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Geographic Distribution

The World Health Organization estimates that over 200 million people are infected with schistosomiasis and 400 million more are at risk in at least 76 countries. It is second only to malaria as a cause of morbidity and mortality from parasites.

The importance of schistosomiasis is shown by a 1997 survey of an island on Lake Victoria, Tanzania. Eighty-six percent of 1695 villagers on the island were infected with S. mansoni, with children and adolescents being at highest risk. Forty-three percent of those infected had suffered from bloody diarrhea in the 2 weeks before the survey. The later stages of the infection, such as periportal fibrosis, were rare.

The disease is found naturally in other primates, such as baboons and chimpanzees in Africa and monkeys such as the capuchin, marmoset and macaque in South America and the West Indies. S. mansoni parasitizes wild rodents in Africa and South America: S. japonicum parasitizes about 40 species of mammals, including water buffalo, cattle, horses, pigs, dogs, cats, rodents and goats. It is generally agreed that the animal hosts of S. japonicum have significance to humans as reservoirs of the disease, but their importance in containing the infection varies regionally.

Fig. 2.1 A. Geographic distribution of S. haematobium.

S. haematobium occurs throughout Africa, on the islands of Madagascar and Mauritius, around the southern shores of the Mediterranean, and in the Middle East, including Turkey (Fig. 2.1A). It primarily involves the urinary tract and the portal system, but can affect the colon and lungs as well.

Fig. 2.1 B. Geographic distribution of S. mansoni.

S. mansoni is prevalent throughout Africa, especially in the north, and is endemic also in Arabia, the northern part of South America, Puerto Rico and other islands of the Caribbean (Fig. 2.1B). It was imported into the New World via the slave trade. It affects primarily the colon, portal system and lungs.

Fig. 2.1 C. Geographic distribution of S. japonicum.

S. japonicum occurs in the coastal and river areas of China, particularly the Yangtze Basin and the Shanghai River, Japan, Taiwan, the Philippines, Celebes and other western Pacific islands; it is found in isolated areas of Cambodia, Thailand, Laos and Myanmar (Burma) (Fig. 2.1C). It primarily affects the colon, small bowel, portal system and lungs, but it may be found in the urinary tract.

S. mekongi is found in the Mekong Delta, and in Laos, Cambodia and Thailand.

S. intercalatum is found only in the Democratic Republic of Congo and equatorial Africa, including Gabon, Cameroon, Chad, Guinea, and the Central African Republic. It affects primarily the digestive tract and the portal system.

S. mattheei is found in southern Africa (South Africa and Zimbabwe). S. incognitum is found in India, and S. rodhaini in the Democratic Republic of Congo. Not all authorities agree that these are of significance for human infection.

There is considerable overlap in distribution and combined infections are common, especially with S. haematobium and S. mansoni. Despite this, S. japonicum probably affects the largest number of people because of the density of population within its geographic area.

Owing to immigration, there are thousands of Puerto Ricans and other immigrants in New York and other American cities who harbor S. mansoni or other schistosomes. Similar immigrant pockets of schistosomal infection exist in many other nontropical countries, especially in major cities. Members of the Armed Forces, other travelers and members of organizations such as the Peace Corps and relief workers are all at risk.

In many parts of the world, schistosomiasis is almost a "normal" part of human existence, reinfection is usual, and many of the effects of the disease are not regarded as abnormal by the local populace.

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Copyright: Palmer and Reeder