Conservation scientist Kimberley Hockings was worried. In 2017, photos from camera traps in Guinea-Bissau’s Cantanhez National Park, where she works, revealed several chimpanzees with terrible lesions on their faces. Hockings emailed wildlife veterinarian Fabian Leendertz. “I have NEVER seen this in chimps,” Leendertz, who works at the Robert Koch Institute in Berlin, wrote back. Then a few months later, Leendertz saw a similar photo from his own research site in Ivory Coast, hundreds of kilometers away. Could it be the same disease?
Now, a new preprint by the two researchers gives a surprising answer: Chimps in both West African sites suffer from leprosy, a disease never before documented in wild chimpanzees. The strains in each park appear unrelated, and they are unlikely to have come from contact with humans, the authors argue. The finding could indicate an unknown source of leprosy in the wild and reveal new clues about a still-mysterious disease.
Leprosy is an ancient ailment, but surprisingly little is known about where and when it emerged, or how exactly it spreads. The disease—and the terrible stigma it carries—once afflicted millions of people across the globe. But after a combination of antibiotics became standard therapy in the 1980s, cases plummeted and scientific interest waned. The difficulty of studying leprosy adds to the lack of interest, says co-author Charlotte Avanzi, a microbiologist at Colorado State University (CSU), Fort Collins. The bacteria that cause the disease, Mycobacterium leprae and the recently discovered M. lepromatosis, cannot be cultured in cells in the lab. The only way to multiply the pathogen is to inject it into armadillos or into the footpads of mice.
For years, researchers thought leprosy afflicted only humans. But over the past 2 decades, scientists have also found the pathogen circulating in nine-banded armadillos in the Americas and in red squirrels in the United Kingdom. Both species harbor the same bacterial genotype, called 3I, that has been linked to human infections in medieval Europe. In both cases, the pathogen appears to have jumped from humans to the animals. Scientists have also reported isolated leprosy cases in captive animals, including chimps.
But the story in wild chimpanzees is shaping up to be very different. When a chimpanzee named Woodstock at Taï National Park in Ivory Coast started to show signs of leprosy, Leendertz decided to screen older fecal and necropsy samples from his library for the disease. He found traces of M. leprae in another chimpanzee that had been killed by a leopard in 2009. When researchers sequenced the pathogen’s genome, they found it was of a rare genotype called 2F. In Guinea-Bissau, researchers collecting fecal samples also got lucky: One sample contained enough bacterium to sequence its full genome, which was another rare genotype called 4N/O.
Human diseases can spill over to chimpanzees with devastating consequences. But Leendertz thinks a recent transmission of leprosy from humans to chimps is unlikely, because the disease usually spreads only after prolonged, close contact, and there have been no known leprosy cases among researchers or local assistants. (Although researchers study the chimps, they keep at least 6 meters of distance.) In addition, the genotypes responsible for both outbreaks are rare in humans, the researchers report today on the preprint server bioRxiv. Leendertz will not rule out two separate, ancient infections from humans. But, he concludes, “The most likely scenario is that there is some unidentified leprosy reservoir.”
John Spencer, an immunologist who studies leprosy at CSU, says there is more and more evidence “that Mycobacterium leprae is not limited solely to existence in humans, but has other niches that it has adapted to.”
Past work has hinted at that idea, says Anne Stone, an evolutionary geneticist at Arizona State University, Tempe, who was not part of the study. She has long suspected the leprosy bacterium may thrive in another reservoir, in part because of the leprosy genome’s small size and other quirks. “That’s really a signature of something that needs to live on another organism,” she says. That signature appears to date back millions of years, to a time before humans, suggesting the bacterium had another host before we evolved.
“The data increasingly points to the possibility that something else than humans is actually the main host,” Stone says. That could be an animal the chimpanzees hunt, for instance, or the leprosy bacterium might even live in the environment.
Rodents are a prime contender for the mystery host, Stone says, although amoeba and some insects have also been infected with leprosy in the lab. Leendertz and his colleagues are planning to look at all these possibilities.
It’s an interesting new avenue for leprosy research, Avanzi says. “It’s a very difficult disease,” she says. “Any clue we can get about it from animals or anywhere is really, really helpful.”
For the moment, the infected chimpanzees seem to be coping with their illness, although one is losing weight, Hockings says. Treating them is not really an option, Leendertz says. “Humans have to take antibiotics for months to treat leprosy. You just can’t do that with these wild animals.” For now, the disease does not appear to put the groups as a whole at risk, he says. “But it’s an additional threat, of course, on top of poaching, habitat loss, and other diseases.”