Dear Mr Thomas (evanr),

The main findings of this paper are two-fold. Inside survey-only and reactive areas, increased numbers of active badger setts within 1500 m of a farm were associated with an increased TB risk. Inside proactive areas, the number of M. bovis positive badgers initially culled within 1500 m of a farm was the strongest predictor of the risk of a confirmed TB breakdown. The density of badger setts and the density of badgers culled were not retained, in our analyses, as significant predictors of the risk of a confirmed TB breakdown.

Previous RBCT publications have demonstrated that the relationship between badger density and cattle TB incidence is not a simple one. For example, 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 [1], yet these areas experienced increases in cattle TB [2,3]. Among badgers, we found that the prevalence of infection was highest in areas of low badger density [4] and in this paper, we show that the incidence of cattle TB in proactive RBCT areas was much more closely related to the number of culled badgers found to be M. bovis infected than to the total number of badgers culled, a measure of the number of badgers living in the area (see also [5]). All of these findings suggest a complex relationship between the density of badgers and the incidence of cattle TB. Culling-induced changes in badger behaviour at different densities alter transmission of disease both among badgers and from badgers to cattle.

Since experimental evidence suggests reduction in badger density can have positive and negative effects on the incidence of TB in cattle herds, reducing the prevalence of M. bovis infection in badgers (such as by effective vaccination of badgers) may be important in reducing TB incidence in cattle herds. There are indications that TB in cattle herds could be substantially reduced, possibly even eliminated, in the absence of transmission from badgers to cattle [5]. These results are based on observational data and a small data set, and therefore provide weaker inference than from a large experimental study. Such an experiment might take the form of monitoring TB incidence among cattle herds in areas randomised to receiving and not receiving badger vaccination, where the magnitude would need to be similar to that of the RBCT (i.e. ten 100km2 areas per randomised ‘treatment’ monitored for 5 years) in order to achieve comparably precise estimates of the effects of badger vaccination on TB incidence in cattle herds [5]. Vaccination is an area of active research for TB control.

References
1. 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.
2. 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.
3. 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.
4. 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.
5. 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.

Flavie Vial1*, W. Thomas Johnston2, Christl A. Donnelly1
1 Department of Infectious Disease Epidemiology, MRC Centre for Outbreak Analysis and Modelling, School of Public Health, Imperial College London, London, United Kingdom
2 Department of Health Sciences, University of York, York, United Kingdom
* E-mail: f.vial@imperial.ac.uk

No competing interests declared.