Predicting the status of wild deer as hosts of Mycobacterium bovis infection in Britain

Abstract

Control of livestock diseases can become complicated when wild animals are involved. The Eurasian badger (Meles meles) is considered the principle wildlife host of Mycobacterium bovis (which causes bovine tuberculosis, bTB) in Great Britain and Ireland, but wild deer have also been implicated. Whether wild deer are likely to perpetuate bTB in cattle depends on the exposure risks they pose, the mode of pathogen transmission, the distances over which the disease can be transported and whether they can maintain infection within their own populations independently of other sources. We evaluated the likely host status of each of four species of wild British deer (red, roe, fallow and Reeves' muntjac) and the badger across a range of densities typically observed in Britain by manipulating the reproductive number equation proposed by Anderson and May (1991). We estimate that roe deer almost certainly act as spillover hosts at densities lower than 30 km−2, red deer below 16 km−2, muntjac below 6 km−2, fallow below 4 km−2 and the badger below 2 km−2. We also estimate that muntjac will almost certainly act as maintenance hosts at densities above 56 km−2, fallow above 47 km−2 and badgers above 24 km−2. For densities between these values, we cannot be certain of the host status of these species, and for red and roe deer we cannot be certain of host status under most natural conditions typically experienced in parts of Britain experiencing high incidence of bTB in cattle. However, enhanced transmission rates resulting from artificially high densities such as might be experienced at supplementary feeding sites may be sufficient to promote independent maintenance of infection. We were not able to confidently assign host status to any species over a wide range of densities, but conclude that this is likely to reflect reality, where host status may be affected as much by, for example, demographic fluctuations as it is by population density. Our results imply densities below which populations of wild deer inhabiting cattle bTB hotspots ought to be maintained in order to control the possibility of them perpetuating the cycle of intra- and interspecific M. bovis transmission.