French scientists have figured out how leprosy evolved and how the migration of human populations spread it across the world. They say that a single bacterial clone, which spread for centuries -- mutating only slightly -- was the origin of leprosy infections.
Legend holds that leprosy was introduced to Europe by Greek soldiers returning home after Alexander the Great's campaign into India in 327 B.C. But a new French study concluded that the disease originated in East Africa or the Middle East and may have migrated outward thousands of years before Alexander.
Although antibiotics brought the disease under control in the developed world in the 1940s, leprosy remains a significant public-health problem in many parts of the globe, particularly India. According to the World Health Organization, about a half-million new cases were detected in 2003.
It primarily affects the skin and central nervous system, particularly the limbs, fingers and toes. Although it's not as contagious as once widely believed, it can cause permanent disability and disfigurement and remains a cause of social stigma.
The first-known mention of leprosy was in an Indian religious text more than 2,600 years ago. By the Middle Ages, it was known and feared worldwide, with victims either killed or driven into remote colonies.
The new research examined slight genetic differences in strains of the bacteria around the world. The genetic code of the leprosy bacterium is filled with a large proportion of damaged, nonfunctional genes, which is why it grows very slowly and is virtually impossible to culture in a lab.
In fact, the samples collected for the study had to be grown in armadillos, the only other animal known to carry the bacteria.
The strains of the leprosy germ were so similar that researchers from the Pasteur Institute in Paris were able to group 175 samples into just four types. Although they focused on genetic sequences known to move around, copy themselves or disappear -- and thus most likely to reflect evolutionary change -- they found strikingly little variation.
"Mycobacterium leprae has the lowest level of genetic diversity of any bacterium that I'm aware of," said Stewart Cole, a molecular microbiologist at Pasteur and lead author of the study, published Friday in the journal Science.
"One clone has infected the whole world," he said. While "India been stigmatized as the cradle of leprosy, the disease just as likely could have arisen in East Africa or perhaps the Near East."
Cole favors an East African origin because the strain found in Ethiopia, type 2, is the rarest, and thus the oldest, of those examined. Type 1 is mostly found in Central Asia, type 3 in Europe.
So for Greeks to have brought the disease home to Europe from India, the bacterium would have had to jump from a type 1 to 2 to 3 in just a few centuries, which seems unlikely for such a slow-changing microbe, Cole said.
The patterns also show that the type of leprosy seen in West Africa and the Caribbean, type 4, is most closely related to the European strain of Type 3, suggesting that Europeans brought the disease to that side of Africa and that the slave trade then spread it to the New World. Type 3 is also found in North America.
"Colonialism was extremely bad for parts of the world in terms of human health," Cole noted.
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