Viewpoint
Viewpoint
Building trust on bovine TB Opinion on how best to control bovine TB remains divided, particularly with regard to badgers. Rosie Woodroffe believes that vets have a constructive role to play in the debate and helping farmers locally Bovine tuberculosis (TB) is a major problem for Britain’s farmers, causing financial ruin to some and significant stress to many. On top of these already-serious concerns, in some parts of Britain, TB is dividing rural communities. Understandably desperate to find a solution to this terrible problem, some farmers seek to cull badgers. At the same time, some other farmers, some non-farming rural residents, and many outsiders vehemently oppose this approach. The media furore surrounding this issue usually emphasises disagreement between farmers and animal rights advocates; the views of practising vets are seldom aired. This is unfortunate, because local vets should be perfectly placed to lead local efforts at TB control. Desperate farmers may (rightly [Donnelly and others 2006]) view badgers as a source of TB and may feel frustration that their ability to control badger numbers is curtailed by legal protection. Local animallovers may claim (again, rightly [Jenkins and others 2007]) that most badgers are healthy; they may therefore perceive badgers as innocent scapegoats. But practising vets should have the insight needed to view such evidence in its entirety, to understand its important nuances, and to help rural communities to reach sustainable solutions to this problem. In fact, local vets may find themselves playing an increasingly important role – both formally and informally – in shaping local TB control efforts. In England, Defra ministers are ‘minded’ to roll out badger culling; but local farmers will need to decide for themselves whether to pursue (and pay for) this option. At the same time, badger vaccination is now widely available, and Rosie Woodroffe is a senior research fellow at the Zoological Society of London, and a visiting professor at Imperial College, London. She was the ecologist on the Independent Scientific Group on Cattle TB from 1998 to 2007, and continues research on TB transmission between badgers and cattle Rosie Woodroffe Institute of Zoology, Regent’s Park, London NW1 4RY e-mail: [email protected] 254 | Veterinary Record | March 8, 2014
new biosecurity tools, such as badger-proof troughs and mineral licks, are also being developed. Local vets’ opinions on the utility of such approaches can and should influence farmers’ decisions about how and whether to adopt them. As well as having the expertise to evaluate these tools, practising vets
‘Vets could play a pivotal role building bridges between wildlife groups offering badger vaccination, and farmers who might benefit from such help’ typically have the trust of local farmers, an invaluable commodity when public debate about TB control has become so polarised that approaches promoted by lobby groups may be viewed with suspicion. Useful freely available resources for vets wanting to become better informed about a contentious and fast-moving issue include a recent objective review of the evidence base for TB control (Godfray and others 2013), and presentations from a recent symposium on TB vaccination (ZSL 2013). As farmers decide how to manage TB transmission from badgers to cattle, the following five points may help vets to advise them: n Badgers do give TB to cattle – but herd-to-herd transmission should not be forgotten There can be no reasonable doubt that badgers help to maintain TB infection in
cattle. A recent analysis (Donnelly and Nouvellet 2013) suggests that badgers are ultimately responsible for at least 38 per cent – and probably around 50 per cent – of confirmed TB breakdowns in high-incidence areas. However, the same analysis suggests that direct transmission from badgers to cattle accounts for less than 25 per cent – and probably only 6 per cent – of these breakdowns. The rest of the impact represents subsequent local spread of infection from herd to herd. These figures highlight the importance of limiting transmission among cattle herds. While managing badgers may have benefits for TB control, such benefits would be greatly improved by renewed efforts to control cattle-to-cattle transmission in the same areas. Indeed, such efforts might also reduce transmission