Slavery and the Spread of Disease

Fig.1. Scanning electron micrograph of a pair of Schistosoma mansoni. 
(Credit: Davies Laboratory Uniformed Services University, Bethesda, MD)

The transatlantic slave trade, which brought millions of Africans to the New World, connected four continents in a massive operation that lasted from the 16th to the 19th centuries. A recent study published in the journal Science Reports revealed that slave traders during these centuries contributed not only to an ongoing and tragic business in human lives, but also helped spread a parasitic disease around the world.

The parasitic fluke Schistosoma mansoni is one of several species that causes schistosomiasis (bilharzia), a Neglected Tropical Disease (NTD) found in almost 240 million people worldwide. This debilitating disease originated in East Africa and currently contributes to 20,000 to 200,000 deaths per year.

Fig.2. Stowage of the British slave ship Brookes, 1789
(Credit: U.S. Library of Congress) 

Slavery and Schistosomiasis

Researchers with the Wellcome Trust Sanger Institute, Imperial College London, and the Royal Veterinary College examined genomes of S. mansoni parasites from Africa and the French Caribbean to map the fluke’s origins, migration, and molecular mechanisms. A comparison with the closely related S. rodhaini, which infects rodents, revealed a common ancestor that existed around 107,000 to 148,000 years ago. They found that this timing corresponds with some of the first archaeological evidence of fishing in Africa.

This detail is important because the parasites begin as free-swimming larvae that infect freshwater snails before transferring to humans. The schistosomes mature inside the human body and release eggs, causing a painful and sometimes fatal disease. Eggs return to freshwater environments through human feces and begin the cycle again. Freshwater fishing would have placed humans in the prime environment for infection by these parasites. The parasites then would have had the opportunity to specialize and adapt to a human host.

After becoming prevalent in East Africa, the researchers believe the flukes spread to West Africa and then to the New World. Genomic comparisons of S. mansoni specimens suggest a split between the West African flukes and their relatives across the ocean between 1117 AD and 1742 AD, overlapping with the transatlantic slave trade. This migration of parasites across the Atlantic would have been possible with the transport of more than 22,000 African slaves from West Africa to Guadeloupe in Caribbean.

In addition to mapping out the parasites’ route, the researchers studied S. mansoni and S. rodhaini for specific adaptations to their respective hosts. S. mansoni contained mutations that allowed the fluke to attach and burrow into human skin, as well as the ability to actively evade the human host’s immune system.

Fig.3. Nursing student from the Arts Medical College of Dire Dawa, Ethiopia distributing 
medications for parasitic diseases in Shinile Woreda, Ethiopia, with the help of the U.S. Army 
418th Civil Function Specialty Team (Credit: Staff Sgt. Kat McDowell/U.S. Air Force, 2010)

Parasites and the Modern Human

Schistosomes have a long history with humanity. The earliest known example of a human and parasite association was a schistosome egg found in Mesopotamian remains from 6,000 years ago. Today, transmission of S. mansoni and other close relatives that cause schistosomiasis have been recorded in 78 countries.

Such prevalence is the focus of the Schistosomiasis Collection at the Natural History Museum in London (SCAN). SCAN holds more than 200,000 individual schistosome larvae, later-stage parasites, and snail hosts from different countries. Last year we spoke with Dr. David Rollinson and Dr. Aidan Emery, parasitology researchers involved in SCAN, and they contributed their expertise to this study on the evolutionary history of S. mansoni.

They and their colleagues’ research contributes to ongoing efforts to address public health concerns and record biodiversity of these flatworm species. Such efforts to understand the history of S. mansoni and to trace the global health impact of human parasites are imperative. Parasitic diseases account for 11 of the 17 NTDs, as well as other more well known diseases like malaria. A historical view of these organisms reveals not only how they change over time and space, but also possible preventative measures in modern times to mitigate a destructive group of pathogens.


References
Crellen, T., et al. (2016). Whole genome resequencing of the human parasite Schistosoma mansoni reveals population history and effects selection. Scientific Reports, vol. 6: 20954. doi:10.1038/srep20954

Glossary

fluke
A group of parasitic flatworms in the class Trematoda.
Neglected Tropical Diseases (NTDs)
Diseases that affect around 1 billion people living in poverty and/or without adequate sanitation in tropical and subtropical countries. Members of this diverse group of diseases often share a lack of awareness and funding towards their prevention, an outsized impact on economies and health, and a prevalence in poor countries.