Strongyloides stercoralis is a nematode and human pathogenic parasitic roundworm causing the disease called strongyloidiasis. These slender cylindrical worms have a long oesophagus and uterus intertwined, giving the appearance of a twisted thread, hence their common name of “dwarf thread-worm”.
They are unique amongst nematodes, being capable of both parasitic and free-living reproductive cycles. Only parthenogenetic female worms are parasitic, living in the small intestinal mucosa of various mammals, birds, reptiles and amphibians and are capable of of reproducing in the soil (warm mist conditions) without necessarily needing a host.
Transmission involves a geo-helminth phase, where rhabditiform larvae in the soil form infective filariform larvae which penetrate the skin of their hosts. Sometimes larvae develop into male and female worms which undergo one or more free-living cycles in the soil before producing infective larvae again.
Strongyloides stercoralis has been reported in other domestics animals, including cats and dogs. However, it seems that the species in dogs is typically not Strongyloides stercoralis, but a related species know as Strongyloides canis.
it is a very rare infection in developed countries meanwhile In developing countries, it is less prevalent in urban areas than in rural areas where sanitation standards are poor.The parasitic worm can also be found in areas with subtropical and tropical climates.
The parasite appears to have a high prevalence in some areas of central america and Brazil. It is also endemic in Africa, but with a prevalence (1% or less).
An estimates of the number of people infected with these parasites varies with one estimate putting the figure at 370 million worldwide. In some subtropical and tropical countries,the local prevalence can exceed 40% .
Morphology of strongyloides stercoralis
- The Male worm grows to only about 0.9 mm that is 0.04 inch in length
- Females worms can grow from 2.0 to 2.5 mm.Longer than the male.
- Both sexes possess a tiny buccal capsule and cylindrical esophagus without a posterior bulb.
- In the free-living stage, the esophagi of both sexes are rhabditiform. Males can be distinguished from females by two structures: the gubernaculum and spicules
Transmission and Life cycle of strongyloides stercoralis
Transmission of dwarf threadworm is similar to that of Hookworm but here,Larva not eggs are recovered from the soil or stool except
- Through direct skin penetration by the filariform worm in the soil or by autoinfection
- Zoonotic infections is also common by contact with dogs and cat.
- It can also be transmitted walking barefoot in warm climates, where sanitation is poor with contaminated soil and through contaminated food.
Life cycle of strongyloides stercoralis
- Larvae of strongyloides are mostly deposited in stools of infected people.Therefore the larvae are the diagnostic features.
- If not properly handle and release in the environment by defecation,uses of untreated faeces as fertilizer and they end up contaminating the soil and the filariform larva which penetrate the skin of humans mostly through the feet.
- Initially, itching and a rash may occur at the site of infection or penetration by the filariform worm.
- In the host,Larvae migrate,pass up to the trachea and are swallowed.
- Larvae becomes mature worms in the small intestine and lays eggs
- Eggs hatch to rhabditiform larvae in the intestine.
- These larvae are then passed out in faeces or become filariform causing autoinfection as in the case of Strongyloides felis
- The Autoinfection feature makes strongyloidiasis due to Strongyloides stercoralis an infection with several unusual feature
Signs and Symptoms of strongyloidiasis infection
- Many infected people are asymptomatic but some symptoms of strongyloidiasis includes :
- Chronic diarrhea
- larva currens which is the appearance of a red line that moves rapidly (more than 5 cm or 2 in per day), and then quickly disappears.This is a pathognomonic for auto infective larvae and can be used as a diagnostic criterion for strongyloidiasis due to Strongyloides stercoralis.
- mild hemorrhage especially at the site of penetration
- Also spontaneous scratch-like lesions may also be seen on the on the body.
- In conditions where the parasite reaches the lungs, Patient may have burning sensation at the level of the chest followed by wheezing and coughing along with pneumonia-like symptoms (Löffler’s syndrome).
- In some cases,the intestines could eventually be invaded, leading to burning pain, the damage of tissues, sepsis, and ulcers.
- Yellow mucus may be seen in stool with a recognizable smell.
- In some severe cases, edema may result in obstruction of the intestinal tract, as well as loss of peristaltic contractions
Laboratory Diagnostic of Strongyloides stercoralis
- The laboratory diagnosis of strongyloides stercoralis infection is by finding the motile larvae in fresh stool samples microscopically.
- It can also be concentrated using the formol ether technique
Detection of Strongyloides stercoralis larvae in faeces using the water emergence techniques
This technique is simple and also efficient.A fresh semi-formed or form stool samples (not more than 2 hours old) is required for this technique
- Using an applicator stick or a wooden stick,make a central deep depression in the formed stool sample.
- Fill the depression with warm water temperature should not be over 37° C
- Now incubate stool sample at 35-37 °C for at least 1 – 3 hours.In Order for the larvae to migrate out of the sample if present
- Using a pipette,probably a bulb or plastic bulb pipette,transfer some of the water to a clean slide to make a wet mount covering it with a cover slide or you can still transfer all the water into a test tube and centrifuge then make a wet mount with the sediment.
- Examine the wet mount microscopically for motile larvae using the 10X objectives.
Prevention of Strongyloidiasis infections
The prevention of strongyloidiasis infection is the same with hookworm infections
The infective larvae of strongyloides stercoralis develop and survive in the soil,an environment of damp dirt, particularly warm soil. Therefore the main ways of precaution are those dictated by practising good hygiene behaviors such as :
- Avoid defecating in the open, but rather do in toilets.
- Avoid the use of untreated human excreta,raw sewage as fertilizer in agriculture.
- Avoid walking barefoot in known infected areas as the larva penetrate the skin
- Remember to deworm pet dogs and cats.
- Using a barrier such as gloves,boots aprons etc to prevent the skin from touching the soil when sitting on the ground or working especially farmers,bricklayers,lumbermen,gardeners etc
- Avoiding consuming soil that may be contaminated with hookworm by properly washing fruits,cassava and other soil products
- Covering children’s sandboxes
- Covering moist soil requiring frequent contact with a waterproof liner
Treatment of Strongyloidiasis infections
Prevention is better than cure by improved sanitation thats is proper disposal of feces, practicing good hygiene (washing of hands), etc.,
- The drugs of choice is Ivermectin
- Thiabendazole can be use but has side effect such as dizziness, vomiting, nausea, malaise and a lower efficacy,
- Second drug of choice is albendazole.