Distribution and Prevalence

The disease has an almost worldwide tropical distribution, but is mercifully absent from the Indian subcontinent. S. haematobium is highly endemic in the Nile Valley, and has an irregular distribution in the Middle East and North Africa (Map VIII. 124.1). It occurs in most West and Central African countries, along the coastal countries of East Africa from Somalia to Natal, and in the islands off the east coast. S. mansoni is highly endemic in the Nile Delta and now seems to be spreading into the Nile Valley (Map VIII. 124.2). In Africa it has a distribution similar to (although more irregular than) that of S. haematobium. But, unlike S. haematobium, S. mansoni also occurs in South America (Brazil, Surinam, and Venezuela) and in some islands in the Caribbean (Dominican Republic, Puerto Rico, St. Lucia, and others) (Map VIII. 124.3); the parasite was transported from Africa to the New World as an unintended by-product of the slave trade. Oriental schistosomiasis, caused by S. japonicum, is endemic to the Yangtze Valley and many coastal provinces of mainland China (Map VIII. 124.4). It also occurs in Central Sulawesi and the Philippines, and there are

Map VIII.124.1. Distribution of Schistosoma haematobium in Africa and the Middle East. (From Farley 1991.)
Map VIII. 124.2. Distribution of Schistosoma mansoni in Africa and the Middle East. (From Farley 1991.)
Map VIII.124.3. Distribution of Schistosoma mansoni in the Western Hemisphere. (From Farley 1991.)
Map VIII.124.4. Distribution of Schistosoma japonicum and Schistosoma mekongi. (From Farley 1991.)

a series of smaller foci in Malaysia, Thailand, and Japan.

Over 200 million people are said to be infected with the disease, although the data on which such figures are based are extremely variable and unreliable. There is little doubt, however, that in villages of the Nile Delta and other areas where there is constant contact of the human host with water, the prevalence can be as high as almost 100 percent. On a worldwide scale, a recent World Health Organization survey revealed that in 42 countries surveyed, 21 percent of the population was infected.

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