BioEd Online

Culling badgers to prevent them spreading tuberculosis (TB) to cattle does work, say researchers trying to solve a long-running controversy in the UK - but only if the culling is aggressive. That's because any remaining badgers around culled areas tend to behave in an erratic fashion, roving around more widely and further spreading disease.

Badgers (Meles meles) are known to be one of the main reservoirs of the bacterium Mycobacterium tuberculosis, which causes TB. But it isn't clear how the bug spreads from these animals. It is possible that badgers contaminate cattle feed when raiding farm supplies, or that curious cows catch the disease by investigating dead badgers in their fields.

Since 1975 the UK government has culled more than 20,000 badgers in an attempt to protect cattle. But the disease has persisted in UK herds, and has even become markedly more common in the last 20 years. Animal welfare groups have campaigned against culling, and scientific studies trying to find out whether it works or not have produced conflicting results. One government-sponsored trial was stopped because it was doing more harm than good (see ' UK halts TB trial') , but studies in Ireland have shown that culling is effective there.

Social disruption

Two new contributions to the debate from scientists funded by the UK Department of Agriculture, Food and Rural Affairs (DEFRA) suggest an explanation for the conflicting results. A paper published online by Nature¹ shows that extensive culling in an area of nearly 100 km2 in western England has reduced TB cases in cattle by nearly 20% compared to a similar area where badgers were left in peace. But in a 2 km-wide ring around the culling zone, TB cases went up by 29%. Work from the same group in the Journal of Applied Ecology² suggests that this is the result of the remaining badgers behaving oddly and travelling further from their normal territories.

So the apparent efficacy of any given culling programme will depend both on the extent of the culling and the area over which the programme is studied.

"Removing badgers from an area works because they do spread TB to cows," says Christl Donnelly of Imperial College, London, an author on both papers. "But it also has negative effects because it disrupts the dynamics of social groups that remain."

Telltale beads

Donnelly and her colleagues, led by Rosie Woodroffe of the University of California, Davis, investigated the effect of culling on badger groups in 13 areas where it was practised. They tracked the badgers by providing them with 'treats' made of treacle, peanuts and coloured plastic beads. After nature had taken its course, the beads showed the researchers where the animals had been.

Badgers live in family-based groups that rarely venture out of their home territory. But the giveaway beads showed they roam further in areas that are culled or if there is culling nearby. "The movement of badgers becomes more chaotic," says Donnelly. "Some badgers appear to wander around aimlessly, and others make more trips outside their territory." She says increased badger traffic means they cross paths with cows more often, so there is more chance TB will spread.

Justifiable killings?

Tim Roper, a behavioural ecologist at the University of Sussex, says the work clarifies why less-intensive culling, such as the trial that was stopped in 2003, does more harm than good. "Before we had only flimsy evidence," he says, "but now I am convinced."

"It's no longer possible for anyone to say badgers don't have anything to do with TB," says Roper. "But a lot of argument will centre around the magnitude of the badgers' effect and whether it is big enough to justify culling."

Farmers' groups and animal-welfare organisations are strongly opposed on the subject of culling. "It's a mixture of scientific, political and economic issues," says Donnelly, and the fact that only strict culling appears to be effective won't make a policy decision any easier. "I certainly don't envy those that have to make a decision; getting across the science to groups with such polarized views is very difficult."