The Spread of Schistosomes
Globetrotting Parasites that Continue to Spread and Cause Disease
Jun 1, 2007 Rosemary Drisdelle
Schistosoma sp., blood parasites, cause schistosomiasis, a devastating parasitic disease that is common in some parts of the world. So far, we’ve been unable to defeat it—in fact, we’ve often spread it and helped it to increase.
Where are schistosomes found?
Schistosoma species establish themselves wherever a suitable species of aquatic snail exists to serve as an intermediate host (read about the life cycle of Schistosoma spp. in “What is Schistosomiasis?”):
- Schistosoma mansoni occurs in Africa, the Middle East, the Caribbean, and in Central and South America.
- Schistosoma japonicum is restricted to the Far East.
- Schistosoma haematobium occurs in Africa and the Middle East, and in regions of India and Portugal.
- A few other species infect humans but, limited to small geographic areas, they are not often found.
- In addition, schistosomes of birds all over the world cause swimmer's itch in humans, an irritating skin rash that is not, in itself, life threatening.
Why are schistosomes more common today than in the past?
Schistosomes probably originated in Africa and, for S. japonicum, the Far East. Human migration and the slave trade spread them to the Middle East, India, the Caribbean, and Central and South America. The fact that they have not spread farther than this probably indicates that suitable snail species are not present in other regions (though the unprecedented spread of species around the globe occurring today may open new doors for these parasites in the future).
Meanwhile, in areas where Schistosoma spp. already existed, we have aided them in many instances through the construction of dams and other water projects. Snails, and therefore schistosoma, do not thrive in fast running rivers and turbulent waters—their habitat is slow moving or still water where aquatic plants proliferate and provide snails with food:
- Damming rivers creates large lakes with still, weedy shorelines.
- Water reservoirs and aquaculture ponds increase the amount of snail habitat in human communities.
- Irrigation canals are perfect environments for water plants and snails as well, as are flooded rice paddies.
Once snails are there, the stage is set for schistosomiasis—add human waste containing schistosoma eggs to the water and a local epidemic will result. Thus, water projects intended to provide water for agriculture and other necessities have, in many cases, brought disease instead.
Should travelers worry about catching schistosomiasis?
Travelers to endemic areas—places where the parasite occurs regularly in the population and the environment—should be cautious around bodies of fresh water. Anywhere where the water is relatively still and plants grow at the water’s edge and in the shallows is a perfect environment for the aquatic snails that Schistosoma sp. need to complete their life cycle. Lakes behind dams, water reservoirs, irrigation canals, small ponds and natural still bodies of water are all suspect—human skin should not come in contact with the water.
Anyone who has traveled to an endemic area can be checked for schistosomiasis—there are blood tests available, and stool and urine samples can be checked for eggs.
Related content:
Other Topics in Microbiology:
Trichomonas vaginalis Parasite
Sources:
Beaver, Paul Chester, Rodney Clifton Jung, and Eddie Wayne Cupp. Clinical Parasitology 9th ed. Philadelphia: Lea and Febiger, 1984.
Garcia, Lynn S. and David A. Bruckner. Diagnostic Medical Parasitology 3rd ed. Washington: ASM Press, 1997.
Roberts, Larry S. and John Janovy Jr. Foundations of Parasitology 6th Ed. Boston: McGraw Hill, 2000.
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