It turns out that leprosy, that ancient affliction steeped in biblical lore, has a deeper and more unexpected backstory than previously thought. While historians and scientists long believed the disease traveled to the Americas aboard European ships, fresh evidence is now painting a different picture, one with roots that stretch back thousands of years before the first European footsteps ever touched American soil.
A groundbreaking study published in Science has turned a major chapter of medical history on its head. A global team of researchers has found genetic traces of Mycobacterium lepromatosis, a lesser-known cousin of the more infamous Mycobacterium leprae, in pre-Columbian human remains from North and South America. This startling revelation challenges the widely held notion that leprosy was solely an Old World import.
An Ancient Illness With Modern Revelations
Leprosy, more formally known today as Hansen’s disease, has long held a place in the annals of human suffering. The illness attacks the skin, nerves, and mucous membranes, causing progressive disfigurement and, if untreated, permanent disability. In its most classic form, the condition is caused by M. leprae. But a new bacterial player has entered the scene: M. lepromatosis, first described in 2008.
Unlike its more famous microbial sibling, M. lepromatosis has a much more elusive profile. It’s harder to detect, more difficult to study, and was once thought to be regionally confined. Most cases appear to cluster in parts of North and South America, and until recently, scientists assumed it was a more modern variant.
But that assumption is now being reevaluated.
A Clue in the Bones
Nicolas Rascovan, a researcher at the Pasteur Institute in Paris, wasn’t specifically hunting for ancient leprosy when his team stumbled across genetic fragments of M. lepromatosis in centuries-old human remains. Their initial find came from a skeleton unearthed in North America, one that predated any known European interaction with the continent.
“This discovery really surprised us,” Rascovan said. “It raised an immediate question: how long had this pathogen been around in the Americas before European arrival?”
To answer that, the team embarked on a meticulous investigation. They analyzed both contemporary clinical samples and ancient DNA from archaeological remains dating back over a thousand years. Among the samples, three individuals—two from North America and one from South America—tested positive for M. lepromatosis. The evidence firmly placed the bacteria in the Americas centuries before the arrival of European settlers.
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Rewriting the Origins of Leprosy in the Americas
The prevailing belief for decades was that leprosy, like smallpox and measles, arrived in the Americas with European colonizers. While this still holds true for M. leprae, the presence of M. lepromatosis tells a different tale.
“This changes the narrative entirely,” Rascovan explained. “Our results indicate that M. lepromatosis was already circulating in the Americas long before Europeans stepped ashore, possibly evolving in the region for thousands of years.”
In other words, the New World had its own strain of leprosy—indigenous, ancient, and quietly persisting for millennia.
Why This Matters for Modern Medicine
While leprosy may sound like a disease from the distant past, it’s not entirely a relic. Globally, over 200,000 new cases are still reported each year. The United States sees about 200 annual cases, often linked to contact with wild animals like armadillos. Though modern antibiotics can treat the disease, it remains a persistent challenge in parts of the world with limited healthcare access.
The researchers’ findings introduce a new layer of complexity. They discovered that although many modern cases of M. lepromatosis stem from a relatively recent and genetically uniform lineage, other strains appear to have diverged thousands of years ago. Some may have been quietly evolving in the Americas for more than 9,000 years.
This revelation has led scientists to believe that M. lepromatosis may have been lurking within unknown animal hosts for centuries, eluding detection. The traditional form of leprosy has been linked to armadillos in the U.S., but it’s possible that M. lepromatosis has its own mysterious reservoir species.
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Could M. lepromatosis Be the Next Big Bacterial Surprise?
Although M. leprae remains the dominant strain globally, M. lepromatosis may warrant greater attention. Its long-standing presence, genetic diversity, and potential to evolve independently could indicate it behaves quite differently from its more well-studied counterpart.
“We may be looking at an emerging pathogen that’s been under the radar,” said Rascovan. “Its spread, its evolution, and its interaction with both humans and animals could follow a very different pattern than what we’ve observed with M. leprae.”
This theory is already finding support in surprising places. One strain of M. lepromatosis is thought to have reached the British Isles in the 19th century, where it continues to infect red squirrels—a peculiar twist in the microbe’s story.
From the Americas to the World—But How?
Even with the current data, some mysteries remain. How did M. lepromatosis make its way out of the Americas, if it began there? Did it travel silently across trade routes or stow away in animal hosts? Researchers suspect the bacteria reached Asia at some point, given the presence of cases there today, but no one knows for sure how it got there.
Unraveling this bacterial journey could shed light not only on ancient human migration but also on the movement of pathogens that have quietly shaped human history.
The Bigger Picture: A Neglected Disease, a Neglected Pathogen
Despite the historical weight that leprosy carries, it’s often overlooked in modern discussions about infectious diseases. That’s even more true for M. lepromatosis, which has largely existed in the shadow of its better-known relative.
Rascovan emphasized this point: “This isn’t just a case of a forgotten disease. It’s about a forgotten microbe—one that may still be shaping human and animal health in ways we’re only beginning to understand.”
The team’s next steps involve locating potential animal hosts that could be serving as reservoirs for M. lepromatosis. Identifying these could help prevent future outbreaks and illuminate how the pathogen travels between species and across continents.
Why This Research Matters Beyond the Lab
The story of M. lepromatosis serves as a reminder that our understanding of disease history is far from complete. With every new discovery, we peel back another layer of the complex relationship between humans and microbes—some of which have been with us for tens of thousands of years.
It also raises broader questions about what else might be hiding in ancient remains or unexplored regions of our microbial ecosystem. Could other diseases we assume to be imports have actually originated or independently evolved in different parts of the world?
As Rascovan and his team continue to chase down clues from both ancient bones and living patients, one thing is clear: the history of leprosy is more complicated—and far more fascinating—than anyone previously believed.
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Conclusion: Time to Rethink What We Thought We Knew
In science, the past has a habit of surprising the present. The discovery of M. lepromatosis in pre-contact American remains forces us to reconsider assumptions that had long been taken as truth. The idea that all leprosy came to the Americas via European colonizers has now been upended by hard genetic evidence. What was once a linear narrative now reveals itself to be more of a web—interconnected, ancient, and still largely mysterious.
And so, a disease that once evoked fear and stigma is again demanding our attention, not because of a surge in cases, but because of what its story says about us, our history, and our microscopic companions.
