Strongyloides stercoralis. Source: CDC
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Strongyloidiasis

Strongyloidiasis is a human parasitic disease caused by the nematode (roundworm) Strongyloides stercoralis. Other Strongyloides include S. fülleborni, which infects chimpanzees and baboons and may produce limited infections in humans. more...

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Life Cycle

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 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, or into infective filariform larvae. The filariform larvae penetrate the human host skin to initiate the parasitic cycle.
  • 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 socio-economic groups.

Clinical features

Frequently asymptomatic. Gastrointestinal system 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.

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Ivermectin approved for two parasitic diseases - strongyloidiasis; onchocerciasis - Update - Brief Article
From FDA Consumer, 3/1/97

The antiparasitic animal drug ivermectin was approved by FDA to treat two human parasitic diseases, strongyloidiasis and onchocerciasis.

Strongyloidiasis is common in many tropical countries and is occasionally acquired in some areas of the United States. Infection is usually confined to the small intestine. It can persist for many years, causing abdominal pain, diarrhea, and elevated levels of white blood cells. In people with weakened immune systems, the infection can spread through the body and be fatal.

Onchocerciasis, commonly known as river blindness, is most often found in Africa and South and Central America. The parasite is transmitted by the bite of a black fly, which deposits immature forms of the parasite under the skin. There, the worms mature and the adult females produce more larvae. The immature worms migrate under the skin, causing intense itching. They can also migrate to the eyes, causing inflammation and blindness.

Ivermectin is better tolerated than other drugs approved to treat these conditions, and it is more effective than the previously approved treatment for onchocerciasis. In clinical studies, a single dose of ivermectin cured between 64 and 100 percent of patients with strongyloidiasis who had normal immune systems. In studies of onchocerciasis, a single dose of the drug reduced the number of larvae under the skin an average of 83 percent at three days and 99.5 percent at three months. Adverse reactions to the drug differ depending on which disease is being treated but can include skin rashes, itching, dizziness, diarrhea, and nausea.

Merck & Co. Inc., of West Point, Pa., sells ivermectin in the United States under the trade name Stromectol. (Elsewhere it is marketed as Mectizan.) Before its approval on Nov. 25, 1996, ivermectin had been available here for limited human use as an investigational drug.

Ivermectin has been used worldwide since 1987 to treat river blindness. More than 5.2 million people have received the drug through the World Health Organization's Onchocerciasis Control Program and Merck's Mectizan Donation Program.

(See also "Treating Tropical Diseases" in the January-February 1997 FDA Consumer.)

COPYRIGHT 1997 U.S. Government Printing Office
COPYRIGHT 2004 Gale Group

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