A long-standing historical narrative has held that leprosy, one of the world’s oldest and most stigmatized diseases, was introduced to the American continent by European colonizers during the age of exploration. This "Columbian Exchange" theory suggested that the arrival of settlers, and the subsequent transatlantic slave trade, brought various pathogens—including the bacteria responsible for leprosy—to populations that had no prior exposure. However, groundbreaking research from an international coalition of scientists is now overturning this consensus. According to a study to be published in the journal Science on May 29, 2025, a second species of leprosy-causing bacteria has been circulating in the Americas for at least a millennium, predating the arrival of Christopher Columbus and European settlers by several centuries.
The research, led by scientists from the Laboratory of Microbial Paleogenomics at the Institut Pasteur, in association with the CNRS and the University of Colorado, reveals that Mycobacterium lepromatosis has a much deeper and more complex history on the American continent than previously understood. This discovery not only shifts the timeline of the disease’s presence in the New World but also provides a significant new perspective on how human pathogens evolve, diversify, and migrate across vast distances. By analyzing nearly 800 samples ranging from ancient archaeological remains to modern clinical cases, the team has reconstructed a narrative of a pathogen that was already widespread across both North and South America long before the first European ships appeared on the horizon.
Leprosy, also known as Hansen’s disease, remains a significant public health challenge today. Classified as a neglected tropical disease, it continues to affect approximately 200,000 people annually, with new cases reported in various regions across the globe. For decades, the medical and scientific community primarily attributed the disease to the bacterium Mycobacterium leprae. It was only relatively recently, in 2008, that a second species, Mycobacterium lepromatosis, was identified in a Mexican patient in the United States. This second species was later found in 2016 to be infecting red squirrels in the British Isles, suggesting a wider ecological footprint than initially suspected. The new study published in Science focuses specifically on this second species, tracing its genetic lineage back through time to uncover its hidden history in the Western Hemisphere.
The investigation was a massive undertaking, involving over 40 scientists from international institutions, including microbiologists, geneticists, and archaeologists. Central to the study’s success was the collaboration with various Indigenous communities, ensuring that the research was conducted with respect for ancestral remains and cultural heritage. By examining the DNA extracted from human remains found in archaeological excavations—some dating back over 1,000 years—the researchers were able to piece together the genetic puzzle of M. lepromatosis. These ancient samples were compared against modern clinical data to create a comprehensive map of the pathogen’s evolution and movement.
"This discovery transforms our understanding of the history of leprosy in America," explained Dr. Maria Lopopolo, the study’s first author and a researcher at the Laboratory of Microbial Paleogenomics at the Institut Pasteur. According to Dr. Lopopolo, the data confirms that a form of the disease was already endemic among Indigenous populations well before the 15th century. This realization challenges the colonial-centric view of disease history and underscores the importance of utilizing advanced genomic tools to re-examine the past. The presence of the bacteria in individuals who lived a millennium ago suggests that the pathogen was a stable, if perhaps under-recognized, part of the microbial landscape of the pre-Columbian Americas.
