For generations, the history of leprosy in the Americas has been framed as a consequence of the "Columbian Exchange," the vast transfer of plants, animals, culture, and human populations—along with their diseases—between the New World and the Old World following the voyages of Christopher Columbus. It was a long-held scientific and historical consensus that leprosy, a chronic infectious disease that has haunted human civilization for millennia, was introduced to the American continent by European colonizers and the subsequent transatlantic slave trade. However, groundbreaking research is now fundamentally altering this narrative, revealing that the history of the disease in the Western Hemisphere is far older and more complex than previously understood.
A major international study led by scientists from the Institut Pasteur, the CNRS, and the University of Colorado, in collaboration with a wide array of institutions across America and Europe, has uncovered evidence that a recently identified second species of bacteria responsible for leprosy, Mycobacterium lepromatosis, has been present in the Americas for at least 1,000 years. This timeline places the pathogen on the continent several centuries before the first European ships sighted American shores. The findings, which represent a significant shift in the fields of paleopathology and microbial evolution, are set to be published in the prestigious journal Science on May 29, 2025.
Leprosy, also known as Hansen’s disease, remains a significant public health challenge today. Classified by the World Health Organization as a neglected tropical disease, it continues to affect approximately 200,000 people every year through new diagnoses. While the primary cause of the disease has historically been attributed to the bacterium Mycobacterium leprae, the scientific community was introduced to a second causative agent, Mycobacterium lepromatosis, only relatively recently. This second species was first identified in 2008 in a Mexican patient living in the United States and was later found in 2016 to be infecting red squirrels in the British Isles. Despite its discovery, much about the origins, spread, and historical prevalence of M. lepromatosis remained a mystery until now.
To solve this evolutionary puzzle, the research team, spearheaded by the Laboratory of Microbial Paleogenomics at the Institut Pasteur—an entity also associated with the CNRS—and the University of Colorado, embarked on an exhaustive genomic survey. The study was a massive collaborative effort involving over 40 scientists from international institutions, including microbiologists, geneticists, and archaeologists. Crucially, the project was conducted in close partnership with Indigenous communities, ensuring that the study of ancestral remains was handled with the utmost ethical consideration and cultural sensitivity.
The researchers analyzed DNA from nearly 800 samples, a massive dataset that spanned both time and geography. These samples included ancient human remains recovered from archaeological excavations as well as modern clinical cases from patients presenting with symptoms of leprosy today. By comparing the genetic signatures of the ancient and modern bacteria, the team was able to reconstruct the evolutionary history of the pathogen with unprecedented precision. The results provided definitive proof that M. lepromatosis was not only present but was already widespread across both North and South America long before the arrival of Europeans.
