Bovine tuberculosis (bTB) is a disease in cattle that has a serious financial impact on farmers in Britain, as infected animals have to be slaughtered. In 2008, 2,738 herds were infected with bTB, costing the government over £100 million. Wild badgers can become infected with bTB and are known to transmit the infection to cattle. Because of this, UK governments have tested various means of badger culling to control bTB infection in cattle over the past 30 years.
The Secretary of State for Environment decided against badger culling to control cattle TB in England in 2008. However, the Welsh Assembly Government now proposes to implement a badger cull using methods very similar to those used in the culling trial, though it faces a legal challenge to this proposal.
The researchers behind the new study analysed data from the Randomised Badger Culling Trial (RBCT), a large-scale field trial that was undertaken in 1998 by Defra to assess the effectiveness of badger culling. The results showed that although the incidence of cattle bTB reduced during culling and in the first years after the final cull, these reductions subsequently declined. The benefits were undetectable within four years after the final cull.
Professor Christl Donnelly, senior author of the study from the MRC Centre for Outbreak Analysis and Modelling at Imperial College London, said: "Bovine TB is a big problem in Britain and the disease can profoundly affect farmers' livelihoods. We know that bTB is transmitted between cattle and badgers, so the Randomised Badger Culling Trial was set up to find out if culling badgers would help control the spread of the disease. There has been some controversy over badger culling as a bTB control method and it has been unpopular with the general public.
"Although badger culling reduced cattle bTB during the trial and immediately thereafter, our new study shows that the beneficial effects are not sustained, disappearing four years post-cull. Our new research also suggests that the savings that farmers and the government would make by reducing bTB infections in cattle are two or three times less than the cost of repeated badger culls as undertaken in the trial, so this is not a cost-effective contribution to preventing bTB infections in cattle," added Professor Donnelly.
In the RBCT, ten areas of 100 square kilometres were subjected to badger culling, and compared to ten similar areas with no culling. Culls were repeated annually and ended in October 2005. Previous analyses have shown that during the cull, bTB incidence in cattle within the cull zones decreased, whereas disease incidence in cattle outside cull zones increased, offsetting the benefit.
Today's study shows that after the culling finished, the number of infected herds inside cull areas was on average 37.6% lower than the number of infected herds in non-cull areas. The results also show that this benefit diminished over time after the culling ended, by 14.3% every six months. By months 43-48 after the final cull, there was no remaining beneficial effect. The research also shows that since the culling finished, the number of infected herds in two kilometre zones outside cull areas was comparable to the number of infected herds in areas outside non-cull areas.
The researchers also analysed the financial costs and benefits of badger culling. Over the seven and a half years during which five annual culls would have detectable benefits on the incidence of cattle bTB, culling in an area of 150 square kilometres would be expected to prevent the infection of 22.6 herds of cattle. The average cost of an infected herd has been estimated to be £27,000, meaning badger culling would save £610,200. However, the cost of a badger cull over a 150 square kilometre area would be between £1.35 million and £2.14 million, using cage trapping, snaring or gassing.