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OverviewEdit

            Strongyloides stercoralis is a nematode that infects up to 100 million people worldwide. This nematode infects various mammals including humans, dogs, and cats and the adult typically resides in the grooves of the epithelial cells in the small intestine. The virulence of Strongyloides stercoralis can range anywhere from being relatively asymptomatic to being lethal in immunocompromised hosts. Typical symptoms associated with infection of this nematode include abdominal discomfort, throat irritation, rash at the point of entry, and potentially hyperinfection syndrome.

          Strongyloides stercoralis can be found worldwide due to its tough body cavity, variety of mammalian hosts, habitat preference, and life cycle. This nematode is primarily found in tropical and sub-tropical environments, but can be found on all continents. Strongyloides stercoralis’ ability to transition between parasitic and free-living lifecycles is also unique to this nematode, ultimately penetrating the skin of its mammalian host to develop into its adult form.

S. stercoralis worm

Infective filariform larval stage.

DescriptionEdit

            Strongyloides stercoralis has various life stages that include: and egg, rhabditform larvae, filariform larvae, and adult forms. The adult form that is found in the small intestines of its mammalian host ranges between 0.9 to 1.5mm in length and has a long, cylindrical body. Free-living males can be up to 0.9mm, and larvae can grow to 800µm. Like most nematodes, Strongyloides stercoralis is made up of four tissue layers: the epicuticle, exocuticle, mesocuticle, and endocuticle. The mouth of the nematode opens to the buccal capsule which is small relative to other nematodes. The buccal capsule is connected to the elongated esophagus which lacks a posterior bulb unlike other nematodes. The esophagus is connected to the intestine which is divided into three regions and connects to the rectum.

         
Adult female

Female adult.

Adult Male

Adult male

Strongyloides rhabditi

Rhabditi larval form

Strongyloides filariform

Filariform larva

The body capsule, or pseudocoel, also contains the reproductive organs. For males, vas deferens extend throughout the body cavity from the testes and widen to form the seminal vesicles. Males also have copulatory spicules and a gubernaculum. Females, on the other hand, typically have two ovaries with an oviduct extending from it connecting the ovaries to the uterus via a seminal receptacle. There is also a difference between the free-living and parasitic forms of female nematodes; free-living female nematodes have uteri at the posterior end of the nematode containing more eggs than the parasitic females which have their uteri located more equatorial. 
Strongyloides lifecycle

Life cycle of S. stercoralis

Life CycleEdit

Strongyloides stercoralis is a nematode with a unique life cycle, as it can alternate between free-living and parasitic life cycles. A person can become infected with this nematode if they come in contact with the infective filariform larvae that can penetrate human skin and enter the circulatory system, and ultimately end up the in the lungs. The host then coughs up and swallows the larvae which allow the larvae to enter the small intestine where they develop into female adults. The female worms are small and they live in tunnels between enterocytes in the small intestine. In the small intestine, the female adults produce eggs via parthenogenesis which develop into rhabditiform larvae and are excreted into the environment through feces. In a process known as auto-infection, the rhabditiform larvae develop into filariform larvae which penetrate the intestinal mucosa and can increase the number of infections in the host. At this point in S. stercoarlis’ life cycle, the rhabditiform larvae can either develop into the directly transmitted infective filariform larvae by molting twice, or molt four times and produce male and female adults that are free living. In the external environment, these free living adults can mate and produce eggs that at any point can develop into the infective filariform larvae and enter the parasitic life cycle by penetrating a mammalian host. 

Map of strongyloides prevalence

Geographical distribution of Strongyloides stercoralis by country

PathologyEdit

Strongyloides stercoralis is found on all continents except for Antarctica but primarily found in the tropics and sub-tropics. World-wide, potentially 30-100 million people are infected,  disproportionately infecting lower socioeconomic classes and rural areas. Strongyloidiasis is more prevalent in countries with poorer hygiene, as infection occurs when a person comes in contact with human waste.  Symptoms of S. stercoralis are typically mild or even asymptomatic, leading to individuals being unaware of their infection. Currently, the longest recorded infection of S. stercoralis is 65 years. Symptoms include a dry cough, intestinal problems, and a rash at the point of entry; however, hyper-infection syndrome can develop in persons that are immunocompromised and can be life-threatening. Disseminated infection occurs in result to a hyper-infection, and the larvae can be found all over the body, particularly in the sputum and skin and has an 87% rate of mortality of the host.

Treatment and PreventionEdit

Luckily, Strongyloides stercoralis can be diagnosed by the presence of larvae in the stool, serology, and or polymerase chain reaction. S. stercoralis is effectively treated by the use of ivermectin, thiabendazole, or albendazole, with ivermectin being the most effective. Ivermectin functions by selectively binding to glutamate-gated chloride ion channels in nerve and muscle cells causing cell death. A single dose of 200µg/kg bodyweight is effective for uncomplicated infections; however, in impoverished endemic countries it is often difficult to procure the drug.

 Unfortunately, there has been little public health measures to prevent infections of Strongyloides stercoralis. Countries that have improved sanitation and waste disposal have experienced a great reduction in the number of cases, however, most of the infections are found in impoverished areas where public health measures such as sanitation and waste disposal are much more difficult to implement. If possible, wearing shoes and limiting contact to potentially infected soil can drastically reduce the number of new infections.

Important ResearchEdit

            With between 30 and 100 million people in over 70 countries being infected with Strongyloides stercoralis, it is important that people are accurately diagnosed with strongyloidiasis and treated immediately to reduce the risk of hyperinfection syndrome which has an 87% mortality rate. Unfortunately, due to the low parasite load and irregular larval output, detection of S. stercoralis is missed in 70% of cases where single stool examination was used as the diagnostic tool. Stool examination is the typical diagnostic method, however, multiple stool examinations are often needed to detect the presence of the nematode. Multiple, unstandardized methods have been used to detect the presence of S. stercoralis larvae in stool samples; these methods include: direct smear of feces in saline–Lugol iodine stain, Baermann concentration, formalin-ethyl acetate concentration, Harada-Mori filter paper culture, and nutrient agar plate cultures. Examination of duodenal fluid is another diagnostic method that is 87% accurate, however, it is recommended only for children in need of rapid diagnosis as the test is invasive.  Eosinophilia is another indication of a S. stercoralis infection, but it is mild and unspecific. An ELISA test, a serodiagnostic test for S. strongyloides antibodies, has been found to be between 95 and 99% accurate, but unfortunately is only available in certain clinics. Due to the potential lethality of S. stercoralis infections, it is necessary to develop an accurate and standardized diagnostic test for this nematode, although not enough research is currently being done to evaluate and develop methods. 

ReferencesEdit

Eriksson CD, Steffens R, Siddiqui AA, Berk SL. Diagnosis of Strongyloides

stercoralis infection. Clin Infect Dis 2001;33:1040 –7.

Hallman, J. 2003. "Strongyloides stercoralis" Animal Diversity. Animal Diversity,  2012. Web. 25 Apr. 2014 .

<http://animaldiversity.ummz.umich.edu/accounts/Strongyloides_stercoralis/>

"Strongyloides." Centers for Disease Control and Prevention. Centers for Disease Control and Prevention, 19

July 2013. Web. 22 Apr. 2014. <http://www.cdc.gov/parasites/strongyloides/>.

"Strongyloidiasis." WHO. World Health Organization, 1 Jan. 2014. Web. 25 Apr. 2014.

<http://www.who.int/neglected_diseases/diseases/strongyloidiasis/en/>.

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