By far, S. stercoralis has one of the most complicated life cycles of all the human parasites. It basically has three life cycles. A normal, free living and an autoinfective cycle. Simplified, infective filariform larva penetrate the skin and migrate via the blood to the lungs. In the lungs they break out of the alveoli, migrate to the epiglottis and are swallowed. Mature females live in the crypts of the duodenum and upper jejunun. Reports of adult females in the lungs are not confirmed.(6) In the intestine reproduction is parthenogenetic (development by unfertilized gamete).(4)(5) The female releases eggs that hatch and develop into rhabditiform larva that pass in the feces, maturing into infective filariform larva.
In the free living life cycle the rhabditiform larva passed in the feces mature in the soil to adult males and females. This is known as a free living generation. The eggs produced mature to infective filariform larva. Finding a free living male or female in the feces suggests a diseminated infection, slow bowel or constipation. The rhabditiform larva present developed into a free living generation closely resembling the free living generation in soil but shorter and more blunt in morphology.
Rhabditiform larva present in the intestine may mature to the infective filariform larva within the intestine. This is an autoinfection. In immune compromised patients this may result in a hyperinfection. Here, the great numbers of larva completing a life cycle significantly increase patient morbidity if not treated. Man is the principle host of S. stercoralis.