Quantifying Spatio-Temporal Overlap of Invasive Wild Pigs and Domestic Pig Farms as a Proxy for Potential Disease Transmission Risk

Abstract

Direct and indirect interactions between livestock and free-ranging wildlife creates important risks to animal health and agricultural productivity. The interface between newly established and rapidly spreading invasive wild pigs and the 2,549 domestic pig farms on the Prairie Provinces of western Canada has created important but poorly understood disease transmission risks. We mapped the spatial overlap of wild and domestic pigs to identify the areas of highest risk and associated distribution of diseases of concern using databases of wild pig occurrences and domestic pig farm locations. We also examined spatial and temporal overlap at the individual farm scale using GPS collared invasive wild pigs. Across the provinces of Alberta, Saskatchewan, and Manitoba, spatial overlap of invasive wild pigs with all combined, large-scale domestic pig farms, small-scale domestic pig farms, and domestic wild boar farms was 21%, 21%, 21%, and 53%. Invasive wild pig locations were significantly closer to domestic pig farms and domestic wild boar farms compared to random points on the landscape. The number of wild pig occurrences was greatest within 20 km of domestic pig farms and decreased linearly as distance increased. The Canadian distribution of wild pigs had considerable spatial overlap with recent areas detected with bovine tuberculosis (6,002 km2) in livestock and wildlife and Chronic Wasting Disease (156,159 km2) in wildlife, including mule deer, white-tailed deer, elk, and moose. The single best predictor of invasive wild pig occurrences across the landscape was close proximity to current or recently past existing domestic wild boar farms. The distance of GPS- collared wild pigs was significant for sex, farm type, month, and season and in southeastern Saskatchewan, average distance to domestic pig farms was 5.3 km. The weighted sum of cover type proportions, wild pig distance to domestic pig and wild boar farms, farm type, and farm density identified the relative risk of wild pig presence associated to each domestic pig farm occupied watershed. Risk was highest for small-scale domestic pig farms and lowest for large- scale domestic pig farms. Our findings highlight important potential routes for disease transmission at the invasive wild pig-domestic pig interface and identify areas where biosecurity improvements are urgently needed. While complete eradication of invasive wild pigs in Canada is no longer achievable, improved passive and active monitoring and removal of wild pigs is critical, especially where risks to domestic pig herds is highest.