Some snails can survive out of water for weeks and, if infected, the sporocysts which develop from the miracidia can live with them. A maximum of 98 days has been recorded, but this will vary in different climates and conditions.

Within the snail, the miracidium develops into an adult sporocyst from which thousands of larval cercariae are released (Fig. 2.6). This release from the snail is stimulated and altered by many factors: for example, the snail releases S. mansoni as a response to light, usually about 2 hours after the stimulus. The total output is shed within 5 hours. Therefore, the risk of S. mansoni infection will be much less in the early morning and the late evening than between 10 a.m. and 2 p.m. S. japonicum is shed at night and different stimuli are involved for other species, as well as different time ratios.

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Fig. 2.6 A The recently hatched miracidium of a schistosome. x1000; MIS 218934-42. B The fork-tailed cercaria which swims through water to penetrate the skin of man. C The fork-tailed cerceria of a schistosome. x1000; MIS 218934-44. D Adult S. japonicum flukes trapped in a blood filter placed in the inferior vena cava of a patient with schistosomiasis. This is sometimes used as a way to reduce the adult worm population within the body. (A, C from Marty and Andersen 1995).

The cercariae of S. mansoni and S. haematobium continually swim vertically upward in the water, then sink down to the bottom; those of S. japonicum remain just under the surface water film. Each cercaria has five pairs of glands and penetrates the skin of the host with a combination of lytic secretions and vibration, taking 3-5 minutes. This, too, may be affected by the environment of the cercaria, such as the quality and quantity of the water. In particular, a low concentration of heavy metal ions reduces the rate of penetration and the ability of the cercariae to mature. Penetration occurs through the skin or buccal mucous membrane, but cercaria which are swallowed are destroyed in the stomach.

After penetration, the larvae shed their tails and become schistosomula, then enter the lymphatics and pass through the thoracic duct into the right side of the heart; whether they also enter the veins after penetrating the skin is disputed. From the right heart they then travel through the lungs. If the capillary diameter is too small, the schistosomula can stretch the vessel to move through to the left side of the heart and via the mesenteric capillaries to the portal system and liver; this journey may take from 10-21 days. In the liver they mature and the "intestinal" species return in pairs to the portal system, where they may live for 30 years or more. Maturation can also occur in other organs, especially the lungs, but copulation is uncommon outside the liver. Vesicle plexus schistosomes apparently reach the vesicle veins by collateral circulation.

Adult schistosomes, while alive in the veins, do not cause any inflammation (Fig. 2.7). However, the continued presence of the adult schistosomes is associated with increased but not complete protection against reinfection by cercaria. In general, the dead eggs and dead worms cause more reaction than either do when living, but no stage in the life cycle is entirely harmless and there are minor differences between the individual species.

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Fig. 2.7 A-C. Schistosomes can be recognized in many body tissues. A Transverse sections of the mesenteric veins of a patient, each containing a male worm embracing a female adult S. japonicum (lower right corner). B A transverse section of a coiled adult male S. mansoni in the mesoappendiceal vein (lower left corner). C The magnified view of B, showing the digestive ceca and cuticular tubercles of the male S. mansoni.