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  • Transient pruritic skin rash and lung symptoms.

  • Anorexia, diarrhea, abdominal discomfort.

  • Larvae detected in stool.

  • Hyperinfection in the immunocompromised; larvae detected in sputum or other fluids.

  • Eosinophilia.


Strongyloidiasis is caused by infection with Strongyloides stercoralis. Although much less prevalent than ascariasis, trichuriasis, or hookworm infections, strongyloidiasis is nonetheless a significant problem, infecting tens of millions of individuals in tropical and subtropical regions. Infection is also endemic in some temperate regions of North America, Europe, Japan, and Australia. Of particular importance is the predilection of the parasite to cause severe infections in immunocompromised individuals due to its ability to replicate in humans. A related parasite, Strongyloides fuelleborni, infects humans in parts of Africa and New Guinea.

Among nematodes, S stercoralis is uniquely capable of maintaining its full life cycle both within the human host and in soil. Infection occurs when filariform larvae in soil penetrate the skin, enter the bloodstream, and are carried to the lungs, where they escape from capillaries into alveoli, ascend the bronchial tree, and are then swallowed and carried to the duodenum and upper jejunum, where maturation to the adult stage takes place (eFigure 35–38). Females live embedded in the mucosa for up to 5 years, releasing eggs that hatch in the intestines as free rhabditiform larvae that pass to the ground via the feces. In moist soil, these larvae metamorphose into infective filariform larvae. Autoinfection can occur in humans, when some rhabditiform larvae develop into filariform larvae that penetrate the intestinal mucosa or perianal skin, and enter the circulation. The most dangerous manifestation of S stercoralis infection is the hyperinfection syndrome, with dissemination of large numbers of filariform larvae to the lungs and other tissues in immunocompromised individuals. Mortality with this syndrome approaches 100% without treatment and has been about 25% with treatment. The hyperinfection syndrome is seen in patients receiving corticosteroids and other immunosuppressive medications; patients with hematologic malignancies, malnutrition, or alcohol use disorder; or persons living with AIDS. The risk seems greatest for those receiving corticosteroids.

eFigure 35–38.

Life cycle of Strongyloides stercoralis (small roundworm of humans). The Strongyloides life cycle is more complex than that of most nematodes with its alternation between free-living and parasitic cycles, and its potential for autoinfection and multiplication within the host. Two types of cycles exist. Free-living cycle: The rhabditiform larvae passed in the stool

image (see "Parasitic cycle" below) can either become infective filariform larvae (direct development)
image, or free-living adult males and females
image that mate and produce eggs
image from which rhabditiform larvae hatch
image and eventually become infective filariform larvae
image. The filariform larvae penetrate the human host skin to initiate the parasitic cycle (see below)
image. Parasitic cycle: Filariform larvae in contaminated soil penetrate the human skin
image, and by various, often random routes, migrate to the small intestine
image. Historically it was ...

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