Our immune systems are funny things. An American traveling to Taiwan, for example, might be warned to get a hepatitis vaccine - unless they grew up on a farm. Rabies is even scarier.  If you are bitten by an unknown animal, it requires a series of painful injections because if clinical disease sets in, it is usually fatal.

In the United States, human deaths from rabies have declined over the past century from more than 100 annually to an average of two per year because of aggressive campaigns to vaccinate domestic animals against the disease.  Recent human rabies cases are primarily due to bat bite exposures. Rabies virus infects the central nervous system, ultimately causing disease in the brain and death within a few days of symptom onset. 55,000 people worldwide are estimated to die of rabies each year.

Clearly, avoiding rabies-infected animals is the best way to go but that isn't always possible in developing countries. Researchers from the U.S. Centers for Disease Control and Prevention, in collaboration with the Peruvian Ministry of Health, have analyzed how some people living in two Amazon communities in Peru survived being exposed to rabies virus without receiving vaccination; strong evidence that an immune response may occur in certain communities where people are regularly exposed to the virus, according to a study published today.

The researchers conducted a survey in two communities in a remote section of the Peruvian Amazon where outbreaks of human rabies infections caused by vampire bat bites have occurred regularly over the past two decades. Several of these people who were previously exposed to rabies virus survived without vaccination, although the study cannot determine whether they ever experienced clinical disease.

The study consisted of 92 people, 50 of whom reported previous bat bites. Blood samples were taken from 63 people, and 7 (11 percent) were found to have “rabies virus neutralizing antibodies,” evidence that they had been previously exposed to the rabies virus. Although one person with antibodies reported receiving vaccine previously, the other people with antibodies are unlikely to have received medical care following prior bat bites. It could not be determined when the virus exposures occurred or which animals were responsible, but the history of repeated bat bites reported among persons in this area strongly suggest vampire bats as the source of rabies virus exposure.

 “Nearly all rabies virus exposures that proceed to clinical infections are fatal. Our results support the idea that under very unique circumstances there may be some type of enhanced immune response in certain populations regularly exposed to the virus, which could prevent onset of clinical illness,” said Amy Gilbert, PhD, of CDC’s National Center for Emerging and Zoonotic Infectious Diseases and lead author of the study. “However, a series of injections following an exposure remains the best way to protect people against rabies.”

Although few studies have ever looked, the findings in this paper may not be unique among persons at high risk for rabies virus exposure, such as persons experiencing high levels of bat or carnivore contact in areas where rabies is endemic.

"Evidence of Rabies Virus Exposure among Humans in the Peruvian Amazon" is published in the American Journal of Tropical Medicine and Hygiene.