Journeys of a Pathogen

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Science  27 Nov 2009:
Vol. 326, Issue 5957, pp. 1165
DOI: 10.1126/science.326.5957.1165-a

Leprosy is caused by Mycobacterium leprae, which infects the neurons and mucosa of humans. Strikingly, its genome has decayed extensively, which probably accounts for its slow rate of growth; global variation between strains is limited to four SNP clusters, each having a distinctive geographical prevalence that reflects past human migrations. Monot et al. have sequenced M. leprae strains from Brazil, Thailand, and the United States, and made comparisons with archaeological samples. Genotyping the pathogen can be used to pinpoint a person's origins quite precisely. For example, samples from an Egyptian burial revealed that the corpse had been infected with a European strain of leprosy and was more probably a Roman legionary than an African. Their updated map of leprosy strains supports the out-of-Africa theory for modern humans and indicates that leprosy crossed to Asia twice: once via a southern route into India and through Indonesia to the Philippines, and also via a northern route along the Silk Road in countercurrent fashion to the Black Death. Subsequently, leprosy arrived in South America via European immigrants rather than by pre-Columbian colonization via the Bering Strait.

Nat. Genet. 41, 10.1038/ng.477 (2009).

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