Strongyloides stercoralis is most notably found in the tropics and the subtropics, but it can occur in temperate climates as well. Because of the variety of hosts which it can parasitize, the worm can be found in various areas around the world. In North America, it has primarily been found in large cities, specifically, New York City, Chicago, and Montreal. There is also a high incidence of Strongyloides stercoralis in Asia, Africa, tropical America, the Pacific Islands, and even in parts of the former Soviet Union. (Cheng, 1986; Roberts and Janovy, 2000)
Free-living Strongyloides stercoralis prefer the moist soil of warm climates. In the parasitic generation, the most common habitat in the host is the mucosal lining of the small intestine. Rarely, filariform larvae have been observed to take up residence in the lining of the bronchi and trachea and lay their eggs there after transforming to adults. (Cheng, 1986)
Strongyloides stercoralis is generally very long and cylindrical. It has four layers of cuticle that are typical of most nematodes. The epicuticle is the outer-most layer and the exocuticle is immediately beneath it. The mesocuticle is beneath the exocuticle and is generally composed of very well organized fibrous layers. The endocuticle is the innermost cuticle and although its makeup is similar to the meso, its fibers are not well ordered.
On the body surface, there are amphids which act as chemoreceptors and they open to the outside via pores which are located on the lips or on other anterior extremities. The worm has lateral alae, which can be seen as ridges which extend, in some cases, across the body.
The mouth of Strongyloides stercoralis opens into a buccal capsule, which, in both sexes, is very small. From the buccal capsule, food moves into the esophagus which is very long, cylindrical, and lacking a bulb at its posterior end unlike most other nematodes. The esophagous is connected to the intestine via an esophago-intestinal valve. The intestine is divided into three parts and is lined with a single layer of epithelium. The most anterior region of the intestine is the ventricular region and it has a secretory function. The midregion, and the posterior prerectal region function primarily in absorption. The intestine ends in the rectum and the alimentary tract eventually opens posteriorly through the anus.
The Strongyloides stercoralis body cavity is known as a pseudocoel. The reproductive system of males consists of vas deferens which extend from the testes and they enlarge in the posterior portion of the body to form the seminal vesicle. Males possess two copulatory spicules in addition to a gubernaculum. Females, on the other hand, usually have two ovaries and an oviduct which extends from it. At the terminal end of the oviduct is a seminal receptacle, which connects the oviduct to the uterus. Free-living females have uteri which contain more eggs than parasitic females. In parasitic females, the uteri are convergent and the vulva is in the posterior portion of the body. The free-living female has a vulva that is more equatorial.
Parasitic adult females range in length from 2.0 to 2.5 mm, larvae range in length from 300-800 micrometers, while the free-living male can grow up to 0.9 mm in length and 40 to 50 micrometers in width. (Cheng, 1986; Roberts and Janovy, 2000)
There are three stages in the life-cycle of Strongyloides stercoralis: free-living, parasitic, and autoinfection. After copulation, the free-living female accumulates eggs in her uterus that contain partially developed larvae. The eggs are laid in the soil where further development occurs. The rhabitiform larvae either become free-living adults or they become infective (filariform) 3rd stage larvae. Only females can enter the next phase of development, the parastitic stage.
To continue development, the filariform larvae must penetrate a host's skin or be ingested. If they enter the host through the skin, they must travel through host tissues in order to reach the final destination, the intestine. There, the females lay parthenogenetic eggs which give rise to rhabitiform larvae. These eventually leave the host via the feces and develop into free-living adults or metamorphose into filariform larvae.
Another variant of the life cycle is the autoinfective phase. Filariform larvae, migrating to the exterior, can reinfect the host by penetrating the skin and traveling in the blood to the lungs. (Cheng, 1986; Roberts and Janovy, 2000)
Females may produce a phermomone to attract males. The male coils around a female with his curved area over the female genital pore. The gubernaculum, made of cuticle tissue, guides spicules which extend through the cloaca and anus. Males use spicules to hold the female during copulation. Nematode sperm are amoeboid-like and lack flagella.
There are three stages in the life-cycle of Strongyloides stercoralis: free-living, parasitic, and autoinfection. After copulation, the free-living female accumulates eggs in her uterus that contain partially developed larvae. The eggs are laid in the soil where further development occurs. The rhabitiform larvae either become free-living adults or they become infective (filariform) 3rd stage larvae. Only females can enter the next phase of development, the parastitic stage. (Barnes, 1987; Cheng, 1986; Roberts and Janovy, 2000)
Rhabitiform larvae of Strongyloides stercoralis molt four times becoming sexually mature adults. However, if the external environment becomes unfavorable, they will metamorphose into non-feeding filariform larvae that can infect humans or some other host. As mentioned earlier, if the filariform larvae enter the host through the skin, they must migrate to the intestine via the host's tissues. For years, it has been thought that the larvae are carried to the lungs by the blood. It is believed that they inhabit the alveoli by traveling through the right atrium and ventricle of the heart. The pulmonary artery eventually brings them to the lungs where they move up the bronchi and trachea and eventually are coughed up and swallowed. The filariform larvae then mature in the intestine. This has only proven to be the case in humans. In dogs the filariform larvae do not seem to travel through the lungs but seem to get to the intestine through any route.
