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Bovine tuberculosis (TB), caused by Mycobacterium bovis, is a disease imposing substantial costs on Britain's cattle industry. Regular testing of cattle, with slaughter of those testing positive, has successfully controlled the infection across much of the developed world. However, control has not been achieved where wildlife populations have become persistently infected (Morris, Pfeiffer & Jackson 1994). In Britain, failure to control cattle TB has been linked to transmission of infection from badgers Meles meles, a wildlife species that thrives in landscapes where cattle are farmed (Neal & Cheeseman 1996). Badger culling therefore formed a component of British TB control policy for many years (Krebs et al. 1997).
M. bovis infections are likewise clustered within cattle populations (Woodroffe et al. 2005c). Clusters of cattle infection are spatially associated with those in badgers, and this association is particularly marked for animals sharing the same M. bovis strain type (Woodroffe et al. 2005c). These findings suggest that interspecific transmission influences the spatial distribution of M. bovis infection, but are not in themselves sufficient to determine whether badger-to-cattle or cattle-to-badger transmission is most important.
Field studies indicate that all transmission pathways (badger-to-badger, badger-to-cattle, cattle-to-badger and cattle-to-cattle) are important components of TB dynamics in Britain (Gilbert et al. 2005; Donnelly et al. 2006; Woodroffe et al. 2006b). However, badger culling is expected to have different effects on each of these pathways, with various possible outcomes for the clustering of infection within badger and cattle populations (Fig. 1), and for the spatial association between the two.
Figure 1. Predicted effects of badger culling on the spatial distribution of M. bovis infection. In undisturbed populations, badger territoriality limits disease spread among badgers (a) and to cattle (b), constraining the spatial scale of infection clustering. Proactive culling lowered badger density and expanded their ranging; this is predicted to reduce clustering of infection in badgers (c). If badgers remained a major source of cattle infections inside proactive areas, reduced clustering of infection in badgers would be mirrored in cattle (d).
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Within badger populations, culling profoundly disrupts social and territorial organization, leading badgers to range more widely (Woodroffe et al. 2006a). This is likely to increase contact rates between badgers, and may explain marked increases in M. bovis prevalence that have been detected in badger populations subjected to culling (Woodroffe et al. 2006b; R. Woodroffe, C.A. Donnelly, P. Gilks et al., unpublished). Immigration of badgers into culled areas from neighbouring lands appears to contribute to this pattern (Woodroffe et al. 2006b).
If culling influences badger-to-badger transmission of M. bovis by disrupting territorial behaviour and expanding home range sizes, infections within the badger population would be expected to become less clustered in response to culling (Fig. 1c). This is because mixing, and hence transmission, is expected to occur between badgers originating at greater distances from one another, breaking up the clusters observed in undisturbed populations. Further, if the majority of M. bovis infections in cattle were acquired from badgers (Fig. 1b), any reduction in the degree of clustering within badger populations would be expected to cause a corresponding reduction in clustering within cattle populations (Fig. 1d). By contrast, badger culling would not be expected to influence infection clustering in cattle populations if most cattle infections were acquired from other cattle.
If culling-induced social disruption of badger populations influences M. bovis transmission as proposed, culling would be expected to reduce the spatial association observed between infections in cattle and badgers, irrespective of whether this association was generated mainly by badger-to-cattle or by cattle-to-badger transmission. This is because the expanded ranging behaviour observed in badgers in culled areas (Woodroffe et al. 2006a) is likely to allow transmission between the two host species over greater distances. In addition, as culling reduces badger density, it is expected to lower the proportion of cattle infections caused by badgers, and hence should reduce the spatial association between infections in the two hosts.
These predictions indicate that describing the impact of badger culling on the spatial distribution of M. bovis infection will be valuable in designing future strategies for cattle TB control. First, such a description will test the hypothesis that alterations to badger spatial organization can influence the geographical distribution of M. bovis infection. This could explain the capacity of badger culling to increase infection rates in cattle where culling is localised in small areas, and on unculled land adjoining widespread culling areas (Donnelly et al. 2003; Donnelly et al. 2006; Donnelly et al. 2007), and could therefore help to determine whether other culling methods may be devised to avoid these detrimental effects. Second, comparing the effects of badger culling on infection clustering in badgers and cattle might potentially shed light on the importance of badger-to-cattle, cattle-to-badger and cattle-to-cattle transmission. This would help to determine the potential value of future TB management strategies targeted at badgers and cattle.
We investigated the effects of badger culling on the spatial distribution of M. bovis infection in badgers and cattle, using data from the Randomised Badger Culling Trial (RBCT), a large-scale field study of badger culling as a strategy to control cattle TB in high-risk areas of England (Donnelly et al. 2003; Donnelly et al. 2006). Previous analyses (Woodroffe et al. 2005c) considered data collected at the start of the RBCT, when badger populations were comparatively undisturbed by culling. Here, analyses are expanded to consider the effects of repeated culling conducted within the same trial areas. Specifically, we predicted that repeated badger culling would reduce the clustering of M. bovis infections within both the badger and cattle populations, and would reduce the spatial association between infections in badgers and cattle.
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The following supplementary material is available for this article.
Appendix S1. Detailed methods and results of the randomised badger culling trial.
Fig. S1. Locations of proactive culling, reactive culling and no culling areas of the RBCT.
Table S1. Dates of proactive culls by triplet and cull number.
Table S2. Approximate dates of reactive culling, by triplet and badger year.
Table S3. Numbers of badgers included in analyses.
Table S4. Numbers of cattle herds included in analyses presented from proactive areas.
Table S5. Clustering of M. bovis infection within badger populations.
Table S6. Median distances to the nearest infected badger or TB-affected herd, measured from badgers and herds with and without evidence of infection, in proactive areas.
Table S7. Robustness of the linear relationship between cull number and M. bovis clustering in badgers.
Table S8. Effect of lesions on M. bovis clustering within badger populations.
Table S9. Clustering of infection within cattle populations in proactive areas.
Table S10. Clustering of infection within cattle populations in proactive areas, excluding contiguous tests.
Table S11. Analysis of the linear relationship between cull number and M. bovis clustering in cattle inside proactive areas.
Table S12. Clustering of M. bovis infections in cattle in proactive areas, accounting for overlapping observation periods.
Table S13. Effect of lesions on clustering of infections in cattle populations in proactive areas.
Table S14. Clustering of M. bovis infection within cattle populations in survey-only areas.
Table S15. Clustering of M. bovis infection within cattle populations ≤ 2 km outside proactive areas.
Table S16. Clustering of M. bovis infection within cattle populations in reactive areas.
Table S17. Spatial association of M. bovis infection in cattle and badgers in proactive areas.
Table S18. Robustness of the linear relationship between cull number and spatial association of M. bovis infections in cattle and badgers.
Table S19. Spatial association between lesioned badgers in proactive areas and infected cattle.
Table S20. Spatial association of lesioned cattle and infected badgers in proactive areas.
Table S21. Spatial associations of M. bovis spoligoypes within and between badger and cattle populations, on successive proactive badger culls.
Table S22. Median distances to the nearest badger or herd with the same, or a different, spoligotype as the index animal.
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