Dr. Mahabub's Microscopy

Live parasite in stool, Strongyloides stercoralis is a nematode, 2.5 mm-long, that is a parasite of humans. The adult parasitic stage lives in tunnels in the mucosa of the small intestine. The genus Strongyloides contains 53 species and S. stercoralis is the type species. S. stercoralis has been reported in other primates and occasionally in dogs. However, it seems that the species in dogs is typically not S. stercoralis, but another species S. canis. Non-human primates are more commonly infected with S. fuelleborni and S. cebus although S. stercoralis has been reported in captive primates. Other species of Strongyloides naturally parasitic in humans, but with restricted distributions, are S. fuelleborni in central Africa and S. kellyi in Papua New Guinea.

Strongyloides larvae exist in 2 forms: filariform infective larvae and a free-living rhabditiform larvae that lives in soil independent of a human host. Infection occurs when exposed skin contacts contaminated soil. The larvae penetrate the skin and migrate via the lymphatics and venules toward the pulmonary circulation. After penetration into the alveoli, the larvae continue their migration up the respiratory tract until they are swallowed. Filariform larvae rest in the small intestine, mature into adult females, and undergo parthenogenic reproduction. Eggs typically mature into rhabditiform larvae within the intestine. Strongyloides is the only helminth to secrete larvae (and not eggs) in feces. Typically, larvae appear in feces approximately 1 month after skin penetration. The excreted rhabditiform larvae may again live freely in soil or be transformed into filariform larvae awaiting another human host.

Under certain conditions such as constipation, the larvae do not exit the host in feces and instead molt into the infective filariform larva within the intestinal lumen. These larvae are then capable of penetrating the bowel wall and traveling throughout the body. The CNS, liver, and lungs are the most common destinations of the autoinfectious larvae. This autoinfectious cycle can be accelerated in immunocompromised hosts and results in a frequently fatal condition known as hyperinfection.

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