Leprosy in America was long thought to originate exclusively from European colonization, but its origins go deeper.
By Helen Petre
Leprosy is thought to be a disease introduced to the Americas during European colonization, along with smallpox and influenza. It is one of the oldest and most stigmatizing diseases in human history, known throughout the world, and described in the Bible. Leprosy is caused by the bacteria Mycobacterium leprae, and no other species were known until 2008, when a second species, M. lepromatosis, was identified in the Americas. A team of researchers from the Pasteur Institute published genetic evidence in the journal Science (May 2025) that shows that M. lepromatosis may have been widespread in the Americas before European colonization.
European diseases
European colonization resulted in devastating consequences for Indigenous Americans through the introduction of guns and other weapons, iron tools, Christianity, Roman law, sugarcane and wheat, cattle and horses, and disease. There is little evidence of disease among native Americans before colonization; the exception being syphilis. It is possible that the lower population density and lack of trade with other continents prevented widespread disease transmission, but for whatever reason, Indigenous Americans appeared very healthy before colonization. When the Europeans came, there was massive disease: smallpox, cholera, influenza, whooping cough, measles, and leprosy. This new research shows that there may have been leprosy in America before colonization, but it was caused by a different species of bacteria than the one that is most common worldwide, even in America today.
The researchers found genetic evidence that a different species of the bacteria that causes leprosy was in 1,000-year-old skeletal remains in Argentina and also in similarly aged skeletal remains in British Columbia. This indicates that the bacterial species was infecting individuals throughout North and South America at approximately the same time, pre-colonization.
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Leprosy in America and globally
Leprosy, also known as Hansen’s disease for the Norwegian physician Gerhard Hansen, causes nerve damage and deformation of the hands and face. There is an inability to feel pain, muscle weakness, and poor eyesight. The disease is spread through human contact, both skin and respiratory, and there appears to be a genetic resistance in some people. The disease can only be confirmed through identifying the bacteria in the skin lesions. It is curable with six months of antibiotics. People still get leprosy; mostly in India, but even in Central Florida in the US. Until 2008, the only species known to cause leprosy was M. leprae, and antibiotics are free from the World Health Organization for anyone with leprosy.
Leprosy is not that contagious, perhaps because some people have immunity. Ironically, people who are in direct contact with those who have been affected with leprosy do not always contract leprosy. However, some do. In Biblical times and up until the 1950s, people with Hansen’s disease were separated from the rest of the world in “leper colonies,” probably because it is a frightening disease. There are many examples in the Bible where people with leprosy were excluded from the community and had to constantly ring bells and yell “Unclean” so others did not inadvertently become infected. So sad.
New species identified
Up until about 2007, few people contracted leprosy, and they were mostly not in America. If someone did contract the disease, they received a free cocktail of antibiotics and after a year or so, they were OK. There was not much interest in leprosy or identifying species of bacteria that cause the disease.
In 2007, Dr. Nair, a physician in Phoenix, contacted Dr. Xiang-Yang Han (one of the researchers behind the recent Science article) to get help diagnosing and treating a Mexican patient. Dr. Han is a cancer doctor, but he knows a lot about DNA and specifically 16S ribosomal RNA, the bacterial ribosomes. Dr. Nair’s patient had extensive leg wounds, developed a high fever and shock, and died after 10 days in intensive care. Dr. Nair analyzed tissue and identified diffuse leprosy, a form first described in Mexico in 1852. Dr. Han had developed a way to analyze small differences in the 16S ribosomal RNA gene. By comparing differences, Han could identify different bacteria, even if they were difficult to grow in the lab, like Mycobacteria species. Han described his technique as identifying a fingerprint of the bacteria.
Han identified the bacteria that caused the man’s death as Mycobacteria, but he found it too different from M. leprae to be M. leprae. It was named M. lepromatosis. So, in 2008, there were two different bacteria known to cause leprosy. Dr. Han published his research in 2008 in the American Journal of Clinical Pathology, describing the genetic fingerprint of M. lepromatosis.
