Introduction to Schistosomiasis

There are a number of Digenean parasitic trematode (flatworms) capable of causing disease in man. Schistosomiasis (Bilharzia) is the most important of these helminth infections, and is caused by members of the genus Schistosoma. These parasites are important pathogens, estimated to be infecting some 200 million people in tropical and subtropical regions. The most important species that infect man are S. japonicum, S.mansoni, S. haematobium. Work in our group concentrates on S.mansoni. Infection occurs through contact with fresh water that contains infective cercariae released from  an intermediate host snail.The cercaria penetrates intact human skin and transforms into the migrating schistosomulum larva, that migrates through the bloodstream to the hepatic portal system to complete the parasites lifecycle. Penetration of the skin by the cercaria (usually from species unable to develop in man, particularly cercaria of species of avian schistosomes may result in a form of dermatitis, cercarial dermatitis, though this is not as important in terms of pathology as egg induced pathology, (see below). Male and female worms differentiate, pair and migrate into the small venules draining the intestine (S. mansoni, S. japonicum) or the bladder (S. haematobium). In these sites, and fully exposed to the host immune system, adultworms live for 3 to 8 years, although cases have been reported of individuals still excreting parasite eggs 30 years after leaving a schistosome endemic area. The female worm produces 300 to 3000 eggs each day. Eggs pass into the lumen of the intestine or bladder and, if deposited in fresh water, hatch to release ciliated miracidia that infect the snail host. However, many eggs also lodge in the definitive host's liver and intestine or bladder, where they cause  the pathology associated with schistosomiasis. In the chronic form of the disease, eggs trapped in the liver elicit the development of a cellular, granulomatous reaction which, with its ensuing fibrosis, gives  rise to the most serious disease symptoms  of infection. Hepatosplenic disease associated with S. mansoni infections results from liver portal tract fibrosis. This progressive fibrosis can lead to obstructive vascular lesions, portal hypertension, ascites and fatal bleeding from oesophagogastic varices.

The number of cases of schistosomiasis  is increasing world-wide. Transmission  is usually associated with poor socio-economic  conditions. The control strategies include control of the intermediate snail host, environmental methods and mollusciciding, chemotherapy; and improved sanitation and health education. The introduction of  effective drugs over the last decade means that the treatment of individual  uncomplicated infections is now much improved, however, the control of transmission on the population level continues to  be problematical. Of particular concern is the high rate of reinfection of young children after treatment. Although transmission in a variety of environments has been reduced by determined control schemes, such successes have been more than counterbalanced by increases that have accompanied social dislocation and mass migrations caused by war, drought and famine, and by the effects of man-made ecological changes, unfortunately including many water resource development projects. A recent example of the latter is an explosive outbreak of S. mansoni infection caused by the damming of the Senegal river  in West Africa.

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Author: Dr. Terry Stewart, Design: Jakub Wawrzyniak