Badgers are an important wildlife reservoir for tuberculosis infection, a disease that leads thousands of cattle to slaughter each year. Now, researchers reporting in the Cell Press journal Current Biology on October 21 have found that the spread of the disease is influenced in surprising ways by infected badgers, and especially by the details of their social lives.
"In wild animals, just as in humans, social networks are very important for disease transmission," says Robbie McDonald of the University of Exeter. "When management changes stable networks, the results for disease control are often counterintuitive and unexpected."
By using special collars that tracked social interactions, first author Nicola Weber, working with colleagues at the National Wildlife Management Centre at Woodchester Park in rural England, found that tuberculosis (TB)-infected badgers were more isolated from their own social groups than were uninfected badgers. However, Weber also found that infected badgers were more likely to form social linkages for the flow of infection to other groups. As a result, infected badgers are less likely to spread the disease within their own groups but more likely to facilitate spread across a network.
"This unusual social arrangement may help explain why TB tends not to spread easily in undisturbed badger populations but also may help explain why, when their social networks are perturbed, infection spreads quickly to other badgers and onwards to cattle," McDonald says.
The researchers refer to these infected animals as "spread capacitors" because they are passive components in the network that can hold and discharge infection but tend to stabilize flow. "That's in contrast to the more familiar notion of 'super-spreader' individuals that are thought to spread infection disproportionately because of their high connectedness in a network," McDonald explains.
The findings suggest that badger management efforts will be most effective when they maintain stability and do not disrupt social networks. And that means it might be more effective in the long term to vaccinate badgers than it is to attempt to cull them.
"The good news is that vaccination does not disturb social structure," McDonald says. "The sort of social structure we have observed -- where relatively few individuals might be responsible for disease spread -- lends itself to vaccination and could lead quite rapidly to herd immunity."
- Nicola Weber, Stephen P. Carter, Sasha R.X. Dall, Richard J. Delahay, Jennifer L. McDonald, Stuart Bearhop, Robbie A. McDonald. Badger social networks correlate with tuberculosis infection. Current Biology, 2013; 23 (20): R915 DOI: 10.1016/j.cub.2013.09.011
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