Ancient DNA contains M. lepromatosis
In 2018, Dr. Rascovan from the Pasteur Institute screened databases of ancient DNA derived from archaeological sites, looking for Dr. Han’s M. lepromatosis. He found it in the remains of a person who lived around 1,000 years ago in British Columbia. His graduate student Maria Lopopolo also found leprosy in America through DNA searches—from two sites in Argentina also dated around that time.
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Modern leprosy patients
Dr. Rascovan decided to screen modern leprosy patients in the Americas to see if maybe their diseases were caused by the new species. The researchers screened 408 leprosy patients from the Americas. They found 34 individuals had the M. lepromatosis species. It seems that leprosy in America can be caused by this new species. More research and testing should be done to understand the geographic distribution of the different species, since both occur in the Americas. Do both occur in the rest of the world too? It is interesting to note that the different species cannot be identified by examining symptoms. The symptoms are drastically different in patients regardless of type of bacteria causing infection. Little is known about why some patients have severe symptoms and others are spared. Little is known about why some people contract the disease and others, who are equally exposed, do not.
Questions remain
The paper clearly shows evidence of M. lepromatosis in the Americas pre-colonization. The disease it causes clearly is here now, as Dr. Nair’s patient died most probably of leprosy caused by M. lepromatosis.
How did it get to America? When? Why don’t we have evidence of leprosy in Indigenous Americans pre-colonization? Is there an animal reservoir, such as armadillos, that carry the bacteria but are not affected? Why was this not noticed, or understood before? Europeans, Asians, and Africans clearly noticed that some people had leprosy probably 1,000 years ago. Why didn’t the Americans? Europeans should have noticed a disease among the Indigenous Americans as disfiguring as leprosy. The man who died of leprosy in 2008 shows that it was very disfiguring. It is possible that it was less symptomatic, or severe, hundreds of years ago, but considering the bacteria was identified in 1,000-year-old remains, it seems likely that there were symptoms.
Since European colonization, leprosy is most often caused by M. leprae even in America . . . Why? Why are people in Europe, Asia, and Africa not infected with M. lepromatosis? It is possible that they are, we just didn’t screen them.
Looking forward to future research
This research, and Han’s original identification of the M. lepromatosis bacteria, are important contributions to understanding the historical disease transmission and potential implications of resistance to leprosy. They are important contributions to the understanding of the evolution of the bacteria that cause leprosy in different geographic regions.
It is interesting that leprosy caused by M. leprae results in a vastly different clinical presentation in different people, even though the bacteria is the same. It is likely that symptoms of leprosy caused by M. lepromatosis are also vastly different and it is possible that there is genetic resistance in some populations. It is also likely that the symptoms of the disease, regardless of causal agent, are indistinguishable. There is much to learn about this dreadful disease, but thankfully there is a cure.
This study was published in the peer-reviewed journal Science.
References
Lopopolo, M., Avanzi, C., Duchene, S., . . . Rascovan, N. (2025). Pre-European contact leprosy in the Americas and its current persistence. Science 0, eadu7144. https://doi.org/10.1126/science.adu7144
University of Texas M. D. Anderson Cancer Center. (2008, November 28). New leprosy bacterium: Scientists use genetic fingerprint to nail “killing organism.” Science Daily. https://www.sciencedaily.com/releases/2008/11/081124141047.htm
Wikipedia. (2025). Leprosy. https://en.wikipedia.org/wiki/Leprosy.
Featured image: Researchers can take ancient DNA found on a sample such as this tooth to reconstruct human and pathogen genomes. The background is a Wiphala flag, representing Indigenous communities of South America. Credit: Nichola Rascovan, Institut Pasteur
About the Author
Helen Petre is a science communicator with Science Connected. She is a retired biologist who continues to learn, write, and teach, hoping to share her experiences and quest for scientific knowledge with a new generation of conservation stewards and scientists.
