Leprosy was present on the American continent before the arrival of European settlers.

Leprosy was present in the Americas before the arrival of European settlers. This is shown by a large-scale investigation in the journal Science , published on May 29, in which more than 50 researchers from France, North America, and South America participated. It sheds new light on the precolonial history of infectious diseases on this continent, isolated from the rest of the world for thousands of years.

Affecting 200,000 new people each year , leprosy, which is characterized by skin and nerve lesions, can be caused by two pathogens. The first, Mycobacterium leprae , known since the ^19th century, was indeed introduced by European settlers in America and remains responsible for the majority of cases on the continent. But it is the second, Mycobacterium lepromatosis , discovered only in 2008 in Mexico, that is the focus of the study.

Until now, scientists had not sequenced only ten genomes of this species, in Mexican patients and in red squirrels from the British Isles (Ireland, England, Scotland), and thus identified two lineages. "It was therefore impossible to know whether the pathogen had been imported from the Americas to Europe or vice versa," underlines Nicolas Rascovan, head of the microbial paleogenomics unit at the Pasteur Institute, who coordinated the investigation.

The scientist made his first discovery in 2017: in a database of ancient DNA, he identified the genetic signature of the bacillus in an individual who had lived in what is now Canada 1,300 years ago. "It was a fairly significant starting point, because it meant that leprosy was present in America before the arrival of Europeans, contrary to what the scientific community believed ," he explains. "But we lacked context."

The team then began working on more than 800 ancient and modern DNA samples in 2020. Maria Lopopolo, the first author of the article, who was a doctoral student in the lab at the time, insisted on also studying human remains from the south of the continent. "When she showed me her results, I initially thought she had made a mistake somewhere. But she was right: she had just found a case of leprosy in Argentina, around 900 years ago. All of a sudden, we discovered that it wasn't just a northern pathogen, but a continental one," the researcher recalls.

The two lineages – Canadian and Argentinian – are thought to have shared a common ancestor two thousand years ago, meaning that the bacteria's spread would have taken only a few centuries. "That's very fast for a continent 10,000 kilometers long," the scientist points out. In total, 26 DNA samples (3 ancient and 23 modern) out of the approximately 800 studied contained the leprosy bacillus and were usable. They allowed the identification of three new lineages within the species Mycobacterium lepromatosis , in addition to the two already known.

"The work presented in this study is remarkable," praises Emmanuelle Cambau, a researcher at the National Reference Centre for Mycobacteria and Mycobacterial Resistance to Antituberculosis Drugs, and a member of the World Health Organization's Leprosy Control Group. She believes the investigation opens up avenues for research in other regions: "A few cases of leprosy due to Mycobacterium lepromatosis have also been observed in Asia. It remains to be studied to determine which branch they belong to and to identify the ancestors from which they might have descended."

A rare occurrence in ancient DNA research: the indigenous communities involved, in Argentina and Canada, gave their consent for the study of human remains and approved the final version of the article. "We are aware that the history of these populations is terrible. So, it is important for us to involve them in the construction of a respectful and useful narrative. We have scientific but also social objectives," explains Nicolas Rascovan.

In a WhatsApp message sent from Las Grutas, a town on Argentina's Atlantic coast, Fernando Ledesma, a member of the Traun Kutral community, expressed his satisfaction with the "very fluid" interaction with scientists and the "back-and-forth of knowledge" it has enabled. "It is essential that we be consulted when our territories and sacred spaces are affected. And that we stop being seen through an outdated vision of science, as if we were prisoners of it instead of being part of it," he said.