A study's mixed messages on the benefits of culling badgers to control tuberculosis (TB) infection among cattle prompted the British government to ask for public comment on the subject. In response to this open consultation, which ends 10 March, the Royal Society for the Prevention of Cruelty to Animals has campaigned to protect the badgers, whereas farmers' groups say a cull is critical.

Christl Donnelly, lead author of the study in question, which appears on page 843 of this issue, is not new to controversy. Since joining the infectious disease epidemiology group at the University of Oxford in 1995, which moved to Imperial College London five years later, the Harvard-trained statistician has analysed the spread of mad-cow and foot-and-mouth disease, and the SARS epidemic. “I enjoy giving scientific advice to policy-makers,” she says. “But it is also very stressful.”

Bovine TB is endemic in some parts of Britain and has been increasing by 18% a year. A control programme has been in place for some years, including trapping and killing badgers, which are the main reservoir of the disease in the wild. But it was never clear whether this practice was helpful.

Donnelly became involved in bovine TB in 1997, as a member of the Krebs committee charged with reviewing existing data on culling. After its review, the committee recommended a large randomized trial of badger-culling strategies on TB incidence, which began in 1998. The study found that widespread removal of badgers in an affected area cut TB in cattle by about 20%, a finding in line with two earlier studies. But, when the scientists decided to look at outlying regions, they found a different pattern altogether.

In a complementary study within trial areas, led by Rosie Woodroffe of the University of California, Davis, peanuts containing coloured beads were deposited outside the badgers' setts, and the researchers monitored where the beads reappeared. The method, known as bait marking, revealed that the badgers that were left after widespread culling wandered farther from their dens than before. The same was true for badgers at the outskirts of an area where the culling took place. “In the periphery there was little reduction in badger density, yet they still felt the effects of the culling,” says Donnelly. “Because of this evidence of disruption in behaviour, we decided to see what happened to TB incidence in those areas.”

This study — the first to look at areas next to those from which badgers were removed — found that in neighbouring areas 30% more herds became infected with TB, presumably as a result of the badgers' wider range of movement. “Each individual herd has the potential to come into contact with more badgers,” explains Donnelly.

Because badgers are protected in Britain, the culling was done by government employees under crown immunity and with permission from the farmers. Donnelly went into the field to observe the trial implementation. “I thought I should be directly aware of what was going on, rather than giving pronouncements from far away,” she says. But for the most part, she analysed data from her office. “When I tell people I work on SARS and TB they jump back thinking I must be infectious,” she laughs. “But most of what I do is in front of a computer.”