Culling badgers should be a low priority for curbing cattle tuberculosis, according to a scientist advising the British government.
New research by Dr Rosie Woodroffe and colleagues suggests that culling raises the rate of TB infection in badgers.
It also demonstrates that cattle infect badgers with the bacterium.
Writing in the journal Proceedings of the National Academy of Sciences, the team suggests cattle testing and movement controls are a priority.
"This research has two important conclusions," said Dr Woodroffe, a researcher at the University of California in Davis and a member of the UK government's Independent Scientific Group on Cattle TB.
"It shows for the first time that there is substantial transmission of TB from cattle to badgers"
"The second conclusion is that repeated culling increases the prevalence in badgers - each time you cull, it goes up and up."
The government is currently considering whether to introduce a cull, though a consultation mounted recently by the Department for Environment, Food and Rural Affairs (Defra) suggested public opinion is firmly against such a move.
New analysis
THE KREBS TRIAL
An initial analysis, released last year, showed mixed results for culling. Now scientists have looked again at the data and tried to piece together a formula describing infection rates in both cattle and badgers.
Key to the new findings was the foot-and-mouth disease epidemic of 2001, which brought a suspension of both cattle TB testing and badger culling.
The following year saw a sharp rise in TB prevalence among cattle, which was expected, and badgers, which was not.
"We saw across seven study areas a rise in the badger TB prevalence - almost a doubling," said Dr Woodroffe.
"No other explanation fits the data."
"Those callous vets who have demanded badger killing should hang their sorry heads in shame"
What appears to be happening is that badgers move more freely and more widely in culled areas, increasing contact with each other and with cattle.
Total elimination
Animal welfare groups greeted the study enthusiastically.
"This research confirms beyond doubt that cattle are the main vectors of bovine TB, readily infecting badgers and other cattle," said Trevor Lawson of the Badger Trust.
"The National Farmers Union (NFU) and other farming lobby groups should now have the courage to call a halt to illegal badger killing and to immediately withdraw their unsupported demands for state-sponsored badger culls.
"Those callous vets who have demanded badger killing should hang their sorry heads in shame."
But the NFU contended that culling has a role.
"The evidence is clear; the less you do about TB in badgers, the more infection you get in both cattle and badgers," said spokesman Anthony Gibson.
Recent months have seen the incidence in cattle falling, which is widely attributed to a strengthening of government controls on testing and killing of infected herds.
And this, concluded Dr Woodroffe, is where the emphasis should lie in policymaking.
"In theory, if you could totally eliminate a badger population in an isolated area, you would eliminate one transmission route; though whether this would be feasible or desirable is another matter," she said.
"But improved cattle controls would have to be top of a policymaker's list; and culling - well, I'm not sure that would be on the priority list at all."
Richard.Black-INTERNET@bbc.co.uk
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