Animal Diversity Web

Geographic Range

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)

• Biogeographic Regions
• nearctic
• palearctic
• oriental
• ethiopian
• neotropical

• Other Geographic Terms
• cosmopolitan

Habitat

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)

• Habitat Regions
• temperate
• tropical
• terrestrial
• freshwater

• Terrestrial Biomes
• desert or dune
• savanna or grassland
• chaparral
• forest
• rainforest
• scrub forest
• mountains

• Aquatic Biomes
• lakes and ponds
• rivers and streams
• temporary pools

• Wetlands
• marsh
• swamp

• Other Habitat Features
• urban
• suburban
• agricultural

Physical Description

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)

• Other Physical Features
• ectothermic
• heterothermic
• bilateral symmetry

• Sexual Dimorphism
• female larger
• sexes shaped differently

• Range length

  0.9 to 2.5 mm

  0.04 to 0.10 in

Development

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)

Reproduction

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)

• Key Reproductive Features
• sexual
• fertilization
  • internal
• oviparous

• Parental Investment
• pre-fertilization
  • provisioning

Behavior

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.

During the autoinfective phase, filariform larvae reenter the host through the lower gut mucosa or the perianal skin. (Cheng, 1986; Roberts and Janovy, 2000)

• Key Behaviors
• parasite
• motile
• sedentary

Communication and Perception

Nematodes within the Secernentea have phasmids, which are unicellular glands. Phasmids likely function as chemoreceptors. Females may produce pheromones to attract males.

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)

• Communication Channels
• tactile
• chemical

• Other Communication Modes
• pheromones

• Perception Channels
• tactile
• chemical

Food Habits

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)

• Primary Diet
• carnivore
  • eats body fluids
• detritivore

• Other Foods
• detritus
• microbes

Predation

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.

Ecosystem Roles

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.

• Ecosystem Impact
• parasite

Economic Importance for Humans: Positive

Strongyloides stercoralis has no positive effects on humans.

Economic Importance for Humans: Negative

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)

• Negative Impacts
• injures humans
  • causes disease in humans
• causes or carries domestic animal disease

Conservation Status

There is no conservation status for Strongyloides stercoralis.

• IUCN Red List [Link]

  Not Evaluated

Other Comments

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)

Contributors

Renee Sherman Mulcrone (editor).

Jason Hallman (author), University of Michigan-Ann Arbor, Solomon David (editor), University of Michigan-Ann Arbor.

References

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.