Daniel Streicker / ASTMH Press
Vampire bats in an abandoned mine in Huanuco, Peru. Researchers are investigating whether some Amazonian villagers may have survived rabies infections, which have a nearly 100 percent fatality rate.
Some Amazonian villagers plagued by vampire bats may have survived rabies infections — something that doctors thought was virtually impossible. The scientists' discovery opens hopes of eventually developing an effective treatment for the nearly always fatal infection.
A study of people living in remote areas of Peru shows that about one in 10 appears to have been bitten by rabid bats, but lived to tell the tale. Rabies is almost universally fatal, with only five documented cases of people surviving. Teams at the Centers for Disease Control and Prevention are now looking further to see if people may survive the deadly virus more often than had been believed.
“They were infected,” said the CDC’s Brett Petersen, an epidemiologist who worked on the study. What’s not clear is whether the people who survived infection ever actually got sick.
Any mammal can get rabies, and the disease causes horrible symptoms as the virus attacks the nervous system. It can make an animal aggressive and violent and can cause severe pain. Victims eventually fall into a coma and die when they can no longer breathe properly.
“Rabies has the highest case fatality rate of any conventional infectious disease, approaching 100 percent,” Petersen and colleagues wrote in their report, published on Wednesday in the American Journal of Tropical Medicine and Hygiene. People can survive if vaccinated immediately after a bite or other exposure to rabies, but there is no real treatment for infection, with very rare exceptions.
The CDC team and officials with Peru’s health ministry traveled to a remote Amazon region where vampire bats regularly feed on cattle and people. It took two days to reach the area by boat.
“Many of [the people] have very basic housing. There are no real barriers for entry to bats. The bats are able to enter the house and have easy access to people,” Petersen said in a telephone interview. The teams interviewed 92 people and got blood samples from 63 of them.
More than half said they had been bitten by a bat at least once and 20 percent said they were bitten more than once a year. The blood tests showed something startling -- seven people, or 11 percent of those tested, had antibodies against rabies. These immune system proteins were the type that can neutralize a virus.
“This shows evidence that these people were exposed to rabies virus previously,” said the CDC’s Amy Gilbert, an epidemiologist who led the study.
It's undetermined whether these seven people had actually become sick. Early symptoms of rabies infection aren’t always clear and can resemble those of a cold or flu. The incubation period can be as long as three months. It might be, Gilbert said, that they were bitten and got a very low dose of the virus, perhaps not enough to make them sick. More study will be needed to find out.
“We know that not all viral introductions into the body will lead to clinical illness,” Gilbert said. A lot depends on how much virus gets in, what part of the body, how deep a bite was, and perhaps if other germs got in at the same time.
What is certain is, rabies is very deadly. “We are still talking about 99.999 percent of cases, approaching 100 percent,” Gilbert said.
The scientists aren't sure the villagers are actually immune to rabies, so the CDC team is recommending that all get vaccinated.
“What we know is that they did develop an antibody response. We don’t know what level of antibodies will provide complete protection,” Petersen said.
More outbreaks of rabies have been reported in the region in recent years, he added.
Rabies was once a global scourge, but widespread vaccination has made it very rare in countries such as the United States. The vaccine is very effective if given before symptoms develop. That's why doctors urge anyone bitten by an animal get medical attention right away. Most US cases – and there are just one or two a year at most – are caused by bat bites.
Globally, the World Health Organization says 55,000 people die of rabies every year, with dogs the source of 99 percent of these fatal bites. Five people have survived documented rabies infections -- all young girls who were given a complex treatment called the "Milwaukee protocol," which involves putting them into a medical coma.
"Our results open the door to the idea that there may be some type of natural resistance or enhanced immune response in certain communities regularly exposed to the disease," said Gilbert. "This means there may be ways to develop effective treatments that can save lives in areas where rabies remains a persistent cause of death."
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First published August 1 2012, 2:01 PM