| The life cycle of S.japonicum involves alternation between sexual(in the vertebrate host) and asexual (in the invertebrate host) generations, comprising seven distinct stages,i.e.egg, miracidium, mother sporocyst, daughter sporocyst, cercaria, schistosomulum and adult worm (Fig. 1.1).
The cercaria penetrates the skin of the vertebrate host and becomes a tailless schistosomulum. The schistosomulum enters the lymphatic system or venous system to pass through the right side of the heart and lungs, from where some go into the circulatory system to reach the mesenteric vessels,while others may pass directly through the diaphragm to reach the liver and portal system.The schistosomula mature and mate in the liver and migrate via the portal veins to the mesenteric veins. Once in their chosen sites,egg-laying begins. The egg penerates the lumen of the host's intestine and is excreted in the faeces. This phase takes about six to ten weeks(Faust & Melency, 1924; Wright, 1981;Laughlin,1984;Manson-Bahr & Bell,1987).
After hatching from the egg in water,the miracidium penetrates the invertebrate host snail and loses its cilia. Through metamorphosis at develops into a mother sporocyst in which a number of germinal cells develop into daughter sporocysts. Eventually the daughter sporocyst leaves the body of the mother sporocyst and migrates to the digestive glands of the snail,where it settles and grows. Within the digestive glands, further germinal cells develop into the next larval stage, the fork-tailed cercaria.The cercaria breaks out of the daugher sporocyst, migrates through the tissues of the snail and escapes to water, a phase which takes about six to eight weeks,depending on water temperature. The miracidium and cercaria are free-swimming stages confined to an aquatic environment.
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