COMPARISON OF FERTILITY-CONTROL AND LETHAL CONTROL OF BOVINE TUBERCULOSIS IN BADGERS - THE IMPACT OF PERTURBATION INDUCED TRANSMISSION

In this paper we use mathematical modelling to consider the broad adva ntages and disadvantages of fertility control over lethal control for bovine tuberculosis in badger populations. We use a deliberately simpl e model, attempting to capture only the key transmission processes. Th e model is parametrized with reference to the long-term Woodchester Pa rk study. Estimates of mortality rate from this study suggest no signi ficant extra mortality risk for animals with evidence of infection as indicated by the presence of anti-Mycobacterium bovis antibodies or M. bovis isolation. We find that large reductions in prevalence are some times the consequence of only moderate reductions in population number s. If we assume that the act of control does not in itself affect tran smission rates, then as far as eradication is concerned, both fertilit y control and mortality control operate through the same epidemiologic al mechanism, the removal of susceptibles: if one is in principle capa ble of keeping a population low enough to be infection free then so is the other. It is necessary to continue either form of control at regu lar intervals to maintain a constant level of infection in the long te rm. If control were to be stopped, return to precontrol levels of badg er population and infection prevalence would be expected within a few years. Fertility control is less effective in reducing population dens ity than lethal control since it can only act, at maximum, to remove o ne age cohort per year. It is also less effective in reducing transmis sion as it can only ever remove susceptibles, while lethal control als o removes infectious badgers. However, if the social disturbance cause d by lethal control does in fact increase contact rates for the remain ing infectious badgers, the relative efficacies of the two strategies become a great deal less clear. While we have no quantitative data on the extent to which social perturbation does act to promote transmissi on, model simulations show that it is possible to develop plausible sc enarios in which the lethal control may actually act to increase the a bsolute numbers of animals infected, while reducing the number of unin fected animals to very low numbers.