Dr. Maria Lopopolo, the study’s first author and a researcher at the Laboratory of Microbial Paleogenomics at the Institut Pasteur, emphasized the magnitude of these findings. She noted that this discovery transforms the current understanding of the history of leprosy in America, showing that a form of the disease was already endemic among Indigenous populations well before the colonial era began. This realization challenges the traditional view of the Americas as a "pristine" environment regarding certain infectious diseases prior to 1492 and highlights the long-standing biological relationship between Indigenous peoples and the pathogens in their environment.
One of the most striking aspects of the study involved the use of advanced genetic techniques to reconstruct the full genomes of M. lepromatosis from ancient individuals discovered in locations as geographically diverse as Canada and Argentina. Despite being separated by several thousand kilometers, the ancient strains found in these regions, which date back approximately 1,000 years, were found to be remarkably similar in their genetic makeup.
In the complex evolutionary tree of the genus Mycobacterium, these ancient American strains belong to two distinct branches. However, the study found that these two branches are more closely related to each other than to any other known branch of the bacteria. The scientists concluded that such genetic proximity across such a vast geographic distance could only be explained by a rapid spread of the pathogen across the continent. It is likely that M. lepromatosis moved through human populations across the Americas within just a few centuries, a feat of transmission that suggests the pathogen was highly adapted to the movement and social structures of the continent’s early inhabitants.
Beyond the 1,000-year-old samples, the genomic analysis also identified several previously unknown lineages of the bacteria. Among these was an ancestral branch that appears to have diverged from the rest of the species’ known diversity more than 9,000 years ago. Remarkably, this ancient lineage has not vanished into history; the study found that it continues to infect humans in North America today. This discovery points to an incredibly deep and long-lasting diversification of the pathogen on the American continent. It also suggests that there is likely a vast, largely unexplored reservoir of genetic diversity within these mycobacteria that remains to be discovered by science.
The study’s findings also reached across the Atlantic, shedding new light on the 2016 discovery of M. lepromatosis in red squirrels in the United Kingdom. At the time, the presence of the American-associated pathogen in British wildlife was a source of scientific curiosity. The new analysis suggests a surprising reversal of the typical colonial narrative: rather than the disease moving from Europe to the Americas, the strains found in the UK squirrels appear to be part of an American lineage that was introduced to the British Isles during the 19th century.