Nematodes in general have papillae, setae and amphids as the main sense organs. Setae detect motion (mechanoreceptors), while amphids detect chemicals (chemoreceptors). (Barnes, 1987; Roberts and Janovy, 2000)
The three most important definitive hosts for Strongyloides stercoralis are humans, dogs, and cats. They frequently use other mammals as hosts as well, though with much less frequency. Parasitic females feed on the tissue of the host's internal organs which includes the intestines as well as the lungs. Free-living adults and rhabitiform larvae feed on organic debris in soil or water.
Pharyngeal glands and intestinal epithelium produce digestive enzymes to feed on the hosts’ body fluids. Extracellular digestion begins within the lumen and is finished intracellularly. (Barnes, 1987; Cheng, 1986)
These parasites are probably not preyed on directly, but are ingested. Larval mortality is high as most of the parasites do not reach appropriate hosts.
The three most important definitive hosts for Strongyloides stercoralis are humans, dogs, and cats. They frequently use other mammals as hosts as well, though with much less frequency.
Strongyloides stercoralis has no positive effects on humans.
Strongyloides stercoralis causes strongyloidiasis in humans. Infection occurs via contact with filariform larvae in water or soil. The disease is most prevalent in areas of poor sanitation such as those that have poor sewage disposal sites.
Strongyloidiasis occurs in three phases: invasive, pulmonary, and intestinal. Filariform larvae that penetrate the skin cause itching at the the entry site. Slight hemorrhage and swelling may appear as well and if bacteria get into the site, inflammtion is likely.
The pulmonary phase of the disease is characterized by wheezing, a burning sensation in the chest, sputum production, as well as other symptoms of bronchial pneumonia. People who are thought to have a pulmonary infection are diagnosed with a chest x-ray. Pulmonary infection can be fatal because it can eventually result in repiratory failure. Elderly people, those with AIDS, and people who have had organ transplants have a very high risk of pulmonary strongyloidiasis. AIDS patients and those with organ transplants are at such a high risk because they are immunosuppressed, an effect most likely due to corticosteroid use in the latter group. The immune system is not able to fight off the parasite and this results in a high rate of autoinfection. Much care should be taken to diagnose pulmonary strongyloidiasis properly because the symptoms of it are very similar to those of asthma and asthma is treated with corticosteroids, which could result in increased autoinfection.
The intestinal phase occurs when the parasite penetrates the intestinal mucosa. Large patches of mucosa can be sloughed off in the process. An aching pain is felt in the abdomen and in chronic cases, relapsing colitis is very likely. Ulceration of the intestine can occur in some cases if not treated properly and this can lead to septicemia which is, more often than not, fatal.
Strongyloidiasis is most often diagnosed by a fecal smear, but in some cases, where infection is not rampant, agar plate culture to detect Strongyloides stercoralis is most effective (more so than the conventional filter paper culture). Sometimes, embryonated eggs may even be seen in the stool of patients. However, different numbers of juveniles pass through the feces each day and as autoinfection occurs, the numbers of juveniles exiting the body decreases markedly.
There are three drugs that are primarily used to treat strongyloidiasis. Each regimen needs to be repeated after one or two weeks because it is difficult to confirm a patient as cured. In a study conducted by Japanese doctors, ivermectin proved to be most effective in treating the disease with a 97% rate of eradication. It was given in a 6 mg single dose. The next most successful drug was albendazole (400 mg/day for three days) with a cure rate of 77.4%. Pyrvinium pamoate once only found to cure about 23.3% of those tested. This drug was given at 5 mg/kg/day for 3 days. Thiabendazole was once thought to be an effective drug, but side effects like nausea, vomiting, and smelly urine have caused the drug to lose favor among doctors. (Roberts and Janovy, 2000; Ting, May 2000; Toma, et al., March 2000; Zaha, et al., Sept. 2000)
There is no conservation status for Strongyloides stercoralis.
It has previously been reported that Strongyloides stercoralis is amazingly divergent from Strongyloides ratti. ssrRNA sequencing found that there was only a 70% similarity between gene sequences. This is unusual because two members of the same genus are never found to be that different. The divergence has recently been found to be a result of the fact that the gene sequence of S. stercoralis used in the study were hybrids of nematode and fungal sequences. It has now been confirmed that the ssrRNA between the two species' is just over 90% similar. (Dorris and Blaxter, July 2000)
Renee Sherman Mulcrone (editor).
