TB Dilemma: Badger refugees complicate culling

European badgers can catch and spread the form of tuberculosis that strikes mainly cattle, but farmers and animal enthusiasts have debated whether killing badgers would protect herds. Two studies now reconcile earlier contradictory findings.

New results from a study in Britain suggest that the boundaries of a study area make a difference, says Christl A. Donnelly of Imperial College London. She and her colleagues looked inside 10 100-kilometer-square culling zones where badgers had been removed, regardless of whether or not cattle were infected. There they found a 19 percent reduction in bovine-TB incidence in the cattle. Yet when the researchers looked at land surrounding the culling zones, they saw a 29 percent TB increase. The government-funded study’s results will appear in an upcoming Nature.

A recent test of badger removal in Ireland, which didn’t look beyond the culling zones, found a drop in cattle-TB incidence. However, earlier results from a different part of the British study had looked at areas in which badgers were killed only if a farm had TB infections. These study areas included both those farms and their surroundings. The culling raised TB incidence in each overall area (SN: 11/29/03, p. 349: Available to subscribers at UK halts badger kill after study of TB).

A second new analysis suggests how killing badgers can boost TB around a culling area. Cutting the population upsets badger society, says Rosie Woodroffe of the University of California, Davis. She looked at badgers in the same study areas that were in the new TB-incidence analysis. After a culling, surviving badgers expand their territories and wander far afield, spreading disease, she and her colleagues report in an upcoming Journal of Applied Ecology.

People can become ill from the bovine-TB bacterium, but pasteurizing milk largely eliminates the risk of catching the disease from cattle. Dairy farmers are required to slaughter infected cows.

Many industrialized countries have almost wiped out bovine TB in cattle, but efforts can be foiled by infections persisting in wild animals, says Gary Witmer of the U.S. National Wildlife Research Center in Fort Collins, Colo. In the United States, otherwise-rare bovine TB flares up periodically in cattle in northern Michigan, for example, where white-tailed deer carry the infection. The U.S. badger falls in a different genus from that of the European badger and doesn’t spread the disease.

Woodroffe says the past findings of TB’s spread after culling inspired her study of badger-society disruption. She and her colleagues offered each badger group food laced with a different color of bead and then mapped where the beads ended up. Bead maps showed that surviving badgers redistributed themselves more widely.

Tim Roper of the University of Sussex in Brighton, England, has also studied badgers as TB carriers. He notes that the new measurements show badger culls reducing TB transmission 19 percent at best, leaving cattle-to-cattle transmission as a major problem.

“There’s got to be tightening up of cattle testing,” Roper says.