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Researchers at the Wellcome Trust Sanger Institute showed that the bilharzia-causing parasite, Schistosoma mansoni, was brought to the New World through slave traders, who got infected when they fished in East African lakes through the 16th to the 19th century.

By looking at the complete DNA sequences of S. mansoni parasites from Africa as well as the French Caribbean, researchers could point out their origins and transport through the world.

S. mansoni, a blood fluke infecting more than 250 million people in the world, leads to 11,000 deaths every year. By using the parasite's genome, published by the Sanger Institute six years ago, scientists compared the genomes of diverse S. mansoni parasites from across Africa and the New World.

The S. mansoni, and its close relative S. rodhani, which infects rodents, seem to have a common ancestor in East Africa between 107,000 to 148,000 years ago.

"The timing of the separation of the two species coincidences with the first archaeological evidence of fishing in Africa," Thomas Crellen, first author of the study, said in a press release. "The parasite develops in freshwater and infects people by burrowing through their skin. The introduction of fishing would have meant that people spent more time in the water, greatly increasing their chances of being infected."

From the 16th to 19th century, about 22,000 Africans were carried from West Africa to Guadeloupe on French slave ships, with the parasites.

Some genetic variations got selected over time, with a few variations that made the parasites thrive and live within humans.

"When we looked for the differences between human-infecting S. mansoni DNA and its rodent infecting cousin S. rodhaini, we found two important variations. We found that changes to two genes in S. mansoni's DNA - VAL21 and an elastase gene -appear to be important in allowing the fluke to enter and live in humans," said James Cotton, senior author of the study. "VAL genes produce proteins that cause allergic responses, so it is possible that the variation in VAL21 helps the fluke to hide from our immune systems. The elastase gene helps the parasite to burrow in to the body, by breaking down elastin - a major component of human skin."

The study was published in the Feb. 16 issue of Scientific Reports.