Strongyloides stercoralis
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| Strongyloides stercoralis Bavay, 1876 |
Strongyloides stercoralis is the scientific name of a human parasitic roundworm causing the disease of strongyloidiasis.
This 2 mm-long worm is found both free-living in soil, and as a parasite of humans, other primates, and dogs. The free-living male and female worms are not infectious, however, their offspring all develop to infectious larvae. These infectious larvae penetrate the skin when there is contact with the soil. Some of them enter the superficial veins and ride the blood vessels to the lungs, where they enter the alveoli. They are then coughed up and swallowed into the gut, where they parasitise the intestinal mucosa (duodenum and jejunum). However, research in dogs has shown that most of the larvae that penetrate the skin migrate randomly through the body until they reach the small intestine. Only females will reach reproductive adulthood in the intestine. Female strongyloides reproduce through parthenogenesis. The eggs hatch in the intestine and young larvae are then excreted in the feces. It takes about two weeks to reach egg development from the initial skin penetration. By this process, S. stercoralis can cause both respiratory and gastrointestinal symptoms. Adult worms can live up to a year in dogs.
Autoinfection can also occur when the larvae which hatch from the eggs in the small intestine have time to mature to infectious larvae before being excreted. They penetrate the wall of the lower ileum or colon or the skin of the perianal region, enter the circulation again, up to the lungs, and back down to the small intestine thus repeating the cycle. Because of autoinfection, humans have been known to still be infected up to 50 years after they were first exposed to the parasite. Immunosuppressive drugs, such as those used for tissue transplantation, (especially corticosteroids) can increase the rate of autoinfection to the point where there is an overwhelming number of larvae migrating through the lungs, and in many cases this can prove fatal. Additionally, diseases such as HTLV-1 (Human T-cell Lymphotropic Virus 1), which enhance the Th1 arm of the immune system and lessen the Th2 arm, increase the disease state. While there are a number of drugs which will kill the adult worms, these drugs have little effect on the majority of these autoinfective larvae during their migration through the body.
