Snail-mediated Helminthiases

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With the exception of angiostrongyliasis which is a nematode infection transmitted accidentally to man in the course of its complex life cycle in rodents, the important group of snail-transmitted helminths that infect man are all trematodes (flukes) which undergo a complicated cycle involving various species of land or aquatic snails. The three common schistosome infections of man are responsible for a vast amount of general ill health and contribute in certain areas to the heavy mortality rate found among adolescents and young adults. Since infection occurs by penetration of the skin by water-dwelling infective stages (cercariae) transmission tends to increase in parallel with the increase of land utilisation by irrigation. Hence schistosomiasis is a positive obstacle in the way of agricultural and economic development in many parts of the developing world.

The intestinal liver and lung flukes have a somewhat more complicated life cycle in that the cercariae develop a resting stage (the metacercaria) which, in turn, must be ingested by man in order to infect him. As the metacercariae are found usually on various water plants, on fish or on crustaceans, infection with these worms tends to be localised to areas where particular local eating habits bring man and the metacercaria together. Thus Clonorchiasis is restricted to parts of Southeast Asia where raw fish is commonly ingested, fasciolopsiasis where, for example, water caltrops and water chestnuts are eaten uncooked, and paragonimiasis where the appropriate crustaceans form part of man's diet in one culinary delicacy or another. Many of these flukes are primarily parasites of animals other than man, eg Fasciola hepatica is essentially a parasite of sheep that infects man when he eats watercress or other aquatic plants from contaminated, wet pastures. They are thus zoonoses and, like many zoonoses, are relatively uncommon with notable, highly localised exceptions. In certain villages in northeast Thailand, for example, Fasciolopsis buski has been found to infect nearly 100% of the population and Opisthorchis viverrini 90%. An idea of the numbers of human infections with these trematodes is given in Table.

the digenetic trematodes of medical importance and their prevalence

 


» Schistosomiasis
» Schistosoma mansoni
» Schistosoma haematobium
» Schistosoma japonicum
» Schistosomes Found Uncommonly in Man
» The intestinal flukes
» Fasciolopsiasis
» Liver fluke infections
» Fasciola hepatica and Fasciola gigantica
» Paragonimus westermani
» Angiostrongyliasis


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