Immunity against rabies without vaccination

A newly published study suggests that some people living in two Amazon communities in Peru survived being exposed to rabies virus without receiving vaccination.

There are extensive reports of rabies transmitted from vampire bats (Desmodus rotundus), in cattle and humans throughout Latin America, and communities in the Amazon have experienced a number of outbreaks of rabies from vampire bats in recent years. Due to the successful reduction in canine rabies, 80% of all human rabies cases reported in Peru from 1996 to 2010 were associated with vampire bats.

The researchers from the US CDC, in collaboration with the Peruvian Ministry of Health, surveyed 92 members of two remote Amazonian communities in Peru and asked them about their knowledge about rabies transmission and history of exposure to vampire bat bites. All respondents had basic or no understanding of rabies and only 23% knew that rabies was transmitted by animal bite. Over half reported that they had been bitten by bats, but only 3 people had previously received rabies vaccination.

The survey found that one community had a higher level of reported bat bites, possibly related to the lack of cattle there (a previously described phenomenon) or to differences in their proximity to bat roosting sites. Also, people under 25 years of age, and people who owned domestic animals that were bitten by bats were also more likely to report being bitten.

Those who consented also provided blood samples which were tested for rabies Virus Neutralizing Antibodies (rVNA) by the rapid fluorescent focus inhibition test (RFFIT) technique and two types of rabies virus binding antibodies (with proven roles in the immune response to vaccination) by indirect fluorescent antibody testing (IFAT). Of these 63 blood samples, 7 showed rVNAs, and a further 2 samples demonstrated rabies antibodies only by IFAT. All 9 seropositive individuals were over 25 years old, all reported bat exposures, and 7 reported bat bites. One person had received rabies vaccination in the past, and for a further two persons the data were unavailable. Considering their remote location and that these communities rarely seek vaccine after a bite, it is likely that the rest had not received rabies vaccination and had developed antibodies after natural exposures.

The presence of rVNAs indicates exposure to virus, but not necessarily active viral infection and replication in the body. Indeed, vigorous rVNA responses are induced by inactivated rabies vaccines. Samples that could have demonstrated central nervous system (CNS) infection in the Peruvian subjects were not taken, but interview questions concerning past illnesses did not suggest any CNS infections. As younger people were more likely to be exposed to bat bites, but older people more likely to produce antibodies, it would appear that repeated exposures lead to the production of detectable antibody levels.

This evidence adds to other findings suggesting that natural immunity can fight off rabies viruses; bats often show rVNAs, unvaccinated wildlife trappers and hunters have shown antibodies to rabies virus, and a handful of unvaccinated human patients have survived clinical rabies. It remains to be investigated whether the Amazonian indigenous populations show enhanced immune responses and genetic resistance to rabies infection that would indicate natural selection resulting from regular exposures to rabies virus. Regardless of the mechanism, the new study confirms a high level of vampire bat exposures and a risk of rabies virus exposure from bats. The authors suggest that pre-exposure vaccination of these and other high-risk communities should be considered in future rabies interventions.

Summarised by Louise Taylor from the publication “Evidence of Rabies Virus Exposure among Humans in the Peruvian Amazon” by Amy Gilbert and colleagues in the American Journal of Tropical Medicine and Hygiene, vol 87(2), 2012, pp. 206–215.