Leprosy's Origins Are Even Weirder Than We Thought

Gizmodo

29-05-2025

New research complicates the narrative of how leprosy reached the Americas.
One of the world's oldest diseases has an even more complex history than expected. Research out today reveals that a form of leprosy was stalking people in the Americas long before Europeans arrived.
A large international team of scientists conducted the study, published Thursday in Science. The researchers found DNA evidence of Mycobacterium lepromatosis—a recently discovered species of leprosy-causing bacteria—infecting people in North and South America over 1,000 years ago, predating European colonization. The findings upturn the prevailing belief that Europeans were responsible for leprosy's introduction to the New World.
Leprosy's reputation for human destruction is so notorious that it's repeatedly referenced in the Bible (some of these references might have been about other conditions, though). These bacteria can cause a chronic infection of our skin and nerve cells, though it may take years for symptoms like lesions and sores to appear. If not treated with antibiotics, the infection can progressively and permanently damage these cells, leading to lifelong disabilities and other serious complications.
Leprosy, also called Hansen's disease, is primarily caused by Mycobacterium leprae bacteria. But in 2008, scientists discovered that a similar species, M. lepromatosis, can cause it, too. Leprosy from M. lepromatosis appears to be rarer than the former, with cases predominantly found in North and South America. And we know very little about it, partly because both species aren't easy to culture in the lab (much like viruses, leprosy-causing bacteria can only replicate by hijacking our cells from the inside).
Study author Nicolas Rascovan, who specializes in the evolution of human diseases, and his team decided to take a closer look at the bacteria after making a discovery of their own.
'Our interest began when we unexpectedly detected M. lepromatosis DNA in the remains of an ancient individual in North America,' Rascovan, a researcher at the Pasteur Institute in Paris, France, told Gizmodo. 'That prompted us to ask how long this pathogen had been circulating in the continent, how much spread there was before European contact, and what diversity it might still hold today.'
The team analyzed hundreds of samples collected from modern-day people (mostly recent leprosy patients), as well as ancient DNA samples recovered from people in the Americas before European contact. Three of these ancient samples tested positive for the bacteria, the researchers found, from people living in what's now Canada and Argentina.
'Leprosy has long been considered a disease brought to the Americas by Europeans, which is true for the case of M. leprae. But our study shows that at least one of the two species that causes it—M. lepromatosis—was already here centuries earlier, and probably evolved locally for thousands of years,' Rascovan explained. 'This essentially changes how we understand the disease's history in the Americas.'
Thanks to antibiotics and improved sanitation, leprosy is rare in much of the world (only about 200 cases are documented in the U.S. every year). But it remains a public health issue in some countries, with more than 200,000 cases reported annually globally. And the researchers' findings may complicate both the past and future of leprosy.
While most modern cases of M. lepromatosis were linked to one specific lineage, or clade, of the bacteria that has recently expanded, the team also identified older clades that have likely been evolving independently in the Americas for over 9,000 years. That suggests these bacteria have been hiding out in still mysterious animal hosts (in the U.S. the classic form of leprosy has sometimes been linked to armadillo exposure). And though M. leprae still causes the majority of leprosy cases worldwide, its less famous cousin certainly has the potential to become a bigger threat.
'Given its diversity and spread, M. lepromatosis could be an emerging pathogen with dynamics different from M. leprae,' Rascovan said.
As is often the case in science, the team's work has raised even more questions for them to answer. They're hoping to unearth the animal reservoirs where the bacteria have potentially been circulating all this time, for instance, and to trace its journey from the Americas to other parts of the world. They now believe that a strain of M. lepromatosis was brought over to the British Isles sometime in the 19th century, where it still infects red squirrels today. But we're still in the dark about how it reached Asia, where cases have been documented.
'All of this suggests that this is not just a neglected disease, but a neglected pathogen—one whose history and spread are only now beginning to be understood,' Rascovan said.