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.