New research has rewritten the history of leprosy in the Americas. Scientists have discovered genetic evidence that a rare strain of the disease, known as Mycobacterium lepromatosis, existed in South America as far back as 4,000 years ago—long before European contact.
This challenges the long-standing belief that leprosy arrived in the Americas during the colonial era.
Leprosy, or Hansen’s Disease, is a chronic illness that can cause skin lesions, nerve damage, and physical disabilities if left untreated.
Today, it is found in over 100 countries, but access to treatment is still unequal, especially in areas with limited healthcare resources.
While many ancient skeletons from Europe and Asia show clear signs of leprosy dating back 5,000 years, no such signs had ever been seen in pre-colonial American remains—until now.
A team of international researchers, led by Kirsten Bos from the Max Planck Institute for Evolutionary Anthropology, worked with colleagues in Argentina and Chile to study ancient bones from archaeological sites in Chile.
Using advanced DNA techniques, the researchers identified and reconstructed two complete genomes of M. lepromatosis in human skeletons that are approximately 4,000 years old.
The discovery was surprising. Leprosy had long been considered a disease introduced by Europeans.
But when doctoral researcher Darío Ramirez of the University of Córdoba first detected a genetic signature related to leprosy in the ancient bones, he was cautious. After detailed analysis, the team confirmed that the DNA belonged to M. lepromatosis—a strain of leprosy that is much rarer today and has not been well studied due to the scarcity of available genomes.
Lesley Sitter, a postdoctoral researcher on Bos’s team, noted that the DNA from the ancient bones was remarkably well-preserved—something rare in samples this old. This allowed the team to reconstruct the full genetic makeup of the ancient bacteria, offering a clearer picture of its evolutionary history.
The findings suggest that M. lepromatosis was present and causing disease in the Americas long before European colonization, and that it may have followed a different path of development from the more common M. leprae strain. While M. lepromatosis has recently been found in red squirrels in the UK and Ireland, in the Americas, it has only been identified in humans.
There’s still much to uncover. Scientists don’t yet know whether M. lepromatosis originated in the Americas or if it arrived with early human migrations from Eurasia. As more ancient and modern genomes are discovered and compared, researchers hope to better understand how this disease spread and evolved across the globe.
Rodrigo Nores, a professor of anthropology at the University of Córdoba, believes this is just the beginning. Now that researchers know what to look for, they expect to find more evidence of ancient leprosy in the Americas.
This groundbreaking study has been published in Nature Ecology & Evolution, opening a new chapter in the story of one of humanity’s oldest diseases.
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