Tracking Peru's Rabid Vampire Bats

A new model attempts to predict the spread of rabies through the Andes Mountains.

Andrew Silver, Contributor

(Inside Science) -- A threat lurks in the lush river valleys of the Andes Mountains in Peru. When some vampire bats drink blood, they pass on the deadly rabies virus. Now, biologists have made a model that could protect livestock by predicting the spread of rabies.

"It's a pretty neglected disease in Latin America," said Julio Benavides, a disease ecologist at the University of Glasgow in Scotland and lead author of the study.

Vampire bats infected at least 1,146 animals with rabies between 2003 and 2014, according to Peru's national service of health for animals and plants, SENASA. They sometimes bite and infect humans, but the Andes have more of their favorite meal: livestock.

Although the bats can appear as far north as Mexico and south as Chile, many Peruvian farmers are unaware of the lethal threat they pose, Benavides said. The farmers who do know might not be able to afford vaccinations. So he and his team created a model to predict where rabies will appear next in the region.

To develop the model, the team compared data from outbreak reports with farmer questionnaires and livestock bat bites between June 2014 and July 2015.

"We made the data talk," he said.

By crunching numbers, the team found the virus spreads in a clear, wave-like pattern. The key turned out to be the shape of the Andes.

Vampire bats mainly share the virus through saliva by biting one another as part of the social system, Benavides said. They don't switch between valleys, however, for two main reasons.

First, the bats have to avoid the cold temperatures at high elevations.

Second, bats usually only fly around 3 to 6 miles from their homes, which is about the length of a single valley. "They are much less migratory than people think," he said.

The team's final model predicts that 11,600 small farms, with at least 339,000 total livestock, would be at risk of rabies from 2015 to 2017. At $1.10 per dose, livestock vaccinations would cost at least $373,000 a year.

Meanwhile, the team's questionnaire showed that only one out of 90 farmers had vaccinated an animal against rabies. Only 25 percent of the farmers in one valley and 3 percent in another even knew bats could transmit it.

"It's a good paper," said Alvaro Aguilar-Setién, a virologist at the Mexican Social Security Institute who was not involved in the research. "The conclusions are right for many, many countries, but it needs to take in consideration other factors."

Benavides agreed that the model works well in the valleys but not as well in other regions.

In open areas without mountains, such as in the Amazon rainforest, vampire bats fly around freely. It would be possible but harder to predict the spread of rabies, Benavides said.

He hopes policymakers will begin planning vaccinations or education efforts in local communities. Peru's government supports killings bats to prevent rabies, usually by setting a bat cave on fire.

A November 2013 study in the Proceedings of the National Academy of Sciences by a group of researchers which includes one of Benavides' team members, however, found that rabies still spreads after bat-killing practices. Biologists don't know exactly why, but one reason could be that young, infected bats become fearful and escape, Benavides said.

SENASA does advise vaccinations after it confirms an outbreak. But by then it is too late, he said.

The new research, funded by the Welcome Trust and Royal Society, appeared June 8 in the Proceedings of the Royal Society B. Benavides and his team are now studying the financial effects of rabies on farmers. For example, more than half save the money from raising livestock for their children's education or household maintenance, and losses due to rabies can drain family funds.

"We are going to get places wrong. And in a way, we know that because models have uncertainty," Benavides said. "I think we take the risk because the spread is so clear."

Andrew Silver is a contributing writer for Inside Science. He has created interactive visualizations for Quanta Magazine and written for outlets such as Science, Physics World and Live Science. Follow him on Twitter @asilver360.