terça-feira, 14 de agosto de 2012
Revenge of the Vampire Bats
Most cases of rabies in Latin America are caused by vampire bats, which bite victims at night and feed on their blood. In addition to infecting humans, the bats also do more than $30 million worth of damage to livestock each year. Governments have typically responded by culling bat colonies, but new research suggests that this approach doesn't work and might be backfiring.
Credit: D. Streicker
Rabies has been on the rise in Latin America, killing dozens of people annually, although exact numbers are hard to come by. "Combine this disease with a bat that sucks blood at night, and you have the stuff of nightmares," says Rosie Woodroffe of the Institute of Zoology in London. A major factor in the increase has been the growing numbers of livestock across the continent, which has allowed vampire bat populations to more than double in some places.
Many Latin American countries started culling bat colonies in the 1970s. Daniel Streicker, a postdoctoral ecologist at the University of Georgia in Athens, was interested in understanding the impact. He and colleagues in Peru spent more than 3 years studying vampire bats at 20 sites throughout the country. They collected blood samples from the bats and, because they could not test them directly for infection, checked them for antibodies to the rabies virus.
As they report online today in the Proceedings of the Royal Society B, every bat colony showed signs of the rabies virus. The rate of exposure was not statistically different in small colonies and big colonies. This matters because the virus does not spread faster through large, densely populated colonies—unlike, say, a cold virus racing through a crowded child care center—reducing the size of a colony by culling should not reduce the rate of rabies, Streicker says.
A disturbing finding was that colonies that were periodically culled, in fact, had higher rates of exposure. In these colonies, about 12% of bats had been exposed to rabies, compared with some 7% in colonies that were never culled. "If anything, we're going in the wrong direction," Streicker says. A possible explanation has to do with how the bats are typically killed. Captured bats are coated with an anticoagulant paste. When they return to their colony, other bats groom them and eat the paste. It kills adults, which are more likely to have acquired resistance to the rabies virus and not spread it. That would increase the number of juveniles, which don't groom other bats and are susceptible to developing rabies.
"This current study confirms the emerging picture that host-pathogen systems are complex and can respond to management in unexpected ways," says Woodroffe, who was not involved in the study. James Wood, an epidemiologist at the University of Cambridge in the United Kingdom, adds that the findings could have "profound implications" for how to control rabies in bats. But he would like to see the results replicated in a controlled experiment before advocating changes in culling practices. In any case, he says, vaccination of people and animals is proven to work and should be expanded. Farmers will probably continue to insist on culling, Streicker notes, because even uninfected bats can harm livestock by causing festering wounds.