbation effect on badger behavioural ecology? (b) is there evidence of a perturbation effect on prevalence of bTB in badgers? (c) are any observed effects of a magnitude relevant to bTB control policy? The results of the Randomised Badger Culling Trial (RBCT) and our own studies indicate that to have any prospect of contributing significantly to controlling bTB in cattle, a badger cull would have to be undertaken over a very large area. Considering the likely very important role of cattle-to-cattle transmission, and the opportunities for solutions in terms of farm management and surveillance, it would be inappropriate (and probably impractical) to undertake such a cull now.