The ancient scourge known as leprosy likely originated in either East Africa or Central Asia and then extended its reach to the east and west in a pattern mirroring human migration, according to a new analysis of its bacterial agent's unusual genetic fingerprint.
Led by researchers at the Pasteur Institute in Paris, the study suggests the disease reached North America and West Africa through infected explorers, traders, or colonialists within the past 500 years, and that it infiltrated the Caribbean and South America via the slave trade in the 18th century.
"Colonialism was extremely bad for parts of the world in terms of human health," said co-author Stewart Cole in a news release accompanying the study in the journal Science.
Recorded in China, India and Egypt as early as 600 BC, leprosy attacks nerve and skin cells, resulting in permanent disabilities if left untreated. The bacterial culprit, Mycobacterium leprae, has proven a difficult study subject due to its extreme sluggishness and inability to grow in anything other than humans, mouse footpads, and the nine-banded armadillo.
"Even if we could grow it in a Petri dish, it would take nine to 12 months to form a colony," said Pasteur Institute technician Marc Monot.
Through a genetic analaysis, however, Monot, Cole and their collaborators found that seven M. leprae strains isolated from patients in 21 countries were nearly identical, suggesting the disease spread from a single clone whose DNA sequence has remained remarkably stable for centuries.
To chart the disease's global expansion, the team analyzed a genetic landmark known as a single nucleotide polymorphism, or SNP. This uncommon variation in DNA letters -- a CTC sequence at one site instead of TTC, for example -- revealed that all of the collected bacterial isolates could be divided into four main groups.
Leprosy was long believed to have arisen in what is now India and reached Western Europe through infected Greek soldiers returning from Alexander the Great's military campaign in the Indian subcontinent. But the genetic analysis suggests that an East African origin is just as likely, and that Alexander's soldiers may have acquired the disease from other campaigns in the Near East.