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

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.

Keywords: Northern Ireland; bTB; bovine tuberculosis; conacre; farm fragmentation; local spread; matched case-control; neighbourhood.

Figure 1

Spatial distribution of case herds and all potential control herds across the 10 DVO areas in NI.

Figure 2

Plots of OR’s and 95% CI for each factor in the four categorical predictors; (A) Farm area, (B) Farm fragmentation, (C) Fragment dispersal, and (D) Contact with neighbours.

Figure 3

Illustration of confounding between the number of bTB positive neighbours and each of (A) farm area, (B) farm fragmentation, (C) fragment dispersal, and (D) contact with neighbouring farms.

Figure 4

A schematic of definitions used in this manuscript. In this example, the index farm is shown in grey, surrounded by land parcels belonging to first-order spatial neighbours in white. The index farm has five land-parcels distributed across two fragments. The distance between the fragment centroids (in km) is shown by the double headed arrow. The contact with neighbouring farms (in km) is the perimeter of the fragments, coloured with the thickest black line.