Strongyloides is a genus of parasitic nematodes that infect a wide variety of vertebrates (mammals, birds, amphibians, reptiles); two species (S. stercoralis and S. fuelleborni) infect humans (Speare, 1989). Some 50 million individuals are infected with Strongyloides spp. worldwide (Grove, 1989). For most individuals there are often no overt signs of infection. However, in individuals that are immunocompromised the infection can disseminate throughout the body and is fatal in the absence of anti-Strongyloides therapy. S. ratti has been used for some years as a model organism for investigating the biology of nematode parasitic infections (Viney, 1999).
Strongyloides spp. have an unusual and complex life cycle involving both free-living and parasitic phases (Figure 20.1).
Parasitic adult females dwell in the gut of the host (which in the case of S. ratti, as its name implies, is the rat). Here they burrow into the intestinal mucosa and reproduce by mitotic parthenogenesis (Viney, 1994), passing eggs via the feces into the external environment. These eggs can either moult through two larval stages into infective third-stage larvae (iL3s) or through four larval stages into free-living adult females and males (Figure 20.1). These adults mate by conventional sexual reproduction and produce progeny which (as above) can develop into iL3s (Viney et al., 1993; Harvey et al., 2000). Infective L3s infect new hosts by skin penetration. Experimental genetic studies found that reproduction of the parasitic females of S. ratti is functionally mitotic (Viney, 1994), and so the progeny of the parasitic female are genetically identical to each other and to their mother. Anecdotal observations had noted a large difference in the lifespan potential of the adult free-living and parasitic female morphs: approximately one week and one year, respectively (Viney, 1994; Gemill et al., 1997).
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