Biology

The lifecycle of Strongyloides is remarkable for a variety of reasons as it shows the intricacies of evolution and flexibility of sexuality.

  • worms in the infected host, usually man, are exclusively female
  • male worms are not parasitic and found in the soil 

Immunological and environmental cues trigger the developmental pathway which allows those female worms to develop further and become male worms.

The Strongyloides' life cycle is more complex than that of most nematodes 

There are 2 types of life cycle

Free-living cycle 
  • rabditiform larvae passed in the stool 
  • either molt twice and become infective filariform larvae (direct development) 
  • or molt four times and 
    • become free living adult males 
    • or females that mate produce eggs 
    • rabditiform larvae hatch. 
  • rabditiform larvae
    • develop into a new generation of free-living adults, 
    • or into infective filariform larvae. 
  • filariform larvae penetrate the human host to initiate the parasitic cycle.
Parasitic cycle

Filariform larvae 

  • found in contaminated soil 
  • penetrate the human skin  
  • are transported to penetrate the alveolar spaces
  • carried through the tree to the pharynx
  • swallowed 
  • reach the small intestine 
  • molt twice and become adult female worms. 
  • female worms live threaded in the epithelium of the small intestine 
    • parthenogenesis produce eggs, 
    • eggs yield rabditiform larvae. 

Rabditiform larvae

  • passed in the stool (see "Free-living cycle" above), 
  • or cause autoinfection
    • the rabditiform larvae become infective filariform larvae
    • filariform larvae can penetrate 
      • the intestinal mucosa (internal autoinfection) 
      • or the skin of the perianal area (external autoinfection); 
    • filariform larvae may follow
      •  the previously described route(see "Free-living cycle" above)
      • or disseminate widely in the body. 

 Autoinfection may explain 

  • persistent infections in persons who have not been in an endemic area for many years 
  • hyperinfections in immunodepressed individuals

Autoinfection in humans with helminthic infections is only seen in Strongyloides stercoralis and Capillaria philippinensis infections.