August 2, 2012
Amazonians Exposed To Vampire Bat Bites Have Natural Immunity To Rabies
Lawrence LeBlond for redOrbit.com - Your Universe Online
US scientists studying indigenous tribes in the Amazon have found six native Peruvian Amazonians who, after being bitten by rabid vampire bats, suffered no ill effects.
Rabies attacks the central nervous system (brain and spinal cord) and is generally fatal in humans without proper medical intervention. While it typically takes two to 12 weeks before flu-like symptoms begin to appear, incubation has been observed in as short as four days and longer than six years in some individuals, depending on location of the wound and severity of the virus introduced.
But a new study suggests that certain populations that are regularly exposed to vampire bat bites develop a natural immunity to the deadly virus.
According to the study, published today in the American Journal of Tropical Medicine and Hygiene, 92 Peruvians were surveyed in two separate communities. Fifty of those polled reported previous bat bites. In blood samples of 63 people, 7 were found to have rabies virus neutralizing antibodies. Of those one had reported a prior rabies vaccination; the other six had not received medical care following bat bites.
“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 study lead author Amy Gilbert of the US Centers for Disease Control and Prevention (CDC). “This means there may be ways to develop effective treatments that can save lives in areas where rabies remains a persistent cause of death.”
The results do not conclusively determine that the antibodies were caused by an exposure to the virus. But the researchers believe the evidence “suggests that (rabies virus) exposure is not invariably fatal to humans” as traditionally believed.
Gilbert said non-fatal exposures may happen more often than some think because “unless people have clinical symptoms of the disease they may not go to the hospital or clinic, particularly where access is limited.”
The general consensus is that “nearly everyone who is found to be experiencing clinical symptoms of rabies dies,” Gilbert said. “But we may be missing cases from isolated high-risk areas where people are exposed to rabies virus and, for whatever reason, they don't develop disease.”
The CDC study was conducted in collaboration with the Peruvian Ministry of Health as part of a larger project to understand better bat-human interactions and its relation to rabies and emerging diseases that may be transmitted by bats.
The study was conducted in the Province Datem del Maranon in the Loreto Department of northern Peru. There, vampire bats thrive on mammalian blood and regularly come out at night to feed. While they typically feed on livestock, they will occasionally take feast upon humans, especially in the absence of their preferred food sources.
Using extremely sharp teeth and an anticoagulant (draculin) that is naturally produced by their saliva, these bats can feed on a sleeping person without waking them up. Because the rabies virus extensively circulates throughout vampire bat colonies in the region, the likelihood of a person becoming infected is very high.
The fact that some people have a natural immunity to this virus without vaccination or treatment offers hope that medical science can find a cure for the fatal disease, which is normally incurable once it develops in the host´s system.
Experts estimate Rabies kills 55,000 people each year in Africa and Asia alone, and appears to be on the rise in China, the former Soviet Republics, southern Africa, and Central and South America. In the United States, human deaths from rabies have declined dramatically over the past century thanks to an aggressive campaign to vaccinate domestic animals against the disease.
James W. Kazura, MD, an infectious disease expert and president of the American Society of Tropical Medicine and Hygiene (ASTMH), said this study offers “continued support to the belief that even the most dangerous of infectious diseases may be amenable to treatment“¦ Continued investment of resources is essential for us to protect the health and well-being of innocent people whose lives and livelihoods are needlessly threatened by infectious diseases like rabies.”
If it turns out there are distinct populations of people with “complete or relative resistance to rabies,” there could be the potential to use whole genome sequencing to help develop new, life-saving treatments for rabies infections, wrote Rodney E. Willoughby, a pediatric disease specialist at Children's Hospital of Wisconsin, in an accompanying editorial.
“Careful, respectful genetic study of these genetically unique populations may provide information on which pathways in human biochemistry and physiology promote resistance to human rabies,” he wrote. “Equally important, knowing that there is a continuum of disease, even for infectious diseases like rabies, should push us harder to try for cures when confronted by so-called untreatable infectious diseases“¦.”
Despite a record of people who have survived an exposure to a deadly disease, Gilbert noted, the fact remains that rabies outbreaks in small communities usually have significantly tragic results. “These are very small villages and, when they witness ten people dying from what is a horrible disease, it is incredibly traumatic.”
“We want to help raise awareness of the problem and try to develop a more proactive response,” Gilbert concluded.