Life cycles of Strongyloides stercoralis:
(1) A filariaform larvae in the environment penetrates the skin or mucous membrane of the new host and reaches the lungs via the venous circulation. It exits into an alveolus and migrates up the tracheobronchial tree until it is swallowed. When it reaches the jejunum it burrows under the mucosa and develops into the adult female.
(2) The adult female releases eggs that become rhabditiform larvae while in the intestines. These may either be evacuated into the stool or mature into the filariaform larvae.
(3) Rhabditiform larvae in the environment develop into free-living adult male and female worms which produce eggs that develop into rhabditiform and filariaform larvae. The filariaform larvae may enter to new hosts to perpetuate the cycle in new hosts.
(4) Filariaform larvae that develop in the bowel lumen may penetrate the bowel wall and enter the venous circulation with perpetuation of the cycle in the same host for decades (autoinfection).
Requirements for infection:
(1) Exposure to soil or environments that have fecal contamination.
(2) A warm and humid environment conducive to the survival of the free-living adult worms.
The infection may be found in temperate, subtropical and tropical regions.
People at risk for initial infection:
(1) farmers
(2) miners or others who dig in the soil
(3) institutionalized patients who are mentally retarded
(4) prisoners exposed to primitive conditions (especially prisoners of war)
(5) low socioeconomic status
(6) alcoholics
Walking barefoot would increase the risk of exposure, especially if there are breaks in the skin.
The filariaform larvae do not do well in water, so swimming or after drinking contaminated water are not considered significant routes of transmission.
