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Introduction of Strongyloides stercoralis

Strongyloides stercoralisis a human pathogenic parasitic roundworm causing the disease strongyloidiasis. The Strongyloides stercoralis nematode can parasitize humans. The adult parasitic stage lives in tunnels in the mucosa of the small intestine. The genus Strongyloides contains 53 species.

Geographic distribution of Strongyloides stercoralis

This infection is associated with fecal contamination of soil or water. so, it is a very rare infection in developed economies than in developing S. stercoralis can be found in areas with tropical and subtropical climates. Estimates of the number of people infected vary with one estimate putting the figure at 370 million worldwide.

Life cycle of Strongyloides stercoralis

It completes into two stages

a) Free-living cycle

b) Parasitic cycle

Free-living cycle of Strongyloides stercoralis

In this cycle, the rhabditiform larvae passed in the 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 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.

Parasitic cycle of Strongyloides stercoralis

The infectious larvae penetrate the skin when it contacts the soil. 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. Urocanic acid concentrations can be up to five times greater in the foot than in any other part of the human body. Some of them enter the superficial veins and are carried in the blood to the lungs, where they enter the alveoli. They are then coughed up and swallowed into the gut, where they parasitize 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, the occurrence of autoinfection in humans with helminthic infections is recognized only in Strongyloides stercoralis and Capillaria philippinensis infections. 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 hyperinflations in immunosuppressed individuals.

Morphology of Strongyloides stercoralis

Male having a size of about 0.9 mm whereas female larger i.e. from 2.0 to 2.5 mm.

Males can be distinguished from females by two these structures:

the spicules

and gubernaculum.

Autoinfection

It is an unusual feature of S. stercoralis. Auto infection is the development of L1 into small infective larvae in the gut of the host. These auto infective larvae penetrate the wall of the lower ileum or colon or the skin of the perianal region, enter the circulation again, travel to the lungs, and then to the small intestine, thus repeating the cycle.

Symptoms

Initially, many people infected are asymptomatic. Anyway, Symptoms include

dermatitis:

swelling,

itching,

larva currens,

and mild hemorrhage at the site where the skin has been penetrated.

Spontaneous scratch-like lesions may be seen on the face or elsewhere.

Diagnosis of Strongyloides stercoralis

Fecal wet preparation: showing larvae (juvenile, rhabditiform, or filariform) as shown above video

Culturing fecal samples on agar plates: Trekking of Strongyloides on an agar plate as shown below video

Serology: serodiagnosis through ELISA and duodenal fumigation.

Molecular test: PCR

Treatment

Choice of drugs are-

1. Albendazole
2. Thiabendazole
3. ivermectin
4. mebendazole
5. and piperazine

Further Readings

1. Medical Parasitology by Abhay R. Satoskar, Gary L. Simon, Peter J. Hotez and Moriya Tsuji
2. Atlas of Medical Helminthology and protozoology -4th edn  -P.L.  Chiodini, A.H. Moody, D.W. Manser
3. Merkell and Voge’s medical parasitology
   9th edition.
4. Parasitology: 12th edition
   By K. D. Chatterjee
5. District laboratory practice in Tropical countries –Part-I.
   By Monica Chesbrough.
6. Isenberg clinical microbiology procedures Handbook
   2nd edition. Vol. 2
7. Atlas of Human Parasitology, Lawrence R Ash, Thomas C. Orihel, 3 rd ed, Publisher ASCP Press, Chicago.
8. Topley & Wilsons’ Principle of parasitology. Editors: M.T. Parker & amp; L.H. Collier, 8 th ed 1990, Publisher Edward Arnold publication, London.
9. Molecular Medical Parasitology. Editors: J. Joseph Marr, Timothy W. Nilsen, and Richard W. Komuniecki, Publisher Academic Press, an imprint of Elsevier Science.