Leprosy Was in the Americas Long Before the Arrival of Europeans

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The narrative around leprosy has been always been that it's this awful disease that Europeans brought to America," says Nicolas Rascovan at the Pasteur Institute in Paris. Well, our discovery changes that."

The vast majority of leprosy cases worldwide are caused by the bacterium Mycobacterium leprae. But in 2008, Xiang-Yang Han at the University of Texas MD Anderson Cancer Center and his colleagues discovered a second causative agent, M. lepromatosis, in two people from Mexico who had leprosy. Since then, scientists have found more cases of this pathogen in the US, Canada, Brazil and Cuba - as well as in four people in Singapore and Myanmar.

Wanting to know more about this understudied pathogen, Rascovan teamed up with Han and other researchers, as well as Indigenous communities, to analyse ancient DNA from 389 people who lived in the Americas before European contact.

They found M. lepromatosis in the remains of one person near the Alaska-Canada border and two others along the south-eastern coast of Argentina, all carbon-dated to about 1000 years ago. The bacteria's genomes varied slightly, hinting at distinct strains separated by around 12,000 kilometres. It spread so fast, on a continental level, in just a matter of centuries," says Rascovan.

DNA from dozens of modern cases - mostly from the US and Mexico - revealed that nearly all contemporary strains are essentially clones, showing only minor changes since ancient times. But the team also identified one rare and unusually ancient strain in a modern person that hadn't turned up in archaeological remains, suggesting that at least two distinct lineages of M. lepromatosis are still infecting people in North America today - alongside the M. leprae strains introduced by Europeans.

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