**4. Life cycle**

The life cycle of *S. stercoralis* includes a parasitic cycle (within human hosts) and an environmental cycle (free-living larvae). The parasitic cycle occurs when the infective filariform larvae penetrate the skin and secrete metalloproteases that facilitate penetration. The main larval route is via the bloodstream to the lungs, where they break into the alveolar spaces within hours after infection, promote haemorrhage, ascend the respiratory tree, are swallowed, and migrate to the intestine. Alternatively, the larvae may migrate directly through connective tissues (Grove, 1994, 1996).

Fig. 1. Life cycle of *Strongyloides stercoralis* (modified from Carvalho & Da Fonseca Porto, 2004, and http://emedicine.medscape.com/article/229312-overview#a0104).

approximately 1 mm, live in the soil, and reproduce sexually; females are slightly larger than males (Speare, 1989). Embryonated eggs are thin-shelled and measure approximately 55 - 60 μm in length and 28 - 32 μm in width. Rhabditiform larvae are the first-stage larvae (210 μm) and develop into free-living larvae or third-stage infective larvae that measure

The life cycle of *S. stercoralis* includes a parasitic cycle (within human hosts) and an environmental cycle (free-living larvae). The parasitic cycle occurs when the infective filariform larvae penetrate the skin and secrete metalloproteases that facilitate penetration. The main larval route is via the bloodstream to the lungs, where they break into the alveolar spaces within hours after infection, promote haemorrhage, ascend the respiratory tree, are swallowed, and migrate to the intestine. Alternatively, the larvae may migrate directly

Fig. 1. Life cycle of *Strongyloides stercoralis* (modified from Carvalho & Da Fonseca Porto,

2004, and http://emedicine.medscape.com/article/229312-overview#a0104).

approximately 490-630 μm and are capable of infecting the host (Schad, 1989).

**4. Life cycle** 

through connective tissues (Grove, 1994, 1996).

The infective larvae reach the small intestine, where they moult twice to become female adult worms. These females reproduce in the absence of males by parthenogenesis. The females are embedded in the intestinal mucosa and produce eggs in the duodenum. The rhabditiform larvae emerge from the hatching eggs and migrate into the intestinal lumen, then pass into the external environment with the faeces. Depending on temperature and humidity, the rhabditiform larvae may have two different life cycles in the environment: an indirect (heterogonic) life cycle, in which the rhabditiform larvae differentiate into freeliving adults and release eggs that hatch and transform into infective larvae, or a direct (homogonic) life cycle, in which the rhabditiform larvae may moult directly into filariform larvae and repenetrate the host skin, restarting the cycle (Schad, 1989).

One characteristic that differentiates *S. stercoralis* from almost all other worms is its capacity to replicate within the host. Rhabditiform larvae in the bowel lumen transform into filariform larvae before excretion and invade the intestinal wall or the perianal skin, permitting ongoing cycles of autoinfection, an important feature of strongyloidiasis (Concha et al., 2005).
