Scientists have discovered that leprosy existed in the Americas a thousand years before European explorers arrived, overturning centuries of medical history about how the disease spread throughout the New World.
At a Glance
- Researchers identified Mycobacterium lepromatosis, a previously overlooked species of leprosy bacteria, in ancient remains from Argentina to Canada
- The discovery proves leprosy was endemic among Indigenous populations at least 1,000 years before European contact
- Genetic analysis of nearly 800 ancient and modern DNA samples shows remarkably similar strains across the continent
- The bacteria may have arrived with early human migrations or existed in unknown animal hosts
- This finding challenges the long-held belief that European explorers introduced leprosy to the Americas
Rewriting Disease History
A groundbreaking study from the Institut Pasteur in Paris has upended conventional wisdom about leprosy’s introduction to the Americas. Researchers working with the French National Centre for Scientific Research and the University of Colorado examined over 800 DNA samples from ancient human remains and recent medical cases, uncovering compelling evidence that a form of leprosy existed in the Western Hemisphere long before Columbus and other European explorers arrived. The bacterium, Mycobacterium lepromatosis, was found in Indigenous remains dating back at least a millennium.
“This discovery transforms our understanding of the history of leprosy in America. It shows that a form of the disease was already endemic among Indigenous populations well before the Europeans arrived,” explains Dr. Maria Lopopolo, one of the study’s lead researchers.
A new Science study shows that the leprosy-causing mycobacterium M. lepromatosis infected humans in the Americas before European contact.
The findings reshape current views of leprosy in the Americas and provide insights into the long-term interactions between humans and… pic.twitter.com/TdAI74kGwj
— Science Magazine (@ScienceMagazine) June 3, 2025
Ancient Genomic Evidence
The research team analyzed nearly 800 ancient and modern DNA samples from North and South America. Using advanced genetic techniques, they reconstructed bacterial genomes from ancient individuals across a vast geographic range—from northern Canada to southeastern Argentina. Unlike Mycobacterium leprae, which typically causes bone lesions that can be identified in skeletal remains, M. lepromatosis affects blood vessels and internal organs, making its detection in ancient populations particularly challenging without DNA analysis.
“We are just beginning to uncover the diversity and global movements of this recently identified pathogen. This study allows us to hypothesize that there might be unknown animal reservoirs,” said Nicolás Rascovan from the Institut Pasteur in Paris.
The genomes extracted from ancient samples were surprisingly similar, indicating a rapid, continent-wide spread of the pathogen. Additionally, researchers discovered that most modern strains are nearly identical to ancient samples, suggesting a long-standing presence in the Americas. The Centers for Disease Control and Prevention notes that leprosy affects nerves, skin, and eyes, and though historically feared, is now treatable with antibiotics.
Origins and Migration Patterns
First identified in Mexican patients in 2008, M. lepromatosis has since been found across the Americas and in isolated cases in Asia and the United Kingdom. Genetic studies suggest that strains found in the British Isles likely originated in the Americas and may have been transported to Europe in the 19th century—reversing the traditionally assumed direction of transmission. This bacterium diverged from M. leprae between 700,000 and 1 million years ago, long before human migration to the Americas.
Researchers are now investigating how the bacterium arrived in the Americas—possibly with early human migrations or perhaps evolving in native animal hosts before spreading to humans. The discovery of an exceptionally rare and ancient strain in a living North American patient suggests multiple lineages of M. lepromatosis may still exist today. This research, conducted with ethical involvement of Indigenous communities, opens new avenues for understanding the history and epidemiology of infectious diseases that have shaped human history.
