A story in Science has been making the rounds on my feeds about a newly-recognized problem in chimpanzee populations: “Leprosy, ancient scourge of humans, found to assail wild chimpanzees”.
This is a compelling example illustrating how researchers have become better able to monitor the health of natural chimpanzee populations using camera traps and metagenomics from fecal samples. It’s also another example of how something rare doesn’t trigger scientists to recognize a phenomenon until two different cases of it emerge, and then scientists rapidly find more. In this case, primatologists noticed two different chimpanzees at two different parks exhibiting symptoms of the disease, hundreds of kilometers apart.
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.
Hansen’s disease, or leprosy, is a long part of human history as the story mentions. Much of this historical connection may simply be the highly visible lesions and the slow disease progression. We don’t have a historical record as clear about diseases that nonetheless have long evolutionary associations with our species. History is short in the evolutionary time scale.
Still, it’s clear that humans have M. leprae strains that are adapted to us, just as we have M. tuberculae strains that have spread worldwide and seem to be human-adapted. How permeable the human population may be for these diseases, and how entrapped they are within humanity as a host population, has to be a major goal of pathogen studies as metagenomics of wild animal populations becomes more widespread.