**3.** *Schistosoma mansoni* **life cycle and maintenance in the laboratory**

Schistosome species are dioecious (having male and female reproductive organs in separate individuals) platyhelminthes and have complex life cycles comprising multiple morphologically distinct phenotypes in definitive mammalian and intermediate snail hosts. *S. mansoni* is one of the most common etiological agents of human schistosomiasis and is the most widely used schistosome model for chemotherapeutic studies.Schistosome infection of humans (or another definitive host) occurs by direct contact with freshwater containing freeswimming larval forms of the parasite, known as cercariae. Cercariae penetrate the intact human skin and transform into schistosomula, which reside in the skin for up to 72 hours before entering a blood vessel. Within the vascular system, schistosomula migrate via complex routes to their nal venous destination, where they mature into male and female adults. The mature flukes dwell in the human portal vasculature, depositing eggs in the intestinal wall that either pass to the gut lumen and are expelled in the faeces or travel to the liver and trigger immune-mediated granuloma formation and peri-portal fibrosis. Egg production commences 5 to 6 weeks after infection and continues for the life of the worm. The life cycle is completed when the eggs passed in the faeces hatch in the water, releasing the larval form miracidia, which then infect freshwater snails of the *Biomphalaria* spp. The infected snails, bearing schistosomal sporocysts, release cercariae into the water, which in turn penetrate the skin of their denitive host (Gryseels et al., 2006).

To complete a life cycle in the laboratory, *S. mansoni* is commonly maintained using rodents, ranging from hamsters to mice, as the definitive hosts and *Biomphalaria glabrata* as the intermediate host snail species (Figure 1). Infections of rodents of the same gender, 3 to 4 weeks of age and weighing 18 to 25 g, with *S. mansoni* are commonly initiated by subcutaneous injection of 100 to 150 cercariae (infective larvae). At 42, 49 or 56 days postinfection, animals are sacrificed with CO2, dissected, and miracidia are hatched from *S. mansoni* eggs taken from animal livers. Each intermediate host snail is exposed to approximately 10 miracidia. All animals should be handled in strict accordance with good animal practice adhering to the institutional guidelines for animal husbandry. Thus, all studies should have a statement from their ethics committee or institutional review board indicating the approval of the research.

The use of *in vivo* animal models in drug discovery and the techniques used for these studies in the laboratory have recently been described in detail elsewhere by Keiser (2010) and Ramirez et al. (2007). This chapter will focus on the techniques used for *in vitro* studies. Many methods have been described that aim to determine the antischistosomal activity of drugs *in vitro*. The assessment of the viability of different stages of schistosome, tegumental changes, oviposition, toxicity in mammalian cells and other parameters are important in the search for antischistosomal substances. The techniques detailed here will be the key to better assess the methodology employed during screening tests.