from cattle to badgers (Woodroffe and others 2006), with knock-on benefits for transmission back to cattle. n Badger behaviour is critically important to TB transmission In south-west England, most farms contain one or two badger territories. Left to themselves, badgers seldom leave these territories, interacting mainly with members of their own social group (Weber and others 2006). This behaviour pattern helps to stop disease spread from farm to farm. Unfortunately, culling disrupts such behaviour, so that each remaining badger ranges across more farms, and is more likely to be infected, than before culling (Godfray and others 2013). These changes help to explain why considerable reductions in badger numbers do not prompt equivalent reductions in cattle TB. n Small-scale badger culling makes TB worse Recognising that badgers can give TB to cattle, farmers may be tempted to conduct their own ‘unofficial’ badger culls. However, there is very strong evidence to show that small-scale culls increase cattle TB rather than reducing it (Godfray and others 2013). Aside from the fact
Viewpoint that it’s illegal, simple self-interest and neighbourliness should be enough to steer farmers away from conducting their own small-scale badger culls. n Large-scale licensed badger culling has uncertain benefits The criteria for granting culling licences have been carefully constructed with the aim of delivering overall reductions in cattle TB (Defra 2011a). To do this, they require killing a very high proportion of badgers, very rapidly, over very large areas. If these criteria can be met, the anticipated ‘overall reductions’ entail winners and losers, with farmers inside the cull zones expected to see somewhat fewer breakdowns, while farmers on adjoining unculled land may experience more TB than they otherwise would (Donnelly and others 2006). In practice, the recent pilot culls in Somerset and Gloucestershire highlight the challenges of meeting the licensing conditions. Problems with killing enough badgers, fast enough, on enough of the land, are likely to have reduced the benefits of culling. At the same time, increased use of cage trapping elevated the costs to farmers, while intense public protest led to substantial policing costs for government. Before the pilots, the Government predicted that culling would cost farmers more, financially, than they would save in reduced TB (Defra 2011b). Further reduced benefits and increased costs may make culling less and less appealing to farmers. n Badger vaccination is promising, but untested The effects of badger vaccination on cattle TB are currently unknown. However, vaccination markedly reduces
badger-to-badger transmission (Carter and others 2012), allowing optimism that badger-to-cattle transmission could also be reduced. Although vaccination does not remove infected animals, background mortality rates of 25 to 30 per cent suggest that most animals already infected at the start of vaccination will die off over the course of three to four years. Crucially, because badgers’ natural territorial behaviour limits disease transmission across the landscape, small-scale vaccination efforts might prove beneficial even though small-scale culling is harmful. Currently, vaccinating badgers requires cage trapping and injecting them. Government policy recognises the high cost of trapping and therefore emphasises development of an oral vaccine. But, wildlife groups are increasingly coming forward to offer badger vaccination at low or even no cost to farmers. A significant challenge is trust: fear of anti-culling protest makes some farmers reluctant to engage with wildlife groups, undermining efforts to roll out (or indeed thoroughly evaluate) vaccination. Vets could play a pivotal role building bridges between wildlife groups offering badger vaccination, and farmers who might benefit from such help. For decades, rural vets have been foot soldiers in the fight against TB. As new information and management tools become available, practising vets can be expected to take a much more proactive role in guiding local efforts at TB control. Meeting this challenge may require continuing education to help vets to keep abreast of a complex and dynamic area of epidemiology – but offers the opportunity of practical, locally appropriate and sustainable approaches to a serious problem.
