CDC: Amazon study finds routine rabies exposures

Some people living in two Amazon communities in Peru survived being exposed to rabies virus without receiving vaccination, according to researchers from the U.S. Centers for Disease Control and Prevention, in collaboration with the Peruvian Ministry of Health. While avoiding rabid animal exposures, and receiving injections administered after a person is exposed to rabies virus remain the best ways to protect against acquisition of this fatal disease, there is 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 study, “Evidence of Rabies Virus Exposure among Humans in the Peruvian Amazon,” is in the August issue of the American Journal of Tropical Medicine and Hygiene. 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.

Rabies is a preventable viral disease most often transmitted through the bite of a rabid animal. In the United States, the vast majority of rabies cases reported to CDC each year occur in wild animals like raccoons, skunks, bats, and foxes. Rabies virus infects the central nervous system, ultimately causing disease in the brain and death. More than 55,000 people worldwide are estimated to die of rabies each year.

Early symptoms of rabies include fever, headache, and general weakness or discomfort. As the disease progresses, more specific symptoms appear and may include insomnia, anxiety, confusion, slight or partial paralysis, excitation, hallucinations, agitation, hypersalivation (increase in saliva), difficulty swallowing, and hydrophobia (fear of water). Death usually occurs within days of the onset of these symptoms.

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. Human and domestic animal contact with bats should be avoided. People who may have had contact with bats should seek medical advice, and such exposures should be promptly reported to local public health authorities.

For more information on rabies, visit CDC’s rabies Web site at http://www.cdc.gov/rabies/.  For general information about this study, visit the ASTMH site at http://www.astmh.org/.

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U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES

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