Strongyloides stercoralis- Do you know what’s in your soil?  

Introduction

            Strongyloides stercoralis is an intestinal nematode that infects millions of people worldwide. It is predominantly found in tropical and subtropical areas.[4] S. stercoralis is unique in its ability to carry out a parasitic life cycle where it persists in a host through an asexual autoinfective cycle, or a free-living life cycles where it lives and reproduces in the soil without a host. S. stercoralis is responsible for causing strongyloidiasis and it can autoinfect the same host over and over without an intermediate host, making strongyloidiasis a very incessant disease. Most infected individuals are asymptomatic making it hard to treat the disease in a timely fashion.

Symbiont Description

            Strongyloides stercoralis is in the phylum nematoda and the class secernentea. They normally appear as slender cylindrical worms that have a long esophagus and uterus intertwined. This gives them the appearance of a twisted thread and therefore is commonly named ‘thread-worms’. S. sterocoralis’  developmental cycle involves the formation of eggs that appear as small oval thin-shelled bodies, free-living and parasitic larvae, free-living male and female adult worms, and  parasitic female worms. The eggs are usually 50-58µm in length by 30-34µm and the free-living larvae that live in the soil (L1 and L2) measure to 350µm. [3] These larvae contain a rhabditiform pharynx that consists of a muscular esophagus for feeding on particulate material.  The infective larvae (L3) measure to 600µm in length and have a filariform pharynx with a long fine esophagus specialized for sucking fluids after penetrating the host tissues. [3] The parasitic worms are all females. They measure from 2-3mm in length and are characterized by their extremely long filariform pharynx and a blunt pointed tail. The free-living worms (male and female) have a rhabditiform pharynx and are smaller in size, measuring up to about 1mm in length. [3]

        

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Host Description

            Depending on the life cycle, S. stercoralis can depend on a human host or it can be host independent, living in the soil of warm tropical and temperate regions. In the parasitic life cycle, S. stercoralis resides in the epithelium of the small intestine of humans. Here, the filariform larvae will molt, become and adult, and reproduce to yield rhabditiform larvae. [1]  

Life Cycle

            The life cycle of Strongyloides stercoralis consists of both free-living and parasitic stages. In the free-living stage, the S. stercoralis resides in the soil as a rhabditiform larva and develop into either filariform larvae or adults. If the larva only molts twice, it becomes the infective filariform larva capable of infecting humans. This is considered to be direct development. If the larva molts four times, it becomes a free living male or female that are capable of mating and producing eggs, from which more rhabditiform larvae hatch, starting the life cycle over again.

           

In the parasitic life cycle, the filariform larvae in the soil can penetrate the human skin just form a human coming in contact with the contaminated soil. From there the larvae are transported to the lungs where they penetrate the alveolar spaces, through the bronchial tree, into the pharynx where they are swallowed and finally reach the small intestine. In the small intestine the larvae molt twice and become adult female worms that thread themselves into the epithelium of the small intestine. Once embedded into the epithelium, the produce eggs by parthenogenesis, which yield rhabditiform larvae. The rhabditiform larvae can them either be passed through the stool and undergo a free-living life cycle, or they can cause and autoinfection. In the autoinfection, the rhabditiform larvae become filariform larvae that penetrate either the intestinal mucosa of the skin of the perianal area. If they penetrate the intestinal mucosa, it is considered an internal autoinfection, if they penetrate the skin of the perianal area it is considered an external autoinfection.  In either case, the filariform larvae follow the same route as the progeny before them; being carried to the lungs, the bronchial tree, the pharynx, and the small intestine where they mature into adults and start the whole life cycle over again, or they scatter widely through out the body.

Ecology

            Strongyloidiasis, which is caused by Strongyloides stercoralis, is frequently asymptomatic. However, some gastrointestinal symptoms include abdominal pain and diarrhea. Pulmonary migration of filariform larvae can cause symptoms, including Loeffler’s syndrome. [4] Some dermatologic symptoms can include urticarial rashes in the buttocks and waist areas. Strongyloidiasis in immunocompromised individuals can cause abdominal pain, distension, shock, pulmonary and neurological complications, and septicemia. It could also potentially be fatal.[1] In these patients the autoinfective cycle can become amplified into a potentially fatal hyperinfection syndrome. Infection can be detected by looking for larvae in stool samples. Strongyloidiasis is difficult to diagnose because the parasite load is low and the larval output is irregular. [2] Although larvae are rarely found in fecal samples, fecal cultures can increase the sensitivity of microscopic diagnosis. They concentrate the larvae or amplifying the populations through a generation of free-living males and females.  Many stool samples may need to be examined because the sample can be so insensitive.

To treat uncomplicated strongyloidiasis ivermectin or albendazole can be used. Thiabendazole is also commonly used, but it has unpleasant side effects, including nausea, vomiting, dizziness, malaise and smelly urine. [4] Ways that you could prevent from acquiring S. stercoralis parasite include wearing of solid shoes in endemic areas, thoroughly washing salad vegetables, prohibiting the use of night soil to fertilize gardens, and sanitarily dispose of feces.

An increasing numbers of immunocompromised individuals throughout the world have called for closer examination of the conditions under which S. stercoralis infection becomes dangerous.[1]  Researchers continue totry to find better approaches to identifying, screening, and treating those at risk. If researchers are successful in finding better screening techniques it will likely decrease the impact of with S. stercoralis infection.

The heterogonic life cycle

Strongyloides stercoralis has an advantage in habitat and in reproduction by having two different life cycles. It can autoinfect the same host over and over without any intermediate host, making strongyloidiasis a very persistent disease. S. stercoralis increase their survival  by taking advantage of both life cycles and adapting in terms of reproduction and other means to accommodate life in a host, such as a human, or in the soil. Also, since reproduction can occur both in the free-living and in the parasitic life cycle, S. stercoralis have the advantage of being able to produce more progeny. This makes cases of infestation more common and the areas in which they can infest less selective, ultimately increasing parasite fitness.

References

[1] Keiser, P. B., and T. B. Nutman. "Strongyloides Stercoralis in the Immunocompromised Population." Clinical Microbiology Reviews 17.1 (2004): 208-17. Print.

[2] Siddiqui, Afzal A., and Steven L. Berk. "Diagnosis of Strongyloides Stercoralis Infection." Travel Medicine 33 (2001): 1040-047. Print.

[3] Springer-Verlag. "Strongyloides." Australian Society for Parasitology. 2001. Web. 1 Feb. 2012. <http://parasite.org.au/para-site/text/strongyloides-text.html>.

[4 ]"Strongyloides Stercoralis." Parasites In Humans. 2010. Web. 01 Feb. 2012. <http://www.parasitesinhumans.org/strongyloides-stercoralis.html>.