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Leprosy in wild chimpanzees

Upon hearing the word “leprosy” most of us immediately have gruesome pictures of disfigured faces and missing fingers flash in front of our eyes, coupled with old stories of leper colonies and the stigma it carried and still carries today. However notorious and historical this disease is, scientific knowledge of the origins and ecology of its causative agent Mycobacterium leprae remains scarce. Until the past twenty years, M. leprae was thought to be an exclusively human pathogen. Recent discovery of its circulation in wild nine-banded armadillos, red squirrels and captive primates has provided preliminary evidence of its ability to infect other species as well. In our most recent preprint, we describe through striking images and a wide range of non-invasive diagnostic methods, the very first instances of leprosy among wild chimpanzees in two national parks more than 1500km away from each other. This study lines up data suggestive of leprosy not being the human originated disease once thought.

The story started in 2017, when Kimberly Hockings, from the University of Exeter contacted the Leendertz lab after remarking some peculiar lesions in several of the wild chimpanzees she was observing via camera traps in the Cantanhez National Park (CNP) in Guinea-Bissau. Shortly after, in 2018, similar pictures arrived from a field site where we have been conducting health monitoring of wild chimpanzees since 2000, the Taï National Park (TNP) in Côte d’Ivoire. The lesions observed resembled human leprosy, but such occurrence was unprecedented. Our team was immediately on the case to corroborate the initial suspicions. After screening fecal samples from CNP and TNP, M. leprae was identified in both field sites. Full genome sequencing showed that both pathogens belong to two different, and equally rare genotypes, 4N/O and 2F, respectively. The phylogenetic distance of the two M. leprae strains, geographical distance of the two affected chimpanzee populations, and the improbability of prolonged human-chimpanzee contact all indicate that, contrary to previous infections in wild animals, these cases are unlikely to be of human origin. The gathered evidence suggests, that it is more plausible that these infections were results of multiple unrelated introductions from an as of yet unidentified animal or environmental reservoir of leprosy, bringing us one step closer to understanding the complex ecology of this unique bacterium.

Most of the affected chimpanzees are still alive at the time of this publication (one leprosy case from 2009 was discovered through retrospective screening of necropsy samples, the cause of death was leopard attack), and are coping relatively well with this debilitating disease. Treatment is unfortunately impossible because it requires repeated antibiotic injections on a long-term basis, which is unfeasible in wild chimpanzee populations.

Don’t hesitate to also read the excellent coverage of this work by Kai Kupferschmidt in Science!

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