The disease has a very complex epidemiology, resulting in part from a quite intricate relationship between the parasite and the snail intermediate host. Not only are there strains of each schistosome species, but also there are a multiplicity of snails that vary in their susceptibility to these strains and have a taxonomy that is in a constant state of revision. Historically this has long been a source of total confusion.

In very general terms, the snail hosts of S. haematobium belong to the genus Bulinus, which has been divided into two subgenera: one with three species complexes, including the B ulinus truncatus-tropicus group; and the other with one species complex, the Bulinus africanus group. The B. truncatus-tropicus species complex act as intermediate hosts to S. haematobium in Egypt and other countries north of the Sahara, whereas the B. africanus species complex generally serve as the most important intermediate hosts south of the Sahara. The genus Biomphalaria acts as the intermediate host for S. mansoni. As in

Bulinus, the genus has been divided into species complexes, with the Biomphalaria pfeifferi group acting as the most important complex in Africa south of the Sahara and in the Middle East, and Biomphalaria alexandrina the host in the Nile Delta. In the Americas, Biomphalaria glabrata acts as the most effective host. Whereas S. haematobium and S. mansoni are transmitted by species of pulmonate freshwater snails, S. japonicum is transmitted by amphibious prosobranch snails belonging to the species Oncome-lania hupensis, of which there are six geographic subspecies. The aquatic prosobranch snail, Tricula aperta, endemic to the Mekong River, acts as the intermediate host of S. mekongi.

Schistosomiasis can be a serious chronic disease in poor rural areas, where children and adults, because of recreational, domestic, religious, and occupational reasons, come regularly into contact with fresh water contaminated with the schistosome cercariae. In most endemic areas, the prevalence of the disease and the intensity of infection (i.e., the number of eggs released, which is an indication of the number of worms carried) peak among those in their teenage years. Indeed in areas of high endemicity virtually all children become infected at some time. Thereafter, a decrease in prevalence and a decline in intensity occur. Even in highly endemic areas the rate of transmission of the disease is low. This follows, in part, from the patchy distribution of the surprisingly low numbers of infected snails.

Reservoir hosts play an important role in Oriental schistosomiasis, where the parasite is naturally transmitted between humans and other vertebrates, including many domesticated animals such as cattle, pigs, and dogs. Although animals are believed to play little if any role in the transmission of the two schistosome species in Africa, rodents are thought to act as important reservoirs of S. mansoni in South America.

The human schistosomes belong to a large family of trematodes, the Schistosomatidae, which also parasitize birds and mammals. In many parts of the world, particularly in the lake country of the central and western United States and Canada, the cercariae of these nonhuman schistosomes can penetrate the skin of humans by accident. Although these cercariae are destroyed in the skin, they nevertheless cause a harmless but very irritating rash - "swimmer's itch" or "schistosome dermatitis."

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