Strongyloides stercoralis is the threadworm parasite of man. There are about 38 species of threadworms found in sheep, swine, goat, ox, deer, camel, rabbit, primates, dogs, and cats. Some species are parasites of birds, reptiles and amphibians. Worms indistinguishable from S. stercoralis have been recovered from dogs and cats.
Parasitic infection with nematodes is typically more of a nuisance disease than a serious medical condition, but this nematode has the unique ability to complete its replication cycle entirely within a human host – and this ability can lead to so-called "hyperinfection" in immunocompromised persons.
These worms have a heterogenetic life cycle which consists of
The heterogonic life cycle is advantageous to the parasite as it affords the opportunity for the parasite to exist and reproduce for one or more generations in the absence of a host. Also the number of infective juveniles may be increased by reproduction of the free-living females and males.
Parasitic females anchor themselves with their mouths to the mucosa of the small intestine or burrow their anterior ends into the submucosa. Reproduction in the host is by parthenogenetic females which lay several dozen eggs each day. Eggs are released into the lumen of the gut or the submucosa where they hatch and juveniles pass into the lumen. These first-stage juveniles are 300-380µm long and are usually passed with the feces. Juveniles develop either to free-living adults or to infective filariform juveniles. Third stage juveniles are the infective stage. They are 490-630µm long. This is a resting stage which does not develop further until it penetrates through skin or is ingested. Following skin penetration they are carried by the blood to the lungs, where they exit into the alveoli, travel up the trachea, are swallowed, and mature in the small intestine. If ingested, migration through the lungs is not necessary.
The free-living adults can produce successive generations of free-living adults. Both free-living and parasitic females can produce juveniles that will become filariform, infective juveniles and juveniles that will mature into free-living adults. Autoinfection is also possible if juveniles have time to molt twice during passage down the digestive tract. In these cases they penetrate the lower gut mucosa or perianal skin, migrate in the typical way, and mature. Both free-living and parasitic females can produce juveniles that will become filariform, infective juveniles and juveniles that will mature into free-living adults. Autoinfection is also possible if juveniles have time to molt twice during passage down the digestive tract. In these cases they penetrate the lower gut mucosa or perianal skin, migrate in the typical way, and mature.
Parthenogenetic females reach a length of about 1/10th of an inch (2.0 to 2.5mm). The buccal capsule of both sexes is small, and they possess a long, cylindrical esophagus that lacks a posterior bulb. The vulva is in the posterior third of the body; the uteri are divergent and contain only a few eggs at a time. The free-living adults both have a rhabditiform esophagus. The male is up to 0.9mm long and 40-50µm wide.
The threadworm of man occurs from about 35° north latitude to 30° south latitude. Generally the distribution is limited to warm moist areas because such climates are favorable to the survival of the juvenile stages. In the United States, strongyloidiasis is relatively uncommon. Endemic concentrations exist in rural areas of the southeastern United States and Appalachia, with prevalence rates close to 4%. Infections acquired in the United States, while not usually associated with larva currens, are not clinically silent; the infected individuals usually have a chronic relapsing illness of mild to moderate severity. Among veterans of the US military forces who served in Southeast Asia, the prevalence of larva currens in those with confirmed strongyloidiasis is high, with studies showing a range of 30-90%.
Strongyloidiasis is endemic in tropical and subtropical countries. The worldwide prevalence is approximately 35 million cases, and rates are as high as 40% in certain regions.
Diagnosis is based on finding juveniles in freshly passed stools, by a direct smear in cases of heavy infection or following concentration by Baermann isolation or zinc flotation with centrifugation. Feces which have been in contact with soil my be contaminated with soil-dwelling rhabditids.
In massive infections death may result unless therapeutic measures are taken. Prevention involves mainly sanitation involving proper disposal of human wastes. Infected persons and animals may be treated with appropriate anthelmintics. (Vermox or Pyrantel Pamoate; Thiabendazole or Cambendazole).
Almost all of the fatal cases of helminthic infection in the United States are caused by this autoinfecting nematode, Strongyloides.
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