From the research paper, Bovine tuberculosis in cattle: reduced risk on wildlife-friendly farms (pdf) by F Mathews (from the Wildlife Conservation Research Unit, University of Oxford) , L. Lovett, S. Rushton and D. W. Macdonald, published in Biol. Lett. (2006) 2, 271–274 doi:10.1098/rsbl.2006.0461 Published online 7 March 2006
4. DISCUSSIONHabitat management appears important to a farm's bTB risk. 'Nature friendly' management practices - the presence of ungrazed wildlife strips, and the greater availability, width and continuity of hedgerow - are all associated with reduced bTB incidence. The results are unlikely to be artefactual: in contrast to other habitat variables, such as deciduous woodland configuration, the boundary characteristics were retained in high-ranking models after adjustment for herd size and the proximity of the nearest infected herd. Further, the top-ranking model, which included hedgerow availability, had more than twice the support of the model containing just these non-habitat variables. Within-farm habitat characteristics were estimated with some random error in our study (because farm perimeter locations were not known), and the true relationships will therefore tend to be even stronger than those we have observed.
Any of the habitat factors associated with bTB risk locally could operate in conjunction with parameters important at a larger spatial scale, such as climate (Wint et al. 2002) and cattle movements (Gilbert et al. 2005). We, as in some ( Johnston et al. 2005), though not all (Griffin et al. 1993) previous studies, found little evidence for badger density being associated with bTB risk. The extent and configuration of deciduous woodland and the amount of pasture - likely determinants of badger densities - were also of little predictive value, as reported previously (White & Benhin 2004). Nevertheless, better indices, particularly farm-level data on bTB prevalence in badgers, may show stronger associations.
Further work is warranted to establish the mechanism linking habitat to bTB risk. Broadly, habitat could influence cattle contact rates or be associated with agricultural management practices in ways relevant to bTB transmission. For example, there may be different rotational patterns on hedgerow-rich farms that could lower the ingestion of potentially contaminated soil (Healy 1968). Favourable habitat could also reduce badger–cattle Mycobacterium bovis transmission. This may initially appear counter-intuitive, since both cattle and badgers preferentially use hedgerows, the former for grazing (cattle have a strong preference for long swards; Hutchings & Harris 1997), and the latter for commuting routes and latrine sites (Stewart et al. 2001). However, when long forage is readily available, as when hedgerow density is high, cattle markedly avoid grass contaminated by active badger latrines (Hutchings & Harris 1997; for further details on mechanism see electronic supplementary material). Also, only the extremities of hedgerows are grazed, with the interior providing areas where cattle cannot access infected badger faeces and urine. Thus, a greater density of hedgerows provides a greater density of land where badger– cattle contact is prevented. The fact that wildlife strips and a lack of hedgerow gaps - which would both reduce badger–cattle contact rates - were also negative correlates of bTB incidence provides some support for this idea.
The reform of the Common Agricultural Policy has decoupled farm subsidies from production, with increased funding being provided through agrienvironment schemes (DEFRA 2005). The baseline 'Entry Level' Environmental Stewardship Scheme rewards favourable boundary feature management, including hedgerow retention and creation, and the formation of wildlife strips. These habitats are important for wildlife conservation (Macdonald & Johnson 2000). Our work suggests that boundary management may also reduce the risk of bTB in cattle, including financially debilitating repeated breakdowns (see electronic supplementary material). Taking, for simplicity, just one parameter contributing to the hedgerow score - total hedgerow length - an increase of 1 km per 100 ha was associated with a decrease in the odds ratio of bTB of about 12.5% (95% confidence interval: 0.3% increase to 26.3% decrease) in univariate analysis. In absolute terms, this equates to the annual risk of bTB changing from the current rate of 9.2% (2152 confirmed incidents in 23 471 herds in 2004) to 8.1% (1901 incidents) for herds in the West of England: an annual reduction of 251 infected herds. Conversely, there is little evidence that increasing farm woodland area or altering its configuration would adversely affect bTB risk.
Managing zoonotic risks to human and animal health is fundamentally important: virtually all emerging infectious diseases originate in wildlife. Superficially, the simplest method of control is to reduce prevalence in the reservoir host by culling. However, effective reductions in population densities can be difficult to achieve, and may be undesirable for species of conservation concern (as for bat-reservoirs of emerging viruses (Dobson 2005), and British badgers are legally protected). Culling may even be counter-productive: the recent evidence from the RBCT in the UK (Donnelly et al. 2006) supports the contention that social perturbation among surviving badgers can increase local bTB risks (Tuyttens et al. 2000). An alternative, and possibly complementary strategy is to establish the ecological conditions associated with the spill-over of disease and to manage these (Dobson 2005). We studied the multifactorial reality of British farmland ecosystems and found, using recent advances in statistical modelling, a link between farmland habitat management and bTB risk. The collective effects of ecological factors were marked. We conclude that managing the landscape in ways that are also beneficial to conservation generally may provide an additional means of controlling bTB.