Dear Mr Thomas (evanr),

The relationship between badger density and cattle TB incidence is not a simple one. Elsewhere, we have shown that the incidence of cattle TB in RBCT areas just before culling commenced was much more closely related to the proportion of culled badgers found to be M. bovis infected than to the numbers of badgers culled, a measure of the numbers of badgers living in the area [1]. Moreover, although the reductions in badger numbers that occurred inside RBCT proactive culling areas were associated with reductions in cattle TB incidence, badger numbers were also somewhat reduced on adjoining unculled lands, and in nearby areas subjected to localised (reactive) culling [2], yet these areas experienced increases in cattle TB [3,4]. Among badgers, we found that the prevalence of infection was highest in areas of low badger density [5]. All of these findings point to a complex relationship between the density of badgers and the incidence of cattle TB, probably mediated by badgers’ changing behaviour at different densities which alters transmission of disease both among badgers and from badgers to cattle.


References
1. Donnelly CA, Hone J (2010) Is there an association between levels of bovine tuberculosis in cattle herds and badgers? Statistical Communications in Infectious Diseases 2: 3.
2. Woodroffe R, Gilks P, Johnston WT, Le Fevre AM, Cox DR, et al. (2008) Effects of culling on badger abundance: implications for tuberculosis control. Journal of Zoology 274: 28-37.
3. Donnelly CA, Woodroffe R, Cox DR, Bourne FJ, Cheeseman CL, et al. (2006) Positive and negative effects of widespread badger culling on cattle tuberculosis. Nature 439: 843-846.
4. Donnelly CA, Woodroffe R, Cox DR, Bourne J, Gettinby G, et al. (2003) Impact of localized badger culling on TB incidence in British cattle. Nature 426: 834-837.
5. Woodroffe R, Donnelly CA, Wei G, Cox DR, Bourne FJ, et al. (2009) Social group size affects Mycobacterium bovis infection in European badgers (Meles meles). Journal of Animal Ecology.

Christl A. Donnelly1*, Helen E. Jenkins2, Rosie Woodroffe3
1 Department of Infectious Disease Epidemiology, Imperial College London, London, United Kingdom
2 Brigham and Women's Hospital and Harvard Medical School, Boston, USA
3 Institute of Zoology, London, United Kingdom

* E-mail: c.donnelly@imperial.ac.uk