Humans may be transmitting hepatitis B virus to Amazon monkeys, raising questions about deforestation and disease spillover

A virus long associated with humans and Old World apes has now been detected in wild New World primates in the Amazon. The findings provide scientists a striking example of how increasing human contact with wildlife may be reshaping disease transmission.

A study published in EcoHealth reported that 17 of 49 primates tested in human-impacted areas of the Brazilian Amazon were positive for hepatitis B virus (HBV), a positivity rate of 34.7%. By contrast, none of 39 primates tested in a remote area along the upper Japurá River carried detectable HBV.

The researchers examined blood or liver samples from 88 wild primates representing 28 species. The animals came from two contrasting parts of the Brazilian Amazon: areas of southwestern Amazonia affected by human activity, including Rondônia and Mato Grosso, and a comparatively remote region in Amazonas state.

The human connection

The genetic evidence provided another important clue. The virus found in the monkeys looked very similar to types of hepatitis B already circulating among people in the region. Researchers examined virus samples from five infected primates and found that they belonged to the same broad groups of HBV found in local human infections, strengthening the possibility that the virus had spilled over from humans to monkeys.

According to the study, this supports the hypothesis that the virus moved from humans into wild primates rather than representing an unrelated primate-specific strain. The researchers also found that human population density significantly predicted HBV infection among the sampled animals.

Jean Boubli of the University of Salford, who led the research, said the finding raises questions about what other consequences human pressure on wildlife may be producing. The university described the discovery as evidence that deforestation and encroachment could increase opportunities for human diseases to reach primates.

Exactly how the monkeys acquired HBV remains unknown. The study notes that transmission could potentially be associated with activities that bring people and primates into closer contact, including hunting, consumption of wild meat and the keeping or trading of monkeys as pets. The researchers stress, however, that the precise transmission route in these wild animals has not been established.

A virus that can seriously affect the liver

Hepatitis B is a viral infection that primarily attacks the liver. According to the World Health Organization, it can cause both short-term and long-lasting infection; chronic hepatitis B can gradually damage the liver and lead to cirrhosis, liver failure or liver cancer. Many people have few or no symptoms, which means the infection can sometimes go unnoticed for years. The researchers say it is still unclear whether HBV causes similar long-term health problems in these Amazonian monkeys.

The infected monkeys examined in this study did not show obvious clinical signs, according to the University of Salford. Researchers therefore say further clinical monitoring is needed to determine whether HBV affects their health, survival or reproduction. It is also not known whether the virus is subsequently spreading between monkeys.

Why the Amazon finding matters

The researchers say the contrast between the two sampling locations is particularly important. While 17 of 49 primates in the human-impacted region tested positive, all 39 sampled from the remote site tested negative. The authors argue that this pattern, together with the genetic similarity to human viruses, supports human-to-primate spillover associated with human encroachment.

Human-primate contact in the Amazon is not limited to deforestation. The paper cites previous estimates that Indigenous people and local communities consume around three million primates annually from three major genera, Alouatta, Ateles and Sapajus, for subsistence. Some species are also traded illegally.