June 13, 2012

Rabies Spread By Vampire Bats Despite Culling Efforts

Brett Smith for redOrbit.com

South American vampire bats continue to spread the rabies virus to both livestock and humans despite widespread culling of the flying bloodsuckers.

According to a new study published in the Proceedings of the Royal Society B by a team of North and South American researchers, the vampire bat population control efforts not only do not mitigate the spread of the virus–they may actually drive up the ratio of bats carrying it.

The researchers, led by Daniel Streicker from the University of Georgia, set up 20 sampling sites across Peru during their study. Over a 40-month period, they took repeated blood samples at each site from individual vampire bats. They also collected data on the population of each bat colony, the size of the corresponding local livestock population, and how often bats were culled, if at all.

"We found that rabies is there no matter what," said Streicker, an ecologist at the university. "The size of the bat colony didn't predict the proportion of bats that were exposed to the virus.”

"That's important because if there is no relationship between bat population density and rabies, then reducing the bat population won't reduce rabies transmission within bats."

For about three decades, the main rabies policy in Peru has been to halt the spread of the virus using poison or explosives and while most bats populations carry rabies–vampire bats, which suck the blood of mammals, are especially adept at transmitting the disease.

In studying the effects of the Peruvian policy, the research team found a surprising trend.

"We detected something that's a little bit worrying," Streicker said. "In areas that were sporadically culled during the course of the study, we saw an increase in the proportion of bats exposed to rabies.”

"Colonies that were culled regularly had slightly lower rabies exposure rates, and those that were never culled had the lowest rates of all. The next thing we have to do is understand the mechanisms for why this happens."

Several theories could explain higher rabies exposure in the sporadically culled bat colonies. Most culling is done by spreading an anticoagulant paste on captured bats that are then released. When other bats in the colony groom the treated bat, they ingest the paste and die. This strategy effectively selects adult bats, which groom each other, but not younger bats that are unlikely to groom their more mature counterparts.

Streicker said a viral resistance or built up immunity could help to explain why population control strategies are seemingly counterproductive. He also noted population density and bat migration as possible explanations.

"There's some experimental evidence that bats that are exposed repeatedly to rabies may develop a degree of immunity," Streicker said. "When you kill off the adult bats that may be immune, you're making space for susceptible juvenile bats (which the team found to have higher rates of rabies exposure than adults)."

"There's also something called the vacuum effect–when the adults are removed, individuals from neighboring roosts might move in to the colony to fill the vacated space."