Editing Strongyloides stercoralis (section)

==Life cycle==
The life cycle of this parasite is more complex than that of most [[nematode]]s, with its alternation between free-living and parasitic cycles, and its potential for autoinfection (the parasite has the ability to complete its life cycle without the involvement of another host) and multiplication within the [[host (biology)|host]]. The parasitic cycle is [[homogonic]], while the free-living cycle is heterogonic. The heterogonic life cycle is advantageous to the parasite because it allows reproduction in the absence of a host.

In the free-living cycle, the [[rhabditiform]] [[larva]]e passed in the [[human feces|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. In the direct development, first-stage larvae (L1) transform into infective larvae (IL) via three molts. The indirect route results first in the development of free-living adults that mate; the female lays eggs, which hatch and then develop into IL. The direct route gives IL faster (three days) versus the indirect route (seven to 10 days). However, the indirect route results in an increase in the number of IL produced. Speed of development of IL is traded for increased numbers. The free-living males and females of ''S. stercoralis'' die after one generation; they do not persist in the soil. The latter, in turn, can either develop into a new generation of free-living adults or develop into infective [[filariform]] larvae. The filariform larvae penetrate the human host skin to initiate the parasitic cycle.
Upon contact with contaminated soil, infectious larvae contained in the soil can penetrate the skin. 

Some of the larvae enter the superficial veins and are carried in the blood to the lungs, where they enter the [[Pulmonary alveolus|alveoli]]. They are then coughed up and swallowed into the gut, where they parasitise the intestinal mucosa of the [[duodenum]] and [[jejunum]]. 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. Only females will reach reproductive adulthood in the intestine. Female strongyloids reproduce through parthenogenesis. The eggs hatch in the intestine and young larvae are then excreted in the feces. It takes about two weeks to reach egg development from the initial skin penetration. By this process, ''S. stercoralis'' can cause both respiratory and gastrointestinal symptoms. The worms also participate in autoinfection, in which 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.{{citation needed|date=June 2024}}  In the case of ''Strongyloides'', autoinfection may explain the possibility of persistent infections for many years in persons not having been in an endemic area and of hyperinfections in immunodepressed individuals.

''Strongyloides'' is thought to be attracted to humans via paired thermosensory neurons.<ref>{{cite journal|doi=10.1016/j.cub.2022.04.010|pmid=35483361|s2cid=9158753 |title=The neural basis of heat seeking in a human-infective parasitic worm |date=2022 |last1=Bryant |first1=Astra S. |last2=Ruiz |first2=Felicitas |last3=Lee |first3=Joon Ha |last4=Hallem |first4=Elissa A. |journal=Current Biology |volume=32 |issue=10 |pages=2206–2221.e6 |pmc=9158753 |bibcode=2022CBio...32E2206B }}</ref> While ''S. stercoralis'' is attracted to chemicals such as carbon dioxide or sodium chloride, these chemicals are not specific. Larvae have been thought to locate their hosts via chemicals in the skin, the predominant one being [[urocanic acid]], a [[histidine]] metabolite on the uppermost layer of skin that is removed by sweat or the daily skin-shedding cycle.<ref name="acid">{{cite journal|doi=10.1073/pnas.0610193104|title=Urocanic acid is a major chemoattractant for the skin-penetrating parasitic nematode Strongyloides stercoralis|year=2007|last1=Safer|first1=D.|last2=Brenes|first2=M.|last3=Dunipace|first3=S.|last4=Schad|first4=G.|journal=Proceedings of the National Academy of Sciences|volume=104|issue=5|pages=1627–30|pmid=17234810|pmc=1785286|bibcode=2007PNAS..104.1627S|doi-access=free}}</ref> Urocanic acid concentrations can achieve at least fivefold greater levels on the foot sole than any other part of the human body.<ref name="acid"/><ref>{{cite journal|vauthors=Kavanagh G, Crosby J, Norval M|title=Urocanic acid isomers in human skin: analysis of site variation|year=1995|doi=10.1111/j.1365-2133.1995.tb02746.x|volume=133|issue=5|pages=728–731|pmid=8555024|journal=British Journal of Dermatology}}</ref>{{verification needed|date=June 2024}}