Culling and cattle controls influence tuberculosis risk for badgers Rosie Woodroffe*������, Christl A. Donnelly�����, Helen E. Jenkins��, W. Thomas Johnston��, David R. Cox��������, F. John Bourne���, Chris L. Cheeseman , Richard J. Delahay , Richard S. Clifton-Hadley**, George Gettinby���,������, Peter Gilks��, R. Glyn Hewinson**, John P. McInerney���, and W. Ivan Morrison��������� *Department of Wildlife, Fish, and Conservation Biology, University of California, One Shields Avenue, Davis, CA 95616 ���Independent Scientific Group on Cattle TB, c o Department for Environment, Food, and Rural Affairs, 1A Page Street, London SW1P 4PQ, United Kingdom ��Department of Infectious Disease Epidemiology, Faculty of Medicine, Imperial College London, St. Mary���s Campus, Norfolk Place, London W2 1PG, United Kingdom ��Nuffield College, New Road, Oxford OX1 1NF, United Kingdom Central Science Laboratory, Sand Hutton, York YO41 1LZ, United Kingdom **Veterinary Laboratories Agency, Woodham Lane, Surrey KT15 3NB, United Kingdom ������Department of Statistics and Modelling Science, University of Strathclyde, Richmond Street, Glasgow G1 1XH, United Kingdom and ������Centre for Tropical Veterinary Medicine, Royal (Dick) School of Veterinary Studies, University of Edinburgh, Easter Bush, Roslin, Midlothian EH25 9RG, United Kingdom Contributed by David R. Cox, July 26, 2006 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 Mycobacte- rium 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 ob- served. 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 unin- tended consequences for transmission, both within and between species. Our findings highlight the need for policymakers to consider multiple transmission routes when managing multihost pathogens. behavior bovine tuberculosis epidemiology Meles meles perturbation Manimal any pathogens that can influence human and domestic health are sustained in wildlife populations (1). Mycobacterium bovis, the causative agent of bovine tuberculosis (TB), is one such pathogen. In Britain, testing and slaughter of infected cattle eradicated the infection in many areas, but control was not achieved where populations of European badgers (Meles meles), widespread but protected wild animals, sustain endemic infection (2). Between 1975 and 1997, 20,000 badgers were culled by the British government in a series of attempts to limit TB transmission to cattle (2). Nevertheless, the national inci- dence of cattle TB has been increasing since the 1980s (2). A large-scale field trial, the Randomised Badger Culling Trial (RBCT), recently showed that although culling badgers reduced cattle TB incidence where widespread (100 km2) culling oc- curred, incidence was increased on neighboring unculled lands (3) and in areas where culling was restricted to small patches of land (average, 5.3 km2) (4). These detrimental effects of badger culling were attributed to disruption of badgers��� territorial organization and expansion of ranging behavior, which were documented in and around culling areas. This social perturba- tion potentially increased contact with cattle (5), but is likely to have also influenced contact rates within the badger population (5, 6). Whereas epidemiological models usually assume that depressing host population density will reduce disease transmis- sion through lower contact rates, social perturbation could lead badger culling to generate only small reductions, or even in- creases, in rates of disease transmission, with concomitant effects on infection prevalence (7). Such effects could be par- ticularly marked where repeated culling and continued immi- gration prevent reestablishment of a stable spatial organization (6, 8). Substantial interannual variation in prevalence has been ob- served in the absence of culling (9), and it is therefore important to distinguish hypothetical culling effects from other factors that could influence TB dynamics in badgers, such as the local prevalence of infection in cattle. However, even though patterns of M. bovis infection in cattle and badgers are associated in space (10), distinguishing badger-to-cattle transmission of infection, which has been demonstrated experimentally (3, 4, 11), from cattle-to-badger transmission is problematic in observational studies, especially where badgers are being sampled destruc- tively. While the RBCT was in progress, measures to control a nationwide epidemic of foot-and-mouth disease (FMD) led to a 9-month suspension of routine cattle TB testing (12). This suspension delayed the removal of M. bovis-infected cattle (Fig. 1a), increasing the potential for spread among cattle (13), and offering an opportunity to assess the risks of transmission to badgers. We used statistical models to investigate the effects of badger culling and cattle controls on the prevalence of M. bovis infection in badgers. We predicted that prevalence would be higher: (i) in areas that had been culled repeatedly (ii) where geographical features allowed badgers to recolonize culled areas, promoting social perturbation and hence elevating contact rates and (iii) when removal of infected cattle had been delayed by suspension of cattle testing. We also