Jason Hallman (author), University of Michigan-Ann Arbor, Solomon David (editor), University of Michigan-Ann Arbor.
living in sub-Saharan Africa (south of 30 degrees north) and Madagascar.
living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.
living in the southern part of the New World. In other words, Central and South America.
living in the northern part of the Old World. In otherwords, Europe and Asia and northern Africa.
living in landscapes dominated by human agriculture.
having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.
an animal that mainly eats meat
an animal which directly causes disease in humans. For example, diseases caused by infection of filarial nematodes (elephantiasis and river blindness).
either directly causes, or indirectly transmits, a disease to a domestic animal
Found in coastal areas between 30 and 40 degrees latitude, in areas with a Mediterranean climate. Vegetation is dominated by stands of dense, spiny shrubs with tough (hard or waxy) evergreen leaves. May be maintained by periodic fire. In South America it includes the scrub ecotone between forest and paramo.
uses smells or other chemicals to communicate
having a worldwide distribution. Found on all continents (except maybe Antarctica) and in all biogeographic provinces; or in all the major oceans (Atlantic, Indian, and Pacific.
in deserts low (less than 30 cm per year) and unpredictable rainfall results in landscapes dominated by plants and animals adapted to aridity. Vegetation is typically sparse, though spectacular blooms may occur following rain. Deserts can be cold or warm and daily temperates typically fluctuate. In dune areas vegetation is also sparse and conditions are dry. This is because sand does not hold water well so little is available to plants. In dunes near seas and oceans this is compounded by the influence of salt in the air and soil. Salt limits the ability of plants to take up water through their roots.
an animal that mainly eats decomposed plants and/or animals
particles of organic material from dead and decomposing organisms. Detritus is the result of the activity of decomposers (organisms that decompose organic material).
animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature
union of egg and spermatozoan
forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.
mainly lives in water that is not salty.
having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.
fertilization takes place within the female's body
marshes are wetland areas often dominated by grasses and reeds.
having the capacity to move from one place to another.
This terrestrial biome includes summits of high mountains, either without vegetation or covered by low, tundra-like vegetation.
found in the oriental region of the world. In other words, India and southeast Asia.
reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.
an organism that obtains nutrients from other organisms in a harmful way that doesn't cause immediate death
chemicals released into air or water that are detected by and responded to by other animals of the same species
rainforests, both temperate and tropical, are dominated by trees often forming a closed canopy with little light reaching the ground. Epiphytes and climbing plants are also abundant. Precipitation is typically not limiting, but may be somewhat seasonal.
scrub forests develop in areas that experience dry seasons.
remains in the same area
reproduction that includes combining the genetic contribution of two individuals, a male and a female
living in residential areas on the outskirts of large cities or towns.
a wetland area that may be permanently or intermittently covered in water, often dominated by woody vegetation.
uses touch to communicate
that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).
Living on the ground.
the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.
A terrestrial biome. Savannas are grasslands with scattered individual trees that do not form a closed canopy. Extensive savannas are found in parts of subtropical and tropical Africa and South America, and in Australia.
A grassland with scattered trees or scattered clumps of trees, a type of community intermediate between grassland and forest. See also Tropical savanna and grassland biome.
A terrestrial biome found in temperate latitudes (>23.5° N or S latitude). Vegetation is made up mostly of grasses, the height and species diversity of which depend largely on the amount of moisture available. Fire and grazing are important in the long-term maintenance of grasslands.
living in cities and large towns, landscapes dominated by human structures and activity.
Barnes, R. 1987. Invertebrate Zoology. Orlando, Florida: Dryden Press.
Cheng, T. 1986. General Parasitology. Orlando: Academic Press.
Dorris, M., M. Blaxter. July 2000. The small ribosomal subunit RNA sequence of Strongyloides stercoralis. International Journal of Parasitology, 30(8): 939-941.
Ohio State University, 2001?. "Strongyloides stercoralis" (On-line). Parasites and Parasitological Resources. Accessed September 28, 2004 at Strongyloides stercoralis.
Roberts, L., J. Janovy. 2000. Foundations of Parasitology. Boston: McGraw Hill.
Ting, Y. May 2000. Pulmonary strongyloidiasis: Case report of two cases. Kaohsiung J Med Sci, 16(5): 269-274.
Toma, H., Y. Sato, Y. Shiroma, J. Kobayashi, I. Shimabukuro. March 2000. Comparative studies on the efficacy of three anthelminthics on treatment of human strongyloidiasis in Okinawa, Japan. SE Asian J Trop Med and Pub Health, 31(1): 147-151.
Zaha, O., T. Hirata, F. Kinjo, A. Saito. Sept. 2000. Strongyloidiasis: Progress in diagnosis and treatment. Internal Medicine (Tokyo), 39(9): 695-700.