An untreated rabies infection is usually seen as a death sentence. But a new study by scientists at the Centers for Disease Control and Prevention (CDC) in Atlanta suggests that may be wrong. In two villages in the Amazon, researchers found that 10% of people tested appeared to have survived an infection with the virus.
The results are "very surprising but convincing," says Hildegund Ertl, a vaccine expert at The Wistar Institute in Philadelphia. The study could be a "game-changer," adds Rodney Willoughby, a pediatrician at the Medical College of Wisconsin in Milwaukee. "If these findings are confirmed and extended, then it would show that rabies can vary in severity, rather than being 100% fatal."
Rabies is believed to kill more than 55,000 people every year in Africa and Asia alone—all of them after a bite from a rabid dog. In Latin America, most human cases are caused by vampire bats. While these animals usually feed on livestock, they are known to bite sleeping humans.
A vaccine against rabies is available, but it is very expensive and usually given only to people who are at high risk, such as veterinarians in rabies-affected countries. The only way to survive an infection—or so scientists believed—is treatment with antibodies and vaccination immediately after a bite from an infected animal.
Still, there have been sporadic reports of people surviving an infection even without those measures. For example, earlier studies provided weak evidence of past rabies infections in Inuit hunters or fox trappers, Ertl says. Some of those people seemed to have antibodies against the rabies virus in their blood, but the amount was too low to be confident about the results, she says. And in May 2011, an 8-year-old girl in California diagnosed with rabies, possibly contracted from a free-roaming cat at her school, survived without treatment. But Willoughby, who saved a girl infected with rabies in 2004 with an experimental treatment, says he is not convinced the California girl really had rabies. "We have other laboratory evidence in the California patient that argues against rabies, but the methodology is under validation," he writes in an e-mail.
The new study suggests that survivors of rabies may be quite common. In May 2010, CDC scientists visited two villages in the Amazon region in western Peru where infections with rabies had been reported several times in the last few years. They interviewed 92 people in 51 households and collected blood samples from 63 of them. The samples were frozen, shipped back to Atlanta, and then screened for antibodies that could bind and neutralize the rabies virus.
Seven of the 63 blood samples tested positive. One of those subjects had told the team he had been vaccinated, but the other six had said they weren't, suggesting their immune system had learned to deal with the deadly virus on its own, the team reports today in The American Journal of Tropical Medicine and Hygiene.
Willoughby suspects that the relative resistance may be unique to the remote Peruvian population. This may be genetic, dietary, or based on unknown coinfection. But although the study sampled a very isolated population, there is no evidence that the Peruvians were genetically special, says Amy Gilbert of CDC's National Center for Emerging and Zoonotic Infectious Diseases and the lead author of the paper. People who had the protective antibodies in their blood tended to be older, however. "We think that the most [likely] explanation is that these people were exposed to the virus multiple times in low doses" through contact with bats, she says. In contrast to the few reported cases of patients surviving an infection, the Peruvians seem not to have fallen ill at all. Symptomless people would not usually go to a clinic after a bat bite—if only because the closest hospital is a long boat ride away—so their infections may often go unnoticed, Gilbert says.
Ertl says the results of the study are "much more robust" than those from the Inuit people, leaving little doubt that some people can indeed survive rabies. But different animal species carry different strains of the virus. "I don't think this could happen in someone with dog rabies," Ertl cautions. "To me it looks like bat rabies is just more wimpy."
And in any case, Ertl cautions, nobody should think of rabies as any less dangerous after this paper—and anybody bitten or scratched by a bat should get the vaccine as soon as possible.