References
Carter, S. P., Chambers, M. A., Rushton, S. P., Shirley, M. D. F., Schuchert, P., Pietravalle, S. & OTHERS (2012) BCG vaccination reduces risk of tuberculosis infection in vaccinated badgers and unvaccinated badger cubs. PLoS One 7, e49833 DEFRA (2011a) The government’s policy on bovine TB and badger control in England. www.gov.uk/government/publications/the-government-s-policy-on-bovinetb-and-badger-control-in-england Accessed March 3, 2014 DEFRA (2011b) Impact assessment: Measures to address bovine TB in badgers. http://archive.defra.gov.uk/foodfarm/farmanimal/diseases/atoz/tb/documents/bovine-tbimpact-assessment.pdf Accessed March 3, 2014 Donnelly, C. A. & Nouvellet, P. (2013) The contribution of badgers to confirmed tuberculosis in cattle in high-incidence areas in England. PLoS Currents Outbreaks doi: 10.1371/currents.outbreaks.097a904d3f3 619db2fe78d24bc776098 Donnelly, C. A., Woodroffe, R., Cox, D. R., Bourne, F. J., Cheeseman, C. L., CliftonHadley, R. S. & OTHERS (2006) Positive and negative effects of widespread badger culling on tuberculosis in cattle. Nature 439, 843-846 Godfray, H. C. J., Donnelly, C. A., Kao, R. R., Macdonald, D. W., McDonald, R. A., Petrokofsky, G. & OTHERS (2013) A restatement of the natural science evidence base relevant to the control of bovine tuberculosis in Great Britain. Proceedings of the Royal Society B-Biological Sciences 280, 20131634 Jenkins, H. E., Morrison, W. I., Cox, D. R., Donnelly, C. A., Johnston, W. T., Bourne, F. J. & OTHERS (2007) The prevalence, distribution and severity of detectable pathological lesions in badgers naturally infected with Mycobacterium bovis. Epidemiology and Infection 136, 1350-1361 Weber, N., Carter, S. P., Dall, S. R. X., Delahay, R. J., McDonald, J. L., Bearhop, S. & McDonald, R. A. (2013) Badger social networks correlate with tuberculosis infection. Current Biology 23, R915-R916 Woodroffe, R., Donnelly, C. A., Jenkins, H. E., Johnston, W. T., Cox, D. R., Bourne, F. J. & OTHERS (2006) Culling and cattle controls influence tuberculosis risk for badgers. Proceedings of the National Academy of Sciences of the United States of America 103, 14713-14717 ZSL (2013) Vaccination in the control of bovine TB. www.zsl.org/science/events/vaccination-in-the-controlof-bovine-tb,773,EV.html Accessed January 13, 2014
doi: 10.1136/vr.g1811
March 8, 2014 | Veterinary Record | 255
Viewpoint
Building trust on bovine TB Opinion on how best to control bovine TB remains divided, particularly with regard to badgers. Rosie Woodroffe believes that vets have a constructive role to play in the debate and helping farmers locally Bovine tuberculosis (TB) is a major problem for Britain’s farmers, causing financial ruin to some and significant stress to many. On top of these already-serious concerns, in some parts of Britain, TB is dividing rural communities. Understandably desperate to find a solution to this terrible problem, some farmers seek to cull badgers. At the same time, some other farmers, some non-farming rural residents, and many outsiders vehemently oppose this approach. The media furore surrounding this issue usually emphasises disagreement between farmers and animal rights advocates; the views of practising vets are seldom aired. This is unfortunate, because local vets should be perfectly placed to lead local efforts at TB control. Desperate farmers may (rightly [Donnelly and others 2006]) view badgers as a source of TB and may feel frustration that their ability to control badger numbers is curtailed by legal protection. Local animallovers may claim (again, rightly [Jenkins and others 2007]) that most badgers are healthy; they may therefore perceive badgers as innocent scapegoats. But practising vets should have the insight needed to view such evidence in its entirety, to understand its important nuances, and to help rural communities to reach sustainable solutions to this problem. In fact, local vets may find themselves playing an increasingly important role – both formally and informally – in shaping local TB control efforts. In England, Defra ministers are ‘minded’ to roll out badger culling; but local farmers will need to decide for themselves whether to pursue (and pay for) this option. At the same time, badger vaccination is now widely available, and Rosie Woodroffe is a senior research fellow at the Zoological Society of London, and a visiting professor at Imperial College, London. She was the ecologist on the Independent Scientific Group on Cattle TB from 1998 to 2007, and continues research on TB transmission between badgers and cattle Rosie Woodroffe Institute of Zoology, Regent’s Park, London NW1 4RY e-mail: [email protected] 254 | Veterinary Record | March 8, 2014
new biosecurity tools, such as badger-proof troughs and mineral licks, are also being developed. Local vets’ opinions on the utility of such approaches can and should influence farmers’ decisions about how and whether to adopt them. As well as having the expertise to evaluate these tools, practising vets
‘Vets could play a pivotal role building bridges between wildlife groups offering badger vaccination, and farmers who might benefit from such help’ typically have the trust of local farmers, an invaluable commodity when public debate about TB control has become so polarised that approaches promoted by lobby groups may be viewed with suspicion. Useful freely available resources for vets wanting to become better informed about a contentious and fast-moving issue include a recent objective review of the evidence base for TB control (Godfray and others 2013), and presentations from a recent symposium on TB vaccination (ZSL 2013). As farmers decide how to manage TB transmission from badgers to cattle, the following five points may help vets to advise them: n Badgers do give TB to cattle – but herd-to-herd transmission should not be forgotten There can be no reasonable doubt that badgers help to maintain TB infection in
