Strongyloides Stercoralis

[Franky]

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. From Wikipedia, the free encyclopedia


The nematode (roundworm) Strongyloides stercoralis. Other Strongyloides include S. fülleborni, which infects chimpanzees and baboons and may produce limited infections in 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 (see "Parasitic cycle" below) can either molt twice and become infective filariform larvae (direct development) or molt four times and become free living adult males and females that mate and produce eggs from which rhabditiform larvae hatch . The latter in turn can either develop into a new generation of free-living adults (as represented in ), or into infective filariform larvae . The filariform larvae penetrate the human host skin to initiate the parasitic cycle (see below) .
Parasitic cycle: Filariform larvae in contaminated soil penetrate the human skin , and are transported to the lungs where they penetrate the alveolar spaces; they are carried through the bronchial tree to the pharynx, are swallowed and then reach the small intestine . In the small intestine they molt twice and become adult female worms . The females live threaded in the epithelium of the small intestine and by parthenogenesis produce eggs , which yield rhabditiform larvae. The rhabditiform larvae can either be passed in the stool (see "Free-living cycle" above), or can cause autoinfection . In autoinfection, the rhabditiform larvae become infective filariform larvae, which can penetrate either the intestinal mucosa (internal autoinfection) or the skin of the perianal area (external autoinfection); in either case, the filariform larvae may follow the previously described route, being carried successively to the lungs, the bronchial tree, the pharynx, and the small intestine where they mature into adults; or they may disseminate widely in the body. To date, occurrence of autoinfection in humans with helminthic infections is recognized only in Strongyloides stercoralis and Capillaria philippinensis infections. In the case of Strongyloides, autoinfection may explain the possibility of persistent infections for many years in persons who have not been in an endemic area and of hyperinfections in immunodepressed individuals.

Geographic Distribution:
Tropical and subtropical areas, but cases also occur in temperate areas (including the South of the United States). More frequently found in rural areas, institutional settings, and lower socioeconomic groups.

Clinical Features:
Frequently asymptomatic. Gastrointestinal symptoms include abdominal pain and diarrhea. Pulmonary symptoms (including Loeffler’s syndrome) can occur during pulmonary migration of the filariform larvae. Dermatologic manifestations include urticarial rashes in the buttocks and waist areas. Disseminated strongyloidiasis occurs in immunosuppressed patients, can present with abdominal pain, distension, shock, pulmonary and neurologic complications and septicemia, and is potentially fatal. Blood eosinophilia is generally present during the acute and chronic stages, but may be absent with dissemination.

Laboratory Diagnosis:
Diagnosis rests on the microscopic identification of larvae (rhabditiform and occasionally filariform) in the stool or duodenal fluid. Examination of serial samples may be necessary, and not always sufficient, because stool examination is relatively insensitive.
The stool can be examined in wet mounts:

* directly
* after concentration (formalin-ethyl acetate)
* after recovery of the larvae by the Baermann funnel technique
* after culture by the Harada-Mori filter paper technique
* after culture in agar plates

The duodenal fluid can be examined using techniques such as the Enterotest string or duodenal aspiration. Larvae may be detected in sputum from patients with disseminated strongyloidiasis.

Treatment:
The drug of choice for the treatment of uncomplicated strongyloidiasis is ivermectin with albendazole* as the alternative. All patients who are at risk of disseminated strongyloidiasis should be treated.