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A group of Peruvians thought to have survived untreated rabies infection have bucked the notion that the virus is universally lethal to humans.

A team led by of the US Centers for Disease Control and Prevention, in collaboration with the Peruvian Ministry of Health, travelled to two communities in a remote part of the Peruvian Amazon. Outbreaks of rabies infection caused by bites from vampire bats over the past couple of decades.

When the team sampled the blood of 63 people from these communities they found that seven of them had “rabies virus neutralising antibodies”. One of these people had had the rabies vaccine before but the other six had not, though they reported having been bitten by bats in the past.

The antibodies are produced when the body is directly exposed to rabies or exposed to a vaccine for the virus. Gilbert and her colleagues concluded that the six unvaccinated people with the antibodies must have been exposed to rabies without dying from it, suggesting they have a natural immunity.

However, the researchers admit that they don’t know whether these people actually developed an infection or were exposed to a small dose of the virus which they were able to fight off. “Despite the fact that we find antibodies, these people are still considered at risk,” says Gilbert. The level of antibodies required to protect someone is unknown, she says.

If further studies confirm that there are populations of people with a greater natural immunity to rabies, they could pave the way for new treatments.

Past fighters

Even if these six patients did manage to fight off the infection themselves, it might not be the first time that natural resistance to rabies has appeared.

When a person suspects they might have been infected with rabies they are advised to seek immediate treatment in the form of a series of vaccines. If they do this within a few days, before exposure causes an active infection, they almost always survive. But they must receive the treatment before symptoms start to show.

A treatment known as the Milwaukee protocol – where unvaccinated people are sent into a drug-induced coma to buy time to fight off the infection – appears to have worked on at least six patients. But it has been tried on at least 35, so it is not clear whether the survivors owe their lives to the Milwaukee protocol or their own immune system’s ability to fight off what could have been a weak strain of the virus.

Journal reference: , DOI: 10.4269/ajtmh.2012.11-0689