The bacterium is very difficult to study. Its genome is filled with damaged, nonfunctional genes, which may explain why it grows so slowly.
Apart from humans, it afflicts only armadillos and the footpads of mice. Scientists cannot grow the bacterium in the laboratory.
"Even if we could grow it on petri dishes, it would take 9 to 12 months to form a colony," Cole said.
So instead Cole and his colleagues compared the genomes of seven strains of the bacterium taken from leprosy patients around the world. The scientists found very little variations between the strains. This, they say, suggests that there was a single clone at the origin of all cases of leprosy.
"Their analysis appears to explain some of what we know about the disease leprosy and the [source] organism that causes leprosy, namely the slow growth of the organism and progression of the disease," said Kenrad Nelson. Nelson is an epidemiologist and leprosy expert at Johns Hopkins University in Baltimore, Maryland, and was not involved in the research.
By analyzing very rare mutations in individual molecules in the bacterium, the scientists were able to trace how the disease spread over the course of human history.
"By comparing the evolutionary scheme for [the bacterium] with the map of known human migrations, we found some striking parallels and differences," Cole said.
Researchers previously believed that leprosy originated on the Indian subcontinent before being introduced to Europe by Greek soldiers returning from the India campaign of Alexander the Great.
The new findings, however, indicate that the disease actually originated in East Africaor perhaps the Middle East.
The scientists say Europeans and North Africans spread the disease to West Africa. From there the slave trade brought it to the Caribbean, South America, and North America.
Colonialism played a major part in the spread of leprosy, Cole said. "It is clear that Europeans introduced the disease to the Americas themselves and by slavery. Other human migrations in history almost certainly did the same thing."
Erwin Schurr, a researcher at McGill University's Centre for the Study of Host Resistance in Montreal, Canada, said the new study's insights into leprosy's natural history were "extremely exciting."
The results "provide critically important clues for an understanding of host-pathogen interaction in leprosy," he said. "Moreover, it will be highly interesting to contrast these findings with similar studies in tuberculosis that suggest a more genetically diverse pathogen pool."
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