Eliminating bovine tuberculosis in cattle and badgers: insight from a dynamic model

Abstract

Bovine tuberculosis (BTB) is a multi-species infection that commonly affects cattle and badgers in Great Britain. Despite years of study, the impact of badgers on BTB incidence in cattle is poorly understood. Using a two-host transmission model of BTB in cattle and badgers, we find that published data and parameter estimates are most consistent with a system at the threshold of control. The most consistent explanation for data obtained from cattle and badger populations includes within-host reproduction numbers close to 1 and between-host reproduction numbers of approximately 0.05. In terms of controlling infection in cattle, reducing cattle-to-cattle transmission is essential. In some regions, even large reductions in badger prevalence can have a modest impact on cattle infection and a multi-stranded approach is necessary that also targets badger-to-cattle transmission directly. The new perspective highlighted by this two-host approach provides insight into the control of BTB in Great Britain.

Keywords: bovine tuberculosis; disease control; transmission dynamics; type reproduction numbers.

Figure 1.

Four scenarios for bovine tuberculosis transmission between cattle and badgers in Great Britain. The horizontal axis R_CC is the number of secondary cases in cattle due directly to cattle and the vertical axis is the proportion of infected cattle (red) and infected badgers (blue) at equilibrium. R_BC, R_CB and R_BB are the number of secondary cases in cattle due to badgers, in badgers due to cattle and in badgers due to badgers (see main text for details). The four scenarios are: (a) low inter-species transmission and unsustained transmission in badgers; (b) intermediate inter-species transmission and unsustained transmission in badgers; (c) low inter-species transmission and sustained transmission in badgers; and (d) intermediate inter-species transmission and sustained transmission in badgers.

Figure 2.

Incidence in cattle as a function of R_CC and R_BB with R_BC = R_CB = 0.05. The coloured areas represent incidence between 0.02% and 12%. White areas indicate combinations of values outside this incidence range. The blue dotted line marks the boundary of incidence greater than 0.02%. The black dashed line indicates the same boundary for R_BC = R_CB = 0.25 and the cross marks the point R_CC = 0.9 and R_BB = 0.6 (see text for discussion).

Figure 3.

The increase in badger prevalence when cattle removal rate is reduced for values of R_BB. The lower line represents the change in badger prevalence when cattle removal decreases from 0.8 to 0.5 years⁻¹ and the upper line represents the change in badger prevalence when the cattle removal rate decreases from 0.8 to 0.2 years⁻¹.

Figure 4.

The time to achieve eradication in cattle (defined as less than five reactors per 10 000 cattle) as a function of R_CC and R_BB with R_BC = R_CB > 0.05. The white region indicates areas where eradication is impossible.

Figure 5.

The time to achieve eradication in cattle (defined as less than five reactors per 10 000 cattle) as a function of (a) R_CC and R_BC with R_BB = 0.9 and R_CB = 0.1; (b) R_CC and R_BC with R_BB = 1.0 and R_CB = 0.1; and (c) R_CC and R_BC with R_BB = 1.1 and R_CB = 0.1. In (a–c), the vertical line denotes R_CC = 0.94 and the horizontal line R_BC = 0.11. The white region indicates areas where eradication is impossible. (d) The relative efficiency of reducing badger-to-cattle transmission compared with cattle-to-cattle transmission once cattle transmission is under control R_CC < 1.