cattle. A recent analysis (Donnelly and Nouvellet 2013) suggests that badgers are ultimately responsible for at least 38 per cent – and probably around 50 per cent – of confirmed TB breakdowns in high-incidence areas. However, the same analysis suggests that direct transmission from badgers to cattle accounts for less than 25 per cent – and probably only 6 per cent – of these breakdowns. The rest of the impact represents subsequent local spread of infection from herd to herd. These figures highlight the importance of limiting transmission among cattle herds. While managing badgers may have benefits for TB control, such benefits would be greatly improved by renewed efforts to control cattle-to-cattle transmission in the same areas. Indeed, such efforts might also reduce transmission from cattle to badgers (Woodroffe and others 2006), with knock-on benefits for transmission back to cattle. n Badger behaviour is critically important to TB transmission In south-west England, most farms contain one or two badger territories. Left to themselves, badgers seldom leave these territories, interacting mainly with members of their own social group (Weber and others 2006). This behaviour pattern helps to stop disease spread from farm to farm. Unfortunately, culling disrupts such behaviour, so that each remaining badger ranges across more farms, and is more likely to be infected, than before culling (Godfray and others 2013). These changes help to explain why considerable reductions in badger numbers do not prompt equivalent reductions in cattle TB. n Small-scale badger culling makes TB worse Recognising that badgers can give TB to cattle, farmers may be tempted to conduct their own ‘unofficial’ badger culls. However, there is very strong evidence to show that small-scale culls increase cattle TB rather than reducing it (Godfray and others 2013). Aside from the fact
Viewpoint that it’s illegal, simple self-interest and neighbourliness should be enough to steer farmers away from conducting their own small-scale badger culls. n Large-scale licensed badger culling has uncertain benefits The criteria for granting culling licences have been carefully constructed with the aim of delivering overall reductions in cattle TB (Defra 2011a). To do this, they require killing a very high proportion of badgers, very rapidly, over very large areas. If these criteria can be met, the anticipated ‘overall reductions’ entail winners and losers, with farmers inside the cull zones expected to see somewhat fewer breakdowns, while farmers on adjoining unculled land may experience more TB than they otherwise would (Donnelly and others 2006). In practice, the recent pilot culls in Somerset and Gloucestershire highlight the challenges of meeting the licensing conditions. Problems with killing enough badgers, fast enough, on enough of the land, are likely to have reduced the benefits of culling. At the same time, increased use of cage trapping elevated the costs to farmers, while intense public protest led to substantial policing costs for government. Before the pilots, the Government predicted that culling would cost farmers more, financially, than they would save in reduced TB (Defra 2011b). Further reduced benefits and increased costs may make culling less and less appealing to farmers. n Badger vaccination is promising, but untested The effects of badger vaccination on cattle TB are currently unknown. However, vaccination markedly reduces
badger-to-badger transmission (Carter and others 2012), allowing optimism that badger-to-cattle transmission could also be reduced. Although vaccination does not remove infected animals, background mortality rates of 25 to 30 per cent suggest that most animals already infected at the start of vaccination will die off over the course of three to four years. Crucially, because badgers’ natural territorial behaviour limits disease transmission across the landscape, small-scale vaccination efforts might prove beneficial even though small-scale culling is harmful. Currently, vaccinating badgers requires cage trapping and injecting them. Government policy recognises the high cost of trapping and therefore emphasises development of an oral vaccine. But, wildlife groups are increasingly coming forward to offer badger vaccination at low or even no cost to farmers. A significant challenge is trust: fear of anti-culling protest makes some farmers reluctant to engage with wildlife groups, undermining efforts to roll out (or indeed thoroughly evaluate) vaccination. Vets could play a pivotal role building bridges between wildlife groups offering badger vaccination, and farmers who might benefit from such help. For decades, rural vets have been foot soldiers in the fight against TB. As new information and management tools become available, practising vets can be expected to take a much more proactive role in guiding local efforts at TB control. Meeting this challenge may require continuing education to help vets to keep abreast of a complex and dynamic area of epidemiology – but offers the opportunity of practical, locally appropriate and sustainable approaches to a serious problem.
