The controversial practice of killing wild badgers to prevent tuberculosis in cattle is unlikely to succeed, according to a new study led by Rosie Woodroffe, an ecologist at the University of California, Davis, and a member of Britain's Independent Scientific Group on Cattle TB.
In Britain, farming interests and badger protectionists have battled for 30 years over the merits of culling badgers to fight cattle tuberculosis, a disease which can occasionally be transmitted to people. Farming ministers are currently deciding whether culling should be continued, following a public consultation on the issue which provoked over 47,000 responses.
Woodroffe's report, published Oct. 2 in the Proceedings of the National Academy of Sciences of the USA, highlights the limitations of badger culling as a control strategy for cattle TB.
Woodroffe examined the outcomes when badgers were culled as part of a seven-year experiment conducted by the British Department for Environment, Food and Rural Affairs. Conventional wisdom suggested that this should reduce transmission among badgers, as well as from badgers to cattle. Instead, after four years of culling, infection rates in badger had doubled.
"Culling disrupts badgers' territorial behavior so they travel further and interact with more other badgers; that is probably why disease spreads more rapidly," said Woodroffe, the leader of a team which also included researchers from Britain's Central Science Laboratory and Veterinary Laboratories Agency. The biggest increases in infection rates happened in culling areas that could most easily be repopulated by badgers migrating in from neighboring land.
Today's report also provides the first evidence of widespread TB transmission from cattle to badgers. Woodroffe's team found that in 2001, when a nationwide epidemic of foot-and-mouth disease in England temporarily halted routine TB testing of cattle, infection rates in badgers doubled. At this time, infected cattle remained in their herds and the researchers suggest that this allowed them to transmit the disease to other cattle and to badgers.
"Our findings helps to explain why badger culling has had limited success as a TB control strategy," noted Woodroffe.
An earlier study by the same research team showed that badger culling reduced TB infection in cattle by just 19 percent inside the culling areas, and actually increased cattle TB on neighboring farms.
"In the past, policymakers assumed that infected badgers were the source of most cattle TB cases, and could be 'cleaned out' by culling," Woodroffe said. "But our results show not only that culling can increase TB in badger populations, but also that cattle themselves play an important role in maintaining the infection."
The paper, "Culling and cattle controls influence tuberculosis risk for badgers," appears online today at PNAS Early Edition, http://www.pnas.org/.
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