Scientists have confirmed the first known cases of wild chimpanzees suffering from leprosy, a bacterial disease that typically infects humans. It remains a bit of a mystery how the chimps caught the infection, but researchers say the findings suggest that leprosy is most likely circulating in chimps and other wild animals much more than previously believed.
An international team of researchers from West Africa, Europe, and the US presented their findings in the journal Nature. The paper was previously published as a preprint in November 2020, which IFLScience covered, but the findings have since undergone peer review and confirmation.
The leprosy cases were documented in two wild populations of western chimpanzees (Pan troglodytes verus) — separated by some 1,000 kilometers (621 miles) — in Guinea-Bissau's Cantanhez National Park and Taï National Park in Côte d'Ivoire.
The outbreaks first came to light through hidden camera traps planted around the Cantanhez National Park that showed that at least four wild chimps had developed unusual lesions on their face, ears, hands, and feet, as well as hair loss and a loss of pigment to their face, remarkably similar to the symptoms of leprosy seen in humans. It was later revealed that similar symptoms were being seen in chimps from a totally separate population at the Taï National Park in Côte d'Ivoire.
Surprised by these observations, scientists confirmed their suspicions using genetic analysis. The team collected poop samples and detected the presence of the bacteria that causes leprosy, Mycobacterium leprae. They also found the bacteria in a necropsy sample taken from an adult female named Zora that had been killed by a leopard in 2009 in Taï National Park.
Further genetic analysis of the M. leprae bacteria obtained from the poop samples revealed some interesting insights. Firstly, the two different outbreaks 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.
Previous cases of leprosy have been reported in captive chimps, but this is the first time the disease has been confirmed in wild populations of chimps. It has also been known to affect a select few wild species such as squirrels and armadillos. All of this begs the question: how did they become infected with the disease?
“There is still so much we don’t know! Very surprising considering how ancient this disease is and the centuries that it has afflicted humankind,” Dr Kim Hockings, lead study author from the Centre for Ecology and Conservation at the University of Exeter, told IFLScience.
Dr Hockings explained that human contact with chimps is uncommon in Cantanhez National Park and Taï National Park. Furthermore, M. leprae is thought to be transmitted by humans with fairly obvious symptoms, yet no leprosy cases have been reported among researchers or local research assistants.
“Although a human source is impossible to rule out, low human contact coupled with the rarity of the M. leprae genotype detected in TNP [Taï National Park] chimpanzees among human populations in West Africa suggests that recent human-to-chimpanzee transmission is unlikely,” Dr Hockings said.
Instead, the team believes it's more likely that the chimps came into contact with the bacteria through their mammalian prey. Alternatively, it’s known that M. leprae can survive in soil and some other mycobacteria can survive in water, suggesting it’s possible the chimps picked up the infection from their natural environment.
It’s also uncertain how this disease might affect chimpanzees in the long term. This is partly because leprosy is a relatively slow-moving disease that takes a long time to impact its host. One female chimpanzee in Cantanhez, for instance, displayed early symptoms of leprosy in 2013 and is still alive and remains part of her chimpanzee community.
Nevertheless, Western chimpanzees are critically endangered with extinction due to a myriad of human-driven threats — the last thing they need is another disease to worry about.
“This is just one of the many pressures that wild chimpanzees and other wildlife are now facing,” Dr Hockings said.
“It’s possible that individuals that are stressed — either from human and environmental stressors in their natural environment or from invasive treatments in biomedical facilities, for example — are more likely to go on to develop leprosy. My future research will look at this in more detail.”