Classed as a neglected tropical disease, bilharzia is thought to infect over 200 million people globally. Caused by a few different types of parasitic flatworm, the potentially fatal disease is most commonly found in rural areas of tropical and subtropical countries. The worms, called Schistosoma or blood-flukes, spend half their life living in freshwater snails, and then burrow into a human host who lives and works in and around contaminated water. Researchers have now tracked the movement of one of the most widespread species of the parasite, Schistosoma mansoni, and found that it reflects one of the darker periods of human history.
By comparing the genomes of the species S. mansoni, which is found both in the Caribbean and in West Africa, researchers were able to trace its spread around the world. They discovered that it probably crossed the Atlantic in the bodies of the thousands of African slaves sent to work on plantations between the 16th and 19th centuries. They were able to show how the genetics of the parasite reflect the murkier aspects of human migration around the world, tracing it from Senegal and Cameroon to the island of Guadeloupe.
The parasite is a worm that burrows into people where it then swims around eating blood cells, until it reproduces. Jessica Lucia/Flickr CC BY-NC-ND 2.0
“Comparing the S. mansoni genomes suggests that flukes in West Africa split from their Caribbean counterparts at some point between 1117 CE and 1742 CE,” says Professor Joanne Webster, who coauthored the paper published in Scientific Reports. “During this period more than 22,000 African people were transported from West Africa to Guadeloupe by French slave ships, and the fluke was carried with them.”
The study went further though, and looked into the evolutionary history of the parasite, which has a close relative that lives in rodents, known as S. rodhaini. They found that the evolutionary split between the two was much later than thought, only occurring between 107,000 and 148,000 years ago. This makes S. mansoni a relatively “new” species in evolutionary terms. Interestingly, this split also coincided with another event in human history.
“The timing of the separation of the two species coincidences with the first archaeological evidence of fishing in Africa,” explains Thomas Crellen, the first author of the study, who works out of Imperial College London. “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.”
They were also able to identify the genetic differences between S. mansoni and S. rodhaini that they think better enables the parasite to infect a human host. One of these is a gene that helps the worms burrow into the body and break down a major component of human skin, elastin. They hope that by studying the genetics of how the parasite infects people, they may be able to find new treatments or preventive measures.