investigated an alternative hypothesis that interannual variation in prevalence was related to climate. Weather conditions have been linked to both geographical (14) and temporal (15) variation in cattle TB, as well as to badger population dynamics (16) and could plausibly influence TB dynamics in badgers. The North Atlantic Oscillation (NAO) is a major determinant of weather conditions in Western Europe and Conflict of interest statement: No conflicts declared. Freely available online through the PNAS open access option. Abbreviations: FMD, foot-and-mouth disease NAO, North Atlantic Oscillation RBCT, Ran- domised Badger Culling Trial TB, tuberculosis. ���To whom correspondence may be addressed. E-mail: rwoodroffe@ucdavis.edu or david.cox@nuffield.oxford.ac.uk. �� 2006 by The National Academy of Sciences of the USA www.pnas.org cgi doi 10.1073 pnas.0606251103 PNAS October 3, 2006 vol. 103 no. 40 14713���14717 APPLIED BIOLOGICAL SCIENCES

influences a wide array of ecological traits (17), predicting ecological responses more reliably than do local weather con- ditions (18). We therefore investigated relationships between the NAO and TB dynamics in badgers. Results Our primary analyses concerned adult badgers taken on succes- sive culls in 10 RBCT trial areas. Analyses of other data sets are provided as supporting information, which is published on the PNAS web site. We constructed a ������base model,������ including covariates likely to influence observed prevalence (see Materials and Methods). This model suggested that, as predicted, the prevalence of M. bovis infection increased with successive culls. This increased preva- lence was particularly marked when multiple operations were used to complete each annual cull (details in supporting infor- mation). However, adding a categorical variable ������year������ to this base model significantly improved model fit ( 2 35.49, df 6, P 0.001) indicating additional unexplained interannual vari- ation. We therefore explored other covariates that might influ- ence M. bovis infection prevalence in badgers. The national incidence of cattle TB was rising throughout the period of the RBCT (Fig. 1b). We therefore hypothesized that M. bovis infection prevalence in badgers might also be increasing. However, a simple linear temporal trend, although significant ( 2 14.46, df 1, P 0.001), left much of the observed interannual variation in prevalence unexplained ( 2 21.03, df 5, P 0.001). The delayed removal of TB-affected cattle caused by the FMD epidemic was associated with a significant rise in M. bovis prevalence in badgers. For each trial area, adding the median interval between the previous year���s cattle tests significantly improved the fit of the base model ( 2 25.38, df 1, P 0.001), with a 1-year delay in cattle testing associated with a 2-fold increase in badger prevalence (odds ratio 2.00, 95% confidence interval 1.52���2.61). Adding this continuous vari- able explained most of the observed interannual variation ( 2 10.11, df 5, P 0.072). Because the intertest interval was markedly higher in 2001 (the year of the FMD epidemic) than at other times (Fig. 1a), the effect of suspended cattle testing could also be represented as a binary variable, distinguishing prevalence in 2002 (after the FMD epidemic) from other years. The fit of the base model was significantly improved by adding this binary FMD variable ( 2 31.94, df 1, P 0.001 odds ratio 1.86, 95% confidence interval 1.50���2.31), leaving no interannual variation in prevalence unexplained ( 2 3.55, df 5, P 0.62). This binary FMD variable was preferred over the continuous intertest interval variable because it could be used consistently across multiple data sets (see supporting informa- tion). The FMD-associated increase in M. bovis infection prev- alence in badgers was observed consistently across trial areas enrolled in the RBCT at the time (Fig. 1c). Adding the previous year���s NAO index improved the fit of the base model ( 2 9.32, df 1, P 0.002 odds ratio 0.48, 95% confidence interval 0.30���0.77) but left much of the observed interannual variation in prevalence unexplained ( 2 26.17, df 5, P 0.001). Adding the FMD variable improved the fit of the base NAO model ( 2 23.89, df 1, P 0.001), but adding NAO did not improve the fit of the base FMD model ( 2 1.27, df 1, P 0.26). These analyses indicate that the FMD variable provided a superior explanation for the interan- nual variation observed. Adding the FMD variable did not affect the trend of increasing prevalence on successive culls. Moreover, adding a linear ������year������ variable to the base FMD model did not improve model fit ( 2 0.12, df 1, P 0.72) and did not affect the increasing prevalence trend associated with repeated culling, suggesting that the changes observed were not due to an underlying gradual increase in prevalence. Model fit was further improved by adding terms for the interactions between repeated culling and the permeability of 1998 1999 2000 2001 2002 2003 2004 2005 0 0.05 0.1 0.15 0.2 0.25 Prevalence Year observed ��95% CI fitted a c b d 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. 14714 www.pnas.org cgi doi 10.1073 pnas.0606251103 Woodroffe et al.