Geographical Distribution of Schistosomiasis
There are three main species of Schistosome that have humans as their definitive hosts.
Schistosoma mansoni
Schistosoma haematobium
Schistosoma japonicum
There are a number of more minor species affecting man, as well as some species that may cause accidental infections, or cercarial dermatitis.
• Distribution of S. mansoni
The parasite S. mansoni is found in many countries in Africa, South America (Brazil, Surinam and Venezuela), the Caribbean (including Puerto Rico, St Lucia, Guadeloupe, Martinique, Dominican Republic, Antigua and Montserat) and in parts of the Middle East. Reservoir hosts are not important for this species of schistosome.
• Distribution of S. haematobium
The parasite S. haematobium is found in large parts of Africa, parts of the Arabia, the Middle East, Khuzestan Province in Iran, Madagascar and Mauritius. In addition a S. haematobium like species has been in the past found in man in a village in Northern India (not shown), but this may now no longer exist. Reservoir hosts are not thought to be of importance for this species.
• Distribution of S. japonicum
The parasite S. japonicum is found in the Far East, particularly China and the Philippines, but not any longer in Japan where successful control programs have been implemented. At one time it was even more widespread in China, but control programs have successfully eradicated it from many areas in Southern China and around Shanghai. In Indonesia it is found in a few isolated valleys in Central Sulawesi. Control of this parasite is complicated in that many reservoir hosts (such as water buffalo) exist. Interestingly there are many strains of S. japonicum, which vary considerably in their pathogenicity, drug sensitivity, morphology and infectivity. In Taiwan a strain exists that is not infective to man.
• Distribution of S. intercalatum
This parasite is endemic in parts of DRC, Gabon, Cameroon, with other small foci in possibly in Central African Republic, Chad, Nigeria, Upper Volta and other parts of Central Africa (though these have only been reported from isolated cases). It is possible that zoonotic infections of S. intercalatum also occur as it has been demonstrated to infect experimentally primates, sheep, goats and other animals, and has also been found in rats in the field. Snail of the genus Bulinus act as intermediate hosts.
• Distribution of S.mekongi
This parasite is very similar in terms of its morphology, clinical manifestations and lifecycle to S. japonicum, and is found in Laos and Cambodia. In Laos dogs have been demonstrated as reservoir hosts. Snails of the genus Tricula act as intermediate hosts.
A similar species, S. malayensis, has recently been reported in Peninsular Malaysia occasionally infecting man, and other S. japonicum - like species may also exist in Thailand.
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In addition a number of animal species of schistosome may occasionally accidentally infect man, reported species including
S. bovis - (normally infecting cattle, sheep and goats in Africa, parts of Southern Europe and the Middle East)
S. mattheei - (normally infecting cattle, sheep and goats in Central and Southern Africa)
S. margrebowiei - (normally infecting antelope, buffalo and waterbuck in Southern and Central Africa)
S. curassoni - (normally infecting domestic ruminants in West Africa) has been reported, although this is disputed.
S. rodhaini - (normally infecting rodents and carnivores in parts of Central Africa)
The cercaria of a large number of non-human infecting schistosome may penetrate human skin, but then die. These can give rise to an allergic condition called swimmers itch, or cercarial dermatitis, a reaction caused by release of antigens by the dying parasites in the skin.
Species that have been implicated in this condition include;
S. spindale - A parasite of ruminants, particularly cattle and water buffalo, in South East Asia.
Austrobilharzia variglandis. - Parasitic in water fowl of North America and Hawaii.
Heterobilharzia americanum - Parasitic in racoons and other mammals in Louisiana, U.S.A..
Schistosomatium douthitti - Parasitic in rodents in North America
Microbilharzia sp. - Parasitic in gulls, ducks and marine wildfowl in East Coast U.S.A. and Hawaii.
Trichobilharzia ocellata - Parasitic in ducks of Europe, Asia and North America.
T. physella - Parasitic in ducks of North America and Japan.
T. stagnicolae - Parasitic in ducks in the Great Lakes area of America.
Gigantobilharzia sp. - Parasitic in passerine birds.