Scientists once believed Europeans brought leprosy to the American continents via infection from the bacterial species Mycobacterium leprae.

But now a new study published in the journal Science reveals that a different form of leprosy-causing bacteria — called Mycobacterium lepromatosis — was already circulating in the Americas for at least one thousand years. 

Leprosy was therefore already affecting American indigenous peoples well before European colonization.

The study authors analyzed more than 800 samples taken from ancient remains in Canada and Argentina.

The genomes of the bacteria taken from the samples were reconstructed, analyzed, and dated. 

Comparisons between the samples showed the bacterial genomes were of distinctive branches of the lepromatosis species at each end of the continent. 

However, they remained genetically similar. This suggested that the bacteria species had spread rapidly across the Americas, probably covering the landmass in just a few hundred years. 

Leprosy is caused by two bacteria species, not one.

Leprosy has been infecting humans for thousands of years. The disease presents as multiple numbing skin lesions. If left untreated, it can result in nerve damage, muscle weakness, paralysis and blindness. Today, leprosy can be treated with antibiotics, but ancient sufferers weren't so fortunate. 

Skeletal records from 2,000BCE have been found in India with traces of the disease caused by Mycobacterium leprae.

These are matched by written records of leprosy cases in ancient literature from Indian, Chinese and African civilizations, as well as stories in the Abrahamic religions. 

Often, these ancient descriptions associated the affliction with stigmas of immorality or ritual uncleanliness. 

But in 1874 the Norwegian doctor Gerhard Armauer Hansen discovered that leprosy was caused by the microscopic organism Mycobacterium leprae. 

In 2008, doctors in Mexico found another leprosy-causing bacteria species — Mycobacterium lepromatosis — in a leprosy patient.

Before this, it was believed leprae was the only pathogen capable of causing the disease. Now both forms of the bacteria are known to cause it.

Nicolas Rascovan, head of the Microbial Paleogenomics Unit at the Pasteur Institut in France led the investigation. He and his colleagues estimate lepromatosis and leprae diverged from a common ancestor about one million years ago. 

"The diversification happened probably independent of humans," Rascovan told DW.

The arrival of the first European fleets to the Americas in 1492 marked the introduction of new diseases to the Americas. Leprosy — in the form of the leprae bacterium — was among them.

Archaeological evidence has shown leprae migrated with human groups out of Africa and into Asia and Europe around 40,000 years ago. 

Its introduction to the Americas, along with other diseases, by Europeans devastated indigenous communities and intensified the impact of pathogens that were already circulating before colonization. 

The discovery of lepromatosis' longer history on the continent further highlights the diversity of pathogens and their complex relationship with humans throughout history, said Rascovan.

"Europeans had a very important impact by bringing this new species [leprae] that was absent in America," he said.

Rascovan hopes the presence of lepromatosis in the archeological record will improve understanding of pre-colonial disease, especially in the absence of written records.

In addition, the study helps understand modern cases of leprosy, especially how it could make the jump from animals like squirrels to humans. 

"Our work is giving the kick start to really start analyzing, monitoring and understanding the diversity of natural reservoirs [disease carriers]," said Rascovan. 

He said monitoring the disease and preventing spillovers from animals to humans should be a priority. 

The disease is still prevalent today — 200,000 cases are reported each year globally. Brazil, India and Indonesia still report more than 10,000 new cases annually, according to WHO data.

Edited by: Fred Schwaller