_New Evidence Suggesting Badger Culling is Counter-Productive Sarah Loftus, 6th January 2012
Badger culling has been considered a means of controlling the spread of disease for 30 years, but is it working?
Photograph by Killianwoods
_ New insights from a study looking at how culling affects the movements of badgers and the spread of bovine tuberculosis amongst them has suggested that culling may in fact help rather than hinder the spread of the disease.
The study, led by Dr Philip Riordan from Oxford University’s Department of Zoology studied the behaviour of badgers in South West England, before and after three culls that occurred there between November 2002 and August 2003, comparing data with that obtained from a control study area in which no culling took place. They classified badgers into social groups; “Removed” if they were the target of culling, “Neighbouring” if they lived immediately adjacent to Removed groups, or “Other” if they were adjacent to Neighbouring groups or beyond, and compared the prevalence of disease within the groups as well as their patterns of movement. The study, published last month in the journal PLoS One, found that although rare, the movement of badgers between groups increased significantly following culling, with badgers tending to move into groups subjected to culling.
Spread of bovine tuberculosis amongst the badgers in the study was monitored by capturing the animals, taking samples of saliva, urine, faeces and pus from wounds or open abscesses and culturing them to detect the presence of Mycobacterium bovis, the bacterium that causes the disease. The prevalence of infection increased across all social groups post-culling, even in the control study area in which no badgers were culled. The biggest increase was seen in Neighbouring groups in the culling area where disease levels increased from 1% to 14%. This increase tended to be greater in groups involved in inter-group movements, whether as donors or receivers. Badger cubs were particularly badly affected with tuberculosis levels rising from 1% to 10.5% after culling. No disease was detected in cubs from the control (non-culling) study area.
Culling as a means to control disease is based on an assumption that spread of the disease is dependent on population density and that there exists a threshold below which the disease can no longer persist. Mycobacterium bovis can infect many species, but since the early 1970s the European badger has been deemed the prime reservoir for bovine tuberculosis in cattle, and has therefore been subject to culling. However, the incidence of the disease has continued to increase. The previous culls were not carried out as scientific studies and so the present study aims to improve our understanding of the effects of badger culling on tuberculosis in cattle.
The “perturbation hypothesis” predicts that culling may affect social behaviour of animals such that the spread of disease is altered, for example by increasing movement of animals between social groups. Since under natural conditions badgers have relatively stable social groups and spread of disease is largely within groups rather than between them, it seems likely that the perturbation hypothesis may well apply in the context of their culling. Although the Oxford group did find evidence of increased movement of badgers between social groups, and a general increase in prevalence of bovine tuberculosis amongst groups involved in inter-group movements, no infection was found in the badgers that moved between groups per se and disease in Neighbouring groups from which the badgers moved also increased. Therefore the authors were unable to show a direct link between inter-group movement and spread of disease. They do however suggest that the low sensitivity of the method of detection of Mycobacterium bovis may be to blame, and that perhaps animals other than those captured for the study may have made inter-group movements and these may have carried disease. They also suggest that the effect of perturbation may act indirectly, for example if the disruption in social structure were to cause stress to the badgers, it may suppress their immune system, making them more prone to disease.
Interestingly, the study showed differences to two previous studies in which it was shown that inter-group movement following culling was predominantly by females, possibly to escape breeding suppression within their original group. The present study showed no sex-specific differences but the authors propose that this may result from the extent of the cull itself; the previous studies were of culls in which virtually all badgers were culled, whereas in the present study, the population was only reduced by 40% so that there were enough individuals left to produce cubs in the next breeding season. It is clear that ecological systems are complex and that this issue is not fully understood.
This study is an example of the importance of having a good understanding of the effects a decision will have before it is made. It is studies such as this that should be used to guide policy making, rather than being used retrospectively to understand why a given policy has failed. When asked to comment on how the study supports the need for evidence based policy making, Dr Riordan said,
"The need for evidence base[d] policy making is clear, but in complex areas such as this, where the evidence can be equivocal, the policy makers have to make difficult judgements. As scientists I think we perhaps sometimes need to reach out a little more to provide interpretations. The danger is that we are then accused of speculation beyond the data, but I think there is currently room for a little more give and take."