Estimates for local and movement-based transmission of bovine tuberculosis in British cattle

Abstract

Both badgers and livestock movements have been implicated in contributing to the ongoing epidemic of bovine tuberculosis (BTB) in British cattle. However, the relative contributions of these and other causes are not well quantified. We used cattle movement data to construct an individual (premises)-based model of BTB spread within Great Britain, accounting for spread due to recorded cattle movements and other causes. Outbreak data for 2004 were best explained by a model attributing 16% of herd infections directly to cattle movements, and a further 9% unexplained, potentially including spread from unrecorded movements. The best-fit model assumed low levels of cattle-to-cattle transmission. The remaining 75% of infection was attributed to local effects within specific high-risk areas. Annual and biennial testing is mandatory for herds deemed at high risk of infection, as is pre-movement testing from such herds. The herds identified as high risk in 2004 by our model are in broad agreement with those officially designated as such at that time. However, border areas at the edges of high-risk regions are different, suggesting possible areas that should be targeted to prevent further geographical spread of disease. With these areas expanding rapidly over the last decade, their close surveillance is important to both identify infected herds qucikly, and limit their further growth.

Figure 1

CIs. Estimates and 95% profile CIs for proportions of infections caused by movement, background and high-risk area transmission. Model likelihood was evaluated at the end of 2004 for (a) the high within-herd and (b) low within-herd transmission models.

Figure 2

Distributions of high-risk areas as estimated for 2004 (low within-herd spread model). Premises in both radius-based and parochial high-risk areas are shown in light grey, premises only in radius-based areas in dark grey, and only in parochial-based areas in black. Elsewhere is shown with a checked background.