Science & Environment

Badger group disruption aids TB flow

Image caption Badgers live in extended social groups

Badgers with TB spread the infection more easily to other badgers and to cattle when social groups are disturbed, a new study confirms.

Vaccination has the potential to reduce the spread of infection without disturbing local populations, scientists report in Current Biology.

The findings help explain why culling badgers can cause TB infection in cattle to rise, as infected badgers roam into new territory, they say.

Pilot culls are underway in England.

Badgers are being culled in west Somerset; with a decision due shortly on whether to extend a pilot cull in west Gloucestershire.

In the study, researchers at the University of Exeter and the National Wildlife Management Centre at Woodchester Park, Gloucestershire, fitted 51 badgers in eight different social groups with electronic collars.

They monitored the range of the wild badgers and their social interactions over the course of a year.

The researchers were able to build a detailed picture of the social network of a wild population of badgers for the first time.

We think of badgers as living in tight knit social groups and "sleeping in a big heap underground", but there is more complexity to it, said Prof Robbie McDonald of the University of Exeter, who led the study.

"What our study shows is that social structure is very important for the transmission of infection in wild badgers," he told BBC News.

"It also suggests that social stability is a good way of mitigating disease spread. So if you can intervene in a system and maintain social stability that's likely to be the best of both worlds."

Vaccination does not disrupt social structure, he said, suggesting the social network in badgers - where relatively few individuals might be responsible for disease spread - lends itself to vaccination and could lead quite rapidly to herd immunity.

"If you are trying to control disease, it may be advantageous not to disrupt social structure and so you could disrupt flow through those sorts of individuals by using vaccination," Prof McDonald added.

"But of course vaccination doesn't do any good to an animal that's already infected."

Policy divide

A 9-year trial found that culling badgers could reduce infection in cattle by 12-16% (after four years of culling and five years of follow-up).

But if too few badgers are killed over too long a time period, TB infections in cattle can rise as badgers roam further afield to establish new social groups.

Prof Rosie Woodroffe of the Institute of Zoology, London, said the study provided clarity about how badgers' social behaviour helps to constrain the spread of TB.

"Genetic studies in the Randomised Badger Culling Trial found that culling increased badgers' movements, with especially marked increases for infected badgers," she said.

"This new study helps to understand how such behavioural changes could prompt the major increases in disease prevalence which we saw as a result of culling.

"The authors are right to highlight that their work shows how the benefits of badger vaccination are likely to be enhanced by badgers' social behaviour, because badger social networks are small and local, so vaccinating small numbers of badgers could still have benefits at a local scale. In contrast, culling disrupts badger social behaviour and spreads disease across the landscape."

The Welsh government has chosen to vaccinate badgers in an effort to control TB in cattle, while the government policy in England is to cull badgers in high TB areas.

Around 28,000 cattle were slaughtered in 2012 at a cost of £100m to taxpayers. Ministers and the NFU say culling is needed to fight bovine TB, but animal welfare and wildlife groups say scientific evidence does not support the policy of shooting badgers in an attempt to control the disease.

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