Investigating Farm Fragmentation as a Risk Factor for Bovine Tuberculosis in Cattle Herds: A Matched Case-Control Study from Northern Ireland.

Georgina Milne,John Mcgrath,Wilma Mcmaster,Jordon Graham,Andrew William Byrne,Raymond Kirke

doi:10.3390/pathogens11030299

Georgina Milne, John Mcgrath + Show 4 more

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https://doi.org/10.3390/pathogens11030299

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Abstract

Bovine tuberculosis remains a challenging endemic pathogen of cattle in many parts of the globe. Spatial clustering of Mycoacterium bovis molecular types in cattle suggests that local factors are the primary drivers of spread. Northern Ireland’s agricultural landscape is comprised of highly fragmented farms, distributed across spatially discontinuous land parcels, and these highly fragmented farms are thought to facilitate localised spread. We conducted a matched case control study to quantify the risks of bovine tuberculosis breakdown with farm area, farm fragmentation, fragment dispersal, and contact with neighbouring herds. Whilst our results show small but significant increases in breakdown risk associated with each factor, these relationships were strongly confounded with the number of contiguous neighbours with bovine tuberculosis. Our key finding was that every infected neighbour led to an increase in the odds of breakdown by 40% to 50%, and that highly fragmented farms were almost twice as likely to have a bTB positive neighbour compared to nonfragmented farms. Our results suggest that after controlling for herd size, herd type, spatial and temporal factors, farm fragmentation increasingly exposes herds to infection originating from first-order spatial neighbours. Given Northern Ireland’s particularly fragmented landscape, and reliance on short-term leases, our data support the hypothesis that between-herd contiguous spread is a particularly important component of the region’s bovine tuberculosis disease system.

Highlights

Bovine tuberculosis, caused primarily by Mycobacterium bovis infection, is a complex and challenging disease of cattle, endemic in many countries across the globe [1]
The average case farm was associated with five additional land parcels than the average control farm, and 21.9% of case farms were “highly fragmented” (8–10 fragments) or “very highly fragmented” (11+ fragments)”, compared to 17.7% of control farms. 30% of case farms had very highly dispersed fragments (>3.05 km between fragment centroids), compared to 26% of control farms, and 41% of case farms had the highest levels of contact (>4.94 km of shared boundary) with neighbouring farms, compared to 33% of control farms
We found that farm fragmentation, fragment dispersal and contact with neighbouring farms are each associated with increased odds of bTB breakdown, which is in concordance with previous studies

Summary

Introduction

Bovine tuberculosis (bTB), caused primarily by Mycobacterium bovis infection, is a complex and challenging disease of cattle, endemic in many countries across the globe [1]. The disease continues to blight the cattle industry in the United Kingdom (UK) and Republic of Ireland (ROI), despite long-running test-and-slaughter eradication programmes [2–4], ancillary testing [5–7], and surveillance at the abattoir for lesions indicative of bTB [8,9]. Whereby farms are distributed across multiple, spatially discontinuous land parcels, is common on the island of Ireland [25,26]. In Northern Irish cattle farms, 35% of businesses were comprised of five or more fragments [27], comparable to the ROI where 32% of sampled farms were comprised of five or more fragments [28]. Farm fragmentation may contribute to the bTB epidemic by providing more opportunities for direct nose to nose contact between cattle from other farms “over the fence” [16,22,23]

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