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On African schistosomiasis

1953; Oxford University Press; Volume: 47; Issue: 6 Linguagem: Inglês

10.1016/s0035-9203(53)80003-9

1878-3503

Julius M. Amberson, Ernst Schwarz,

Parasite Biology and Host Interactions

VII. Summary Two species of Schistosoma are known with certainty at this time to infect man in Africa, (a) S. mansoni with laterally-spined ova and (b) S. haematobium with terminally-spined ova. Whether S. margrebowiei is a human parasite, as has been claimed, requires further proof. It is considered that both the human and animal parasites with terminally-spined ova in Africa should be considered subspecies of S. haematobium. We therefore distinguish (a) S. h. haematobium, (b) S. h. capense (provisionally), (c) S. h. bovis, and (d) the S. h. mattheei group. The S. h. mattheei group includes (a) S. h. intercalatum which is known as a human parasite and suspected to be a parasite of livestock; (b) S. h. mattheei which is known to be a parasite of livestock and suspected to be a human parasite; and (c) S. h. curassoni which is known from cattle, but is too imperfectly known for final appraisal both concerning its taxonomic status and host relationship. As morphological differences of the adult flukes of the haematobium group are varying, overlapping and often doubtful, we consider as the best taxonomic method available at this time appraisal of the shape of the ova. The breadth-length ratio of the ova has been found to be most reliable. Variation graphs of a series of such ratios show characteristic subspecific values. The actual length of the ova is not absolutely characteristic as it varies with the host. Although it is generally admitted that schistosomiasis may affect every part of the body of the vertebrate host, it is also true that preference of site may be characteristic for certain species, subspecies, possibly strains, and may show geographically circumscribed distribution, and variation according to host. The clinical symptoms produced by S. mansoni are predominately intestinal. The clinical symptoms produced in man by S. h. haematobium in North Africa, the Mediterranean area and Anterior Asia, and by S. h. capense in South Africa are predominately urinary. In East Africa and part of West Africa urinary schistosomiasis in man is produced by some form of S. haematobium. It is not clear whether we have to deal with true S. h. haematobium in these areas. In a great part of the Belgian Congo, the French Congo, and southern Nigeria intestinal schistosomiasis without vesicular symptoms is produced by S. h. intercalatum. Schistosomiasis in primates produced by all forms of the haematobium group including S. h. haematobium is primarily of the intestinal type. S. h. haematobium in North Africa, the Mediterranean area and Anterior Asia is primarily a human parasite. It has been shown to develop to maturity in monkeys, hedgehogs, the golden hamster, and possibly the fat-tailed gerbil. It does not develop to maturity, or at least does not produce viable ova, in the house-mouse, the laboratory strains of the Norway rat, the rabbit, and guineapig. It does not infect domestic livestock, like sheep, goats and cattle. S. h. bovis in North Africa, the Mediterranean area and Anterior Asia does not infect man. It is primarily a parasite of livestock, cattle, sheep and goats. No data are available whether it will infect primates, but it is known to develop in the European hedgehog, the house-mouse, the rabbit, the guineapig, the European hamster, and the cat. As the taxonomic status of the haematobium group in West, Central, East and South Africa is uncertain, and as experimental evidence is insufficient, conclusions concerning the host relationship of these flukes are withheld. S. mansoni is known in man and primates. In laboratory animals it is known to develop in the mouse, moderately in the guineapig, in the golden hamster, and in the rabbit. It does not occur in ungulates. The snail intermediate host in Africa of S. mansoni is Biomphalaria alexandrina belonging to the family Planorbidae, subfamily Planorbinae. Two subspecies are admitted, (a) B. a. alexandrina and (b) B. a. pfeifferi. The S. haematobium group is carried in Africa by snails of the genus Bulinus, family Planorbidae, subfamily Bulininae. Three species are known to be involved: (a) Bulinus africanus; (b) B. truncatus; (c) B. forskalii (only in Mauritius). In Portugal it is carried by Planorbarius corneus metidjensis, subfamily Planorbinae. Whether this species is a carrier in the Atlas countries where it occurs together with B. truncatus, is not definitely established. The genera Physopsis Krauss based on B. africanus and Pyrogophysa Crosse based on B. forskalii are not considered valid, but are included in the synonymy of Bulinus O.F. Müller. B. truncatus occurs in North Africa, the Mediterranean area, Anterior Asia, in the East African highlands and in South Africa as far as the Cape Peninsula. It is a species of moderately warm countries, but not adapted to the hot temperatures of the low-lying country in the Equatorial belt. B. africanus is a tropical species, and apparently does not survive below 15°C. (59°F.). Although in intermediate areas both species occur together, in the greater part of their range only one species exists. In the tropical area of Central Africa and the Guinea coast only B. africanus is found. In the northern area as far as 11°N, both S. h. haematobium and S. h. bovis are carried by B. truncatus and will not infect B. africanus. Elsewhere all forms of the S. haematobium group are carried by B. africanus, and will not infect B. truncatus. The remarkable distribution of schistosomiasis in South Africa is identical with that of B. africanus, the infection being absent in the south-western part of the Cape Province where B. truncatus is found, but where B. africanus does not occur. The part played in the transmission of haematobium schistosomiasis by B. forskalii, except in Mauritius, is incompletely known. No data exist concerning B. senegalensis which, on account of its scarcity, does not appear to be of real importance. Measures to control schistosomiasis in Africa by means of snail control have not been successful. The currently known molluscicides have nowhere eliminated the infection. The development of therapeutic and prophylactic agents for the control of schistosomiasis in the vertebrate host should receive more intensive study.