Culling and cattle controls influence tuberculosis risk for badgers

Abstract

Human and livestock diseases can be difficult to control where infection persists in wildlife populations. In Britain, European badgers (Meles meles) are implicated in transmitting Mycobacterium bovis, the causative agent of bovine tuberculosis (TB), to cattle. Badger culling has therefore been a component of British TB control policy for many years. However, large-scale field trials have recently shown that badger culling has the capacity to cause both increases and decreases in cattle TB incidence. Here, we show that repeated badger culling in the same area is associated with increasing prevalence of M. bovis infection in badgers, especially where landscape features allow badgers from neighboring land to recolonize culled areas. This impact on prevalence in badgers might reduce the beneficial effects of culling on cattle TB incidence, and could contribute to the detrimental effects that have been observed. Additionally, we show that suspension of cattle TB controls during a nationwide epidemic of foot and mouth disease, which substantially delayed removal of TB-affected cattle, was associated with a widespread increase in the prevalence of M. bovis infection in badgers. This pattern suggests that infection may be transmitted from cattle to badgers, as well as vice versa. Clearly, disease control measures aimed at either host species may have unintended consequences for transmission, both within and between species. Our findings highlight the need for policymakers to consider multiple transmission routes when managing multihost pathogens.

Fig. 1.

Interannual variation in M. bovis infection in badgers and cattle. Shading indicates the 2001 FMD epidemic. (a) Median interval between cattle tests in proactive areas enrolled in the RBCT, and total infected cattle slaughtered in all proactive areas. (b) National incidence of cattle TB per calendar year. (c) Prevalence recorded in adult badgers in seven proactive areas before (1999 for area A, 2000 for areas B–H) and after (2002) the FMD epidemic. Error bars give exact binomial 95% confidence intervals, and the black line indicates equal prevalence. (d) Prevalence in proactively culled adult badgers, with fitted values from the model shown in Table 1.

Fig. 2.

Effect of repeated proactive culling on M. bovis infection prevalence in adult badgers. Prevalence estimates are derived from the model in Table 1 and are represented as odds ratios for badgers captured in inner (≥2 km inside, solid lines) and outer (<2 km inside, dashed lines) regions of trial areas with the lowest (red, permeability = 0.55), median (green, permeability = 0.94), and highest (blue, permeability = 1.0) observed boundary permeability. Odds ratios are calculated relative to initial culls in outer trial areas.

Fig. 3.

Strain types (spoligotypes) (35) of M. bovis detected in badgers in 10 proactive culling areas (A–J). The solid vertical line indicates the 2001 FMD epidemic. Numbers inside bars give sample sizes. Spoligotypes were available for 1,167 of 1,203 infected badgers.