A spatio-temporal probability model of cattle and African buffalo (Syncerus caffer) contact as a proxy for foot-and-mouth disease risk : a case study at the wildlife-livestock interface of the Kruger National Park, South Africa

Abstract

The work presented here investigates a number of important components in the epidemiology of foot-and-mouth disease (FMD) at the wildlife-livestock interface along the Kruger National Park (KNP) and its adjoining private and provincial nature reserves (APNR). This epidemiological setting is unique and complex, mainly due to the presence of African buffaloes (Syncerus caffer) persistently infected with the Southern African Territories (SAT) strains of the FMD virus. Despite a great deal of experimentation and field research, FMD transmission from both persistently- and acutely-infected buffaloes to cattle is still poorly understood, although it is accepted that the two species have to be in close contact for it to occur. And even then, ‘close contact’ is ill-defined. Comparisons of three intensive study sites in the FMD protection zone, comprising both communally-farmed as well as smallholder cattle rangelands, were conducted to better understand this livestock system and determine livestock distribution patterns. This vast heterogeneous landscape of the FMD protection zone follows a latitudinal gradient of both human density and rainfall (increasing from north to south), which is reflected in similar gradients in cattle densities, the number of households owning cattle, consumption patterns as well as herding practice, whilst inversely associated with cattle sales and calf mortality rates. Distribution studies of the cattle in these study sites revealed a dry season dependence on water, and contraction of home ranges around preferred habitat during the warm wet season. Herding, or the lack thereof, was evident in daily activity budgets of these cattle. Interestingly, seasonal differences in stray buffalo dispersal patterns were the inverse of their domestic counterparts, being condensed along the fence line during the dry season. Stray buffalo events mainly consisted of single animals or very small groups, predominated by adult bulls. Most of these animals were destroyed soon after straying from the FMD infected zone, although some managed to move into areas with unvaccinated cattle, especially along major river courses. Swift removal of stray buffaloes from livestock areas is paramount, since recent outbreaks all share the characteristic of stray buffaloes being reported roaming the outbreak-area over extended periods (months) prior to an outbreak. Combining cattle and stray buffalo habitat suitability estimates revealed areas likely to facilitate contact between these species, which was mostly driven by cattle dispersal into those condensed areas along the fence and rivers more suitable for stray buffaloes during the cool dry season. This is also the season when acute infection with FMD is most likely in buffaloes. Yet, when adjusting cattle habitat suitability to incorporate their density, the concentration of preferred habitat during the warm wet season forms small pockets of high cattle density, amplifying contact risk with stray buffaloes in these areas. Simulation of stray buffalo movements along a least cost path, which incorporates fence permeability, is shown to be a useful visual aid to finding high-risk stray buffalo movement corridors. Contact-risk models performed well in retrospectively identifying high risk areas during recent outbreaks. These risk outputs have value in informing risk-based surveillance and strategic vaccination programs as well as in the delineation of disease control zones.