Strongyloides stercoralis Flashcards
How is S. stercoralis classified?
Metazoa - helminth (nematode)
How is S. stercoralis transmitted
It is a soil-transmitted helminth - soil contaminated with infectious larvae which penetrate the skin of a susceptible host
Less common: blood transfusion, organ transplantation, via breastmilk to infant
What is the life cycle of S. stercoralis?
2 separate life cycles that alternate: free-living cycle (start) and parasitic cycle
- Rhabditiform larvae are passed in the stool of an infected definitive host
- Develop into either infective filariform larvae (direct development) or free-living adult males and females
- That mate and the female worms produce fertilized eggs
- From the embryonated eggs rhabditiform larvae hatch
- And eventually become infective filariform (L3) larvae this takes 3-4 days
- The filariform larvae penetrate the human host skin to initiate the parasitic cycle. This second generation of filariform larvae cannot mature into free-living adults and must find a new host to continue the life cycle
The parasitic life cycle starts here
- Filariform larvae in contaminated soil penetrate human skin when skin contacts soil, and migrate to the small intestine
- It has been thought that the L3 larvae migrate via the bloodstream and lymphatics to the lungs, where they are eventually coughed up and swallowed. However, L3 appears capable of migrating to the intestine via alternate routes (e.g. through abdominal viscera or connective tissue). In the small intestine, the larvae molt twice and become adult female worms
- The females live embedded in the submucosa of the small intestine and produce eggs via parthenogenesis (parasitic males do not exist)
- Which yield rhabditiform larvae. The rhabditiform larvae can either be passed in the stool or can cause autoinfection by reinfecting the same host again by penetrating either the intestinal mucosa or the skin of the perianal area, resulting in autoinfection
How does autoinfection work in S. stercoralis and what is the significance of this?
Filariform reinfects the host and carried to the lungs, pharynx, and small intestine
Significance: untreated cases can result in persistent infection even after many decades of resistance in non-endemic areas and may contribute to the development of hyperinfection syndrome
What type of life cycle has S. stercoralis?
Indirect life cycle
human intermediate host
Soil definitive host
What are other reservoirs of S. stercoralis
soil
domestic dogs / cats
other primates
What is meant by hyperinfection syndrome by S. stercoralis infection
The S. stercoralis parasite has internal infective cysts meaning that the worm load can increase
What is meant by hyperinfection syndrome by S. stercoralis infection
The S. stercoralis parasite has an internal infective cyst meaning that the worm load can increase - Explosive multiplication of infective larvae and thus the worm burden within a host (happens usually in immunocompromised individuals)
What is the incubation time of S. stercoralis?
23-32 days - However the rhabditiform larvae develop into infective filariform larvae in 3-4 days
What is the infective form of S. stercoralis and what stage is diagnosed?
infective form: filariform larvae
Diagnostic form: rhabditiform larvae
What are the symptoms of S. stercoralis?
Acute Strongyloidiasis
Path of the parasite through the skin to the small intestine
Not specific symptoms
Chronic Strongyloidiasis
Can last for years
asymptomatic - gastrointestinal or pulmonary symptoms
Disseminated strongyloidiasis
Larvae migrate beyond the organs of autoinfective cycle
Where is S. stercoralis endemic?
Tropical and subtropical areas (larvae can survive up to 2 weeks in an environment with a temp. 20-28 and high moisture)
How does S. stercoralis ensure its indefinite continuation of the infection with infected people as its reservoir?
auto-infection characteristic
How is S. stercoralis diagnosed?
Microscopy (golden standard)
- Larvae in the feces (appr. 28 days)
- Low sensitivity (<40 eggs/day)
How is S. stercoralis diagnosed?
Microscopy (golden standard)
- Rhabditiform larvae in the feces (appr. 28 days)
- Low sensitivity (<40 eggs/day)
ELISA, IFAT, GPAT, LIPS
- Serological detection of Strongyloides IgG (cannot distinguish between prior and current infection)
Other methods are molecular detection of S. stercoralis DNA through stool
