An ancient disease in humans seems to have jumped the species barrier to one of our closest primate cousins. On Wednesday, researchers detailed finding cases of leprosy among two separate groups of chimpanzees in West Africa—the first ever documented among wild chimps. The origin of these outbreaks is unknown, but it’s thought that the disease remains rare in the wider chimp community.
Leprosy is caused by the bacteria Mycobacterium leprae, making it a relative of tuberculosis bacteria. Similar to viruses, and unlike most other bacteria, leprosy bacteria survive by invading our cells and hijacking their functions in order to replicate. Adding to its peculiar nature, symptoms following infection may not appear for years to two decades. These initially include pale-looking lesions or sores. Over time, the chronic infection can gradually destroy nerve and eye cells as well, leading to symptoms like the loss of sensation, paralysis, blindness, and permanently disfigured limbs.
Our history with leprosy, also called Hansen’s disease, stretches back to Biblical times. But despite the scary connotations it’s long held, leprosy is something of a pushover. The disease is not easily transmissible, usually requiring months of close person-to-person contact, and 95% of people are estimated to be naturally immune to infection. While the bacteria has evolved resistance to the oldest drugs used to treat it in the 1940s, infection remains curable through long-term antibiotic therapy.
Improvements in sanitation and the availability of antibiotics have made leprosy a rare disease globally. But there are still pockets of the world where these resources aren’t as abundant, and about 200,000 cases were reported in 2019. In the 1970s, researchers discovered that humans aren’t the only natural hosts for the bacteria: Armadillos in the Americas now routinely carry it, too, a process that likely started when Europeans colonized the New World and brought the bacteria with them. In a karmic twist, leper armadillos have occasionally infected humans right back.
Studies have shown that captive chimpanzees can also contract leprosy. But in June 2018, researchers studying wild chimps in West Africa noticed strange leprosy-like lesions on an adult male chimp they had named Woodstock. The discovery made them wonder if others in the same community or elsewhere had ever developed similar symptoms.
Looking back through tissue samples obtained after a chimp’s death, they identified a chimp named Zora whose blood contained traces of leprosy bacteria before her death in 2009 from a leopard attack. Subsequent examination of photos of Zora showed skin deformities that could have been caused by leprosy, while her stored fecal samples suggested that the infection had started at least seven years earlier. Ultimately, the group managed to find several other cases of leprosy, confirmed through testing, among two distant chimp communities in the countries Côte d’Ivoire and Guinea-Bissau.
“This is the first confirmation of leprosy in nonhuman animals in Africa,” said lead author Kimberley Hockings, a researcher with the Center for Ecology and Conservation at the University of Exeter in the UK, in a statement from the university. “It’s amazing that it also happens to be in our closest living relative, the chimpanzee, especially considering how well studied chimpanzees are in the wild.”
The team’s findings were published Wednesday in Nature.
Though the prime suspect for how these chimps caught leprosy would be humans, the researchers aren’t so sure. Genetically, the strains found in each chimp community were different from one another, and they’re rarely found in people or even other known animal reservoirs. Neither chimp community spends much time around humans, either, making the already difficult process of transmission less plausible. So it’s possible that leprosy has been making a home in more nonhuman species than currently thought and that the chimps caught it from these unknown hosts. Interestingly enough, scientists in the UK found evidence in 2016 that squirrels could catch leprosy, too.
The good news is that chimp leprosy appears to be rare. Out of the 467 chimpanzees observed over the years in the community where Woodstock lives and others nearby, for instance, the researchers only identified four cases in total. But wild chimpanzees, unlike humans, don’t have access to antibiotics. And the researchers fear that endemic leprosy could still harm these chimps. They say more research is needed to understand the spread and origins of leprosy among wild chimp populations.
“Western chimpanzees are critically endangered, so even the loss of a few individuals could be significant,” said Hockings.