In response to an escalating outbreak of bovine tuberculosis, the U.K. government announced today that it will expand cattle testing and open a 12-week public consultation on badger culling. The announcement coincides with a new nation-wide study that finds that although badgers can spread the disease to cows, culling in some cases makes the problem worse.
Bovine tuberculosis (TB) was a major problem in the first half of the last century in the United Kingdom, infecting up to 40% of dairy cows in the 1930s. About 2000 humans died per year from bovine TB, largely from drinking unpasteurized milk. A policy of testing and slaughtering of infected animals brought the disease under control by the 1960s, but the number of outbreaks in the last three decades has been rising exponentially, according to a recent government report. The U.K. currently spends about $160 million a year combating the disease.
One approach is to kill European badgers (Meles meles), which can contract bovine TB. But conservationists have argued that transmission to cattle is unproven. In response to these uncertainties, the government set up the random badger culling trial in 1998. Preliminary results published in 2003 showed that culling of badgers on and around an infected farm actually increased the number of new cattle infections.
To understand this counterintuitive result, the researchers have now analyzed data on large-scale, indiscriminate culling over 100 square kilometers. These latest results, published online yesterday by Nature, show a 19% decrease of TB incidence in cattle in cull areas vs. no-cull areas-definitively drawing a causal link between badgers and cattle, the team says.
However, when the researchers looked at farms within 2 kilometers of a cull area, they found a 29% increase in TB incidence relative to farms adjacent to no-cull areas. Although the reason is not yet understood, epidemiologist Christl Donnelly of Imperial College London and her co-authors found that culling tends to make the remaining badgers roam more widely. In a paper in the Journal of Applied Ecology--also released online yesterday--the group proposes that this altered behavior exposes more cattle to badgers.
Farming groups say the results confirm that culling can work, while conservationists say the net benefit is too small to continue the practice. But even culling over a larger area "will not cure the problem, only slow it down," says biologist Timothy Roper of the University of Sussex. He says more cattle testing will help, as will a TB vaccine for badgers, which he says is 5 years away.