Estimating the risk of cattle exposure to tuberculosis posed by wild deer relative to badgers in England and Wales

Abstract

Wild deer populations in Great Britain are expanding in range and probably in numbers, and relatively high prevalence of bovine tuberculosis (bTB, caused by infection with Mycobacterium bovis) in deer occurs locally in parts of southwest England. To evaluate the M. bovis exposure risk posed to cattle by wild deer relative to badgers in England and Wales, we constructed and parameterized a quantitative risk model with the use of information from the literature (on deer densities, activity patterns, bTB epidemiology, and pathology) and contemporary data on deer, cattle, and badger (Meles meles) distribution and abundance. The median relative risk score for each of the four deer species studied--red (Cervus elaphus), fallow (Dama dama), and roe (Capreolus capreolus) deer, and muntjac (Muntiacus reevesi)--was lower than unity (the relative risk set for badgers, the putative main wildlife reservoir of M. bovis in England and Wales). However, the 95th percentiles associated with risk estimates were large, and the upper limits for all four deer species exceeded unity. Although M. bovis exposure risks to cattle from deer at pasture are likely to be lower than those from badgers across most areas of England and Wales where cattle are affected by bTB because these areas coincide with high-density badger populations but not high-density deer populations, we predict the presence of localized areas where relative risks posed by deer may be considerable. Moreover, wherever deer are infected, risks to cattle may be additive to those posed by badgers. There are considerable knowledge gaps associated with bTB in deer, badgers, and cattle, and data available for model parameterization were generally of low quality and high variability, and consequently model output were subject to some uncertainty. Improved estimates of the proportion of time that deer of each species spend at pasture, the likelihood and magnitude of M. bovis excretion, and local badger and deer densities appear most important for improving estimates of relative risk in this system.