Strongyloides Stercoralis

Strongyloides stercoralis Objectives:describe the morphology of Threadworm explain the life cycle of Threadworm discuss the clinical feature of Threadworm explain the laboratory diagnosis and treatment of Threadworm Common Name: Threadworm. Is a human parasitic roundworm causing the disease strongyloidiasis. Its common name is threadworm. The adult parasitic stage lives in tunnels in the mucosa of the small intestine. Habitat of Strongyloides stercoralis • It exists in both a free-living form in the soil and as an intestinal parasite. • The adult female worm is found in the small intestine (duodenum and jejunum) of a man whereas the free-living female worms multiply in the environment. • Male worms are always free-living and thought to be non-parasitic. Morphology of Strongyloides stercoralis Parasitic Female Worm • The parasitic female worm is thin, semi-transparent, and colorless and lies embedded within the mucosal epithelium of the proximal small intestine. • It measures about 2.5 mm long, and 0.05 mm wide, whereas the free-living female worm is smaller and thicker (1 mm × 80 μm). • The anterior portion bears a mouth with three small lips. • It has a cylindrical esophagus occupying the anterior one-third of the body, the intestines in the posterior two-thirds, and the mid ventrally situated anus. • The posterior end is extremely pointed. • The reproductive system contains paired uteri, vagina, and vulva. The paired uteri lead to the vulva situated at the junction of the middle and posterior thirds of the body. • The worm is ovoviviparous. • A single female worm will produce up to 50 eggs per day. • The individual worm has a lifespan of 3 or 4 months, but because it can cause autoinfection, the infection may persist for years. * Its uterus intertwined, giving the appearance of a twisted thread, hence their common name is threadworm. Parasitic male worm • They are shorter and broader than females and measure about 0.6-1 mm in length and 40-50μm in width. • They are not seen in human infection because they do not have penetrating power, therefore do not invade the intestinal wall. • • Parasitic males do not exist in infected humans. They possess two spicules at the posterior end, which penetrate the female during copulation. Eggs • • • • Eggs are conspicuous within the uterus of a gravid female. Each uterus contains 8–10 eggs arranged anteroposteriorly in a single row They are thin-shelled, transparent, and oval and measure 50–60 μm in length and 30– 35 μm in breadth Eggs of Strongyloides are ovoviviparous i.e they immediately hatch within the mucosa and emerge into the lumen of the small bowel as noninfectious rhabditiform larvae (first stage larvae), which are excreted in feces. Larva Rhabditiform Larva (L1 stage) • This is the first stage of the larva which immediately hatch out of eggs in the small intestine. • It is the most common form of parasite found in the feces. • It is actively motile and measures 0.25 mm in length and 16µm in breadth. • It has a short mouth, a relatively short muscular double bulb esophagus, and an inconspicuous genital primordium. • The L1 larva migrates into the lumen of the intestine and passes down the gut to be released in feces, in a warm, humid environment, mature, and become free-living adult male and female worms. • It is the diagnostic form found in human feces. Filariform Larva (L3 stage) • This is the third stage of the larva and infective stage. • The L1 larva molts twice to become the L3 larva. • It is long and slender and measures 0.55 mm in length. • It has a short mouth with a long esophagus of uniform width and a notched tail. • These infective L3 stages are long-lived and can persist in the environment until they encounter a suitable host • They enter the human host through the skin like ancylostoma. Mode of transmission • The main source of infection is soil contaminated with human feces. • Penetration of skin by the L3 larva (by walking barefoot). Larva releases hydrolytic enzymes that help in penetration. • Autoinfection (internal autoinfection). • By transplantation of organs such as kidneys to a new host. • Though rare, other routes of transmission of the larva have been reported like in utero, transmammary routes, or zoonotic transmission. Life cycle of Strongyloides stercoralis Host: Involve single host (man) Infective stage: Filariform larva (L3). The life cycle of S. stercoralis is complex because of the multiplicity of pathways through which it can develop. It is unique among human nematodes as it has a free-living soil cycle and a parasitic cycle, in which it can persist for long periods in soil by feeding on soil bacteria, passing through several generations. 1. Free-living soil cycle • The embryonated eggs hatch within the mucosa and emerge into the lumen of the small bowel as noninfectious rhabditiform larvae • The rhabditiform larvae are excreted in the stool of an infected definitive host in a warm, humid environment