Deep in the forests of West Africa, scientists have documented wild chimpanzees with leprosy for the first time. In an unexpected twist, the researchers are fairly confident that the chimps didn't catch the disease from humans, indicating that the disease has sprung from an unknown source.
The leprosy outbreaks have hit at least two wild populations of western chimpanzees (Pan troglodytes verus) in Guinea-Bissau's Cantanhez National Park and Taï National Park in Côte d'Ivoire, the researchers report in a pre-print paper on bioRxiv, which has not yet been peer-reviewed.
The chimps appear to be afflicted in much the same way the disease appears in humans. Using camera traps dotted around the parks, the team captured images of at least four chimps with lesions and disfigurement of the face, ears, hands, and feet.
To confirm the diagnosis, the team collected poop samples and detected the presence of the bacteria that causes leprosy, Mycobacterium leprae. They also detected the bacteria in a necropsy sample taken from an adult female named Zora who had been killed by a leopard in 2009.
Once thought to only infect humans, leprosy is now known to also affect other wild species, such as squirrels and armadillos. While leprosy has previously been seen in captive chimps and other primates, this is the first time it’s been documented in wild populations.
Leprosy has previously been reported in chimpanzees caught in West Africa in infancy and used for medical research in the US and Japan. This analysis revealed that the chimpanzee was infected with the disease during infancy in the wild but didn't present symptoms until many years later. The new preprint, however, appears to show the first confirmed case of leprosy actually manifesting in wild populations.
“The data from Tai suggests a very slow spread: two individuals total in one chimp community, and the chimps are in really close contact with grooming," Leendertz continued.
“We can't say that this disease alone is a conservation issue, but I would like to remind you that the chimps have many other problems – habitat loss, poaching, other diseases – so any additional problem is an issue as such.”
Genetic analysis of the bacteria obtained from the poop samples raised some interesting points. Firstly, the two different sites had two different strains, indicating the outbreaks arose separately. Secondly, the genotypes of the bacterial strain responsible for both outbreaks are extremely rare in humans, suggesting that it’s unlikely the outbreak originated from contact with humans.
Furthermore, leprosy is generally spread through lengthy close contact with an infected individual. These wild chimps scarcely ever came into contact with humans besides the researchers studying them. Notably, no researchers involved with the chimps have ever been diagnosed with leprosy. They also follow strict hygiene measures, such as keeping 7 meters (23 feet) distance and wearing face masks, to reduce the risk of disease jumping from human to primate.
This leaves the question, how did the chimps become infected with the disease? The researchers aren’t sure, but they suspect it came from an unknown animal or environmental source. Unfortunately, they remain currently mystified what that source might be.
“We have no idea, unfortunately, but we are looking into this now. Sampling environmental samples, catching rodents, et cetera,” explained Dr Leendertz.
Finding out, however, could reveal some important insights into the disease. For example, it challenges the long-held assumption that humans are the main reservoir of M. leprae. It also suggests that an unknown factor plays an important role in the life of leprosy.
This article was amended to include information about the case report of wild-caught chimpanzees being infected with leprosy.