References
Carter, S. P., Chambers, M. A., Rushton, S. P., Shirley, M. D. F., Schuchert, P., Pietravalle, S. & OTHERS (2012) BCG vaccination reduces risk of tuberculosis infection in vaccinated badgers and unvaccinated badger cubs. PLoS One 7, e49833 DEFRA (2011a) The government’s policy on bovine TB and badger control in England. www.gov.uk/government/publications/the-government-s-policy-on-bovinetb-and-badger-control-in-england Accessed March 3, 2014 DEFRA (2011b) Impact assessment: Measures to address bovine TB in badgers. http://archive.defra.gov.uk/foodfarm/farmanimal/diseases/atoz/tb/documents/bovine-tbimpact-assessment.pdf Accessed March 3, 2014 Donnelly, C. A. & Nouvellet, P. (2013) The contribution of badgers to confirmed tuberculosis in cattle in high-incidence areas in England. PLoS Currents Outbreaks doi: 10.1371/currents.outbreaks.097a904d3f3 619db2fe78d24bc776098 Donnelly, C. A., Woodroffe, R., Cox, D. R., Bourne, F. J., Cheeseman, C. L., CliftonHadley, R. S. & OTHERS (2006) Positive and negative effects of widespread badger culling on tuberculosis in cattle. Nature 439, 843-846 Godfray, H. C. J., Donnelly, C. A., Kao, R. R., Macdonald, D. W., McDonald, R. A., Petrokofsky, G. & OTHERS (2013) A restatement of the natural science evidence base relevant to the control of bovine tuberculosis in Great Britain. Proceedings of the Royal Society B-Biological Sciences 280, 20131634 Jenkins, H. E., Morrison, W. I., Cox, D. R., Donnelly, C. A., Johnston, W. T., Bourne, F. J. & OTHERS (2007) The prevalence, distribution and severity of detectable pathological lesions in badgers naturally infected with Mycobacterium bovis. Epidemiology and Infection 136, 1350-1361 Weber, N., Carter, S. P., Dall, S. R. X., Delahay, R. J., McDonald, J. L., Bearhop, S. & McDonald, R. A. (2013) Badger social networks correlate with tuberculosis infection. Current Biology 23, R915-R916 Woodroffe, R., Donnelly, C. A., Jenkins, H. E., Johnston, W. T., Cox, D. R., Bourne, F. J. & OTHERS (2006) Culling and cattle controls influence tuberculosis risk for badgers. Proceedings of the National Academy of Sciences of the United States of America 103, 14713-14717 ZSL (2013) Vaccination in the control of bovine TB. www.zsl.org/science/events/vaccination-in-the-controlof-bovine-tb,773,EV.html Accessed January 13, 2014
doi: 10.1136/vr.g1811
March 8, 2014 | Veterinary Record | 255
