A hundred million people around the world may be infected with Strongyloides stercoralis, or threadworm, a tiny nematode in soil and in animal intestines. Most cases of strongyloidiasis occur in tropical and temperate regions where sanitation is poor.
Strongyloides Life Cycle
This parasite has a complex and adaptable life cycle that allows it to persist in human populations and in the environment.
- Infective (filariform) larvae present in the soil infect humans by penetrating the skin, usually the soles of the feet or the buttocks. They may also be swallowed.
- Larvae enter the bloodstream and are carried to the lungs, where they break through into the lung airspace and move up to the throat. They are swallowed and enter the small intestine. Some larvae apparently go straight to the intestine by a more direct route.
- In the intestine, larvae mature to adult worms in the spaces between cells lining the intestine.
- Parasitic adult threadworms produce eggs.
- Eggs hatch, releasing non-infective (rhabditiform) larvae that migrate into intestinal contents and are passed in stool.
- Rhabditiform larvae mature in soil to either adult worms that live free in the environment or filariform larvae capable of infecting a human or animal host.
Interesting Facts About the Strongyloides Life Cycle
Strongyloides stercoralis seems to be designed for survival in many ways:
- Parasitic adult threadworms are all female and they reproduce by parthenogenesis—without male fertilization.
- Parasitic females live for a long time after they stop producing eggs, and may be able to begin producing eggs again to reactivate an old infection.
- Larvae hatching in the intestine can migrate through the intestinal wall, migrate through the blood stream and lungs, and return to the intestine to mature. In a host with a healthy immune system, this “autoinfection,” which can continually increase the number of parasites in the intestine, is kept to a minimum.
- Free living adult threadworms include both males and females, and are capable of reproducing in the soil, maintaining the parasite for an indefinite period in the environment.
- Threadworm infects humans and other primates, and various mammals including dogs and cats.
- Human cases of strongyloidiasis lasting forty years have been documented.
Threadworm Symptoms
Many cases of strongyloidiasis are asymptomatic and people have no idea they’re infected. When symptoms do occur after infection they chiefly involve the skin, lungs, or intestine:
- Itching and swelling of affected skin and sometimes a red track where larvae migrate under the skin.
- An allergic skin rash after repeated exposure.
- Pneumonia-like symptoms when larvae move through the lungs.
- Abdominal pain.
If the immune system is compromised in someone infected with strongyloidiasis (and in cases of corticosteroid therapy), the parasites can multiply to enormous numbers and spread throughout the body in a continuous cycle of autoinfection. Death frequently results from organ damage, infection caused by larvae carrying bacteria from the intestine, or other complications. People receiving medical treatments that suppress the immune system, such as donor organ recipients, should be checked for threadworm infection, especially in places where the parasite is common.
Threadworm Treatment
All cases of threadworm infection should be treated because of the potential for autoinfection and serious disease. The parasite is notoriously difficult to eradicate completely so follow-up testing to confirm that the drug has worked is recommended.
Parasites Similar to S. stercoralis
Sources
Diagnostic Medical Parasitology 3rd ed. Garcia, Lynn S. and David A. Bruckner. Washington: ASM Press, 1997.
Foundations of Parasitology 6th Ed. Roberts, Larry S. and John Janovy Jr. Boston: McGraw Hill, 2000.
“Strongyloidiasis.” Tolan, Robert W. emedicine: Medscapes Continually Updated Clinical Reference. Medscape.com
Rosemary Drisdelle - Rosemary Drisdelle has been published many times as a nonfiction writer and several times as a poet. Her first book, Parasites: Tales of ...
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