An isolated group of natives from the Peruvian Amazon have cast doubt on the theory that is not possible to develop a natural immunity to the disease, and that without immediate treatment fatality is a foregone conclusion.
Researchers at the Centers for Disease Control and Prevention and the Peruvian Ministry of Health, during a search for emerging infectious diseases, observed two communities in the remote region. The groups, Truenococha and Santa Marta, were two and six hours away respectively from the closest health centers, and required a boat to reach.
Amy Gilbert, a postdoctoral fellow at the CDC's National Center for Emerging and Zoonotic Infectious Diseases and co-leader of the work said:
"Generally the communities don't appreciate the risk of rabies from a bite. They don't know how it is transmitted and are unlikely to seek treatment because of the low perceived risk and because care is very far away."
Gilbert and her team interviewed 92 people in total, and collected blood samples from 63. The mean age was 25, and 55 percent were male. The researchers found that ten percent of people living in the remote region appeared to have survived exposure to the virus without any medical intervention -- unsurprising as help is so far away. In addition, 11 percent were found to have antibodies in their blood that would neutralize the disease.
Each participant reported exposure to the bat -- common in the Amazonian region -- through a bite, scratch or skin contact. 50 people reported previous bat bites, but only one of the seven with antibodies reported receiving a vaccination. Gilbert said:
"Generally, most folks presume we don't develop antibodies to respond to rabies exposures. But this was a scenario where clearly there were exposures to the virus that did not lead to disease. I think the same recommendations and advice still hold -- that anyone with a bite exposure to a bat or other carnivore needs to seek out post-exposure [treatment]."
The researchers are not sure how the antibodies developed, but the most likely explanation is that multiple bites of a low rabies dose may have helped their immune systems develop a response -- while preventing a complete nervous system infection. However, it may also be that the isolated groups' genetic makeup played a part, or that the prevalent strains of rabies in the region are less virulent.
Rabies kills over 55,000 people each year across Asia and Africa each year. Vampire bats generally target other animals to feed, but will turn on humans if food is scarce -- and these creatures are one of the main transmitters of the disease.
Through the bite of an infected animal, the virus travels up to the brain through nerve fibers, rather than blood. The further away from the brain, the longer rabies must travel, which provides an opening to seek treatment -- a vaccine and the administration of anti-rabies antibodies.
The standard time for treating rabies is several weeks. After contraction, the individual may experience flu-like symptoms, including a headache, weakness or fever. However, if the virus reaches the central nervous system (CNS) then it becomes a full CNS attack, and symptoms include aggression, slavering, snarling, insomnia, partial paralysis and hydrophobia.
The findings were published in the August 2012 issue of the American Journal of Tropical Medicine and Hygiene.
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