Impact of changes in cattle movement regulations on the risks of bovine tuberculosis for Scottish farms

Abstract

Legislation requiring the pre- and post-movement testing of cattle imported to Scotland from regions with high bovine tuberculosis (bTB) incidence was phased in between September 2005 and May 2006 as part of efforts to maintain Officially Tuberculosis Free (OTF) status. In this analysis, we used centralized cattle movement records to investigate the influence of the legislative change on import movement patterns and the movement-based risk factors associated with new bTB herd breakdowns identified through routine testing or slaughter surveillance. The immediate reduction in the number of import movements from high incidence regions of England and Wales into Scotland suggests that pre- and post-movement testing legislation has had a strong deterrent effect on cattle import trade. Combined with the direct benefits of a more stringent testing regime, this likely explains the observed decrease in the odds of imported cattle subsequently being identified as reactors in herd breakdowns detected through routine surveillance compared to Scottish cattle. However, at the farm-level, herds that recently imported cattle from high incidence regions were still at increased risk of experiencing bTB breakdowns, which highlights the delay between the introduction of disease control measures and detectable changes in incidence. With the relative infrequency of routine herd tests and the insidious nature of clinical signs, past import movements were likely still important in determining the present farm-level risk for bTB breakdown. However, the possibility of low-level transmission between Scottish cattle herds cannot be ruled out given the known issues with test sensitivity, changes in import animal demographics, and the potential for on-farm transmission. Findings from this analysis emphasize the importance of considering how farmer behavioural change in response to policy interventions may influence disease transmission dynamics.