Helminth

Strongyloides stercoralis, an intestinal nematode commonly known as the human threadworm, affects millions of people worldwide.(1) It is endemic in Southeast Asia, Latin America, sub-Saharan Africa, and parts of the southeastern USA.(2) In the USA, the highest prevalence rates are found in eastern Kentucky and rural Tennessee. A unique feature of Strongyloides stercoralis infection is the occurrence of an autoinfection cycle which permits persistence of the parasite years after the normal host has left an endemic area. In this cycle, the rhabditiform larvae in the duodenojujenal portion of the small intestine transform directly into filariform (infective) larvae. The filariform larvae without leaving the body can reinfect the patient by penetrating the intestinal mucosa. This distinctive characteristic of Strongyloides, to persist and replicate within the host for decades, produces minimal or no symptoms. Immunocompromised patients may develop a fulminant illness due to a unique process in the life cycle of Strongyloides in which there are dramatic increases in the number of filariform larvae. In the hyperinfection syndrome, massive numbers of larvae migrate through the intestinal mucosa and into the lungs (the usual migration pattern) and disseminate to involve other organ systems not ordinarily a part of the life cycle of the parasite. Larvae may be found in the central nervous system, kidneys, liver, and almost any other organ.