Biology of the Schistosome Complexes

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  • Parasitology Department, Naval Medical School, National Naval Medical Center Bethesda, Maryland

Summary

Each geographical region is characterized by schistosomes which infect man or lower animals, or both. In Africa and the Middle East, there are two categories of schistosomes, i.e., those of the S. haematobium-type producing terminally-spined eggs and those of the S. mansoni-type producing laterally-spined eggs. The parasite-host relationships of the schistosomes is not clearly understood at the present time. Representatives of the haematobium-complex, i.e., S. bovis, S. intercalatum, S. matthei and probably others occurring in livestock, are of greater concern in human schistosomiasis. Recent discoveries of schistosomes of the mansoni-complex, i.e. S. rodhaini and S. mansoni var rodentorum in rodents and other animals indicate unusual parasite-host relationships with the possibility of concern in the epidemiology of schistosome infections.

Schistosomiasis haematobia in Portugal is unique since the causative agent, S. haematobium, is morphologically similar to its counterpart in Africa but is restricted in its distribution and is transmitted by Planorbis dufourii.

Although a few autochthonous cases of schistosome infection in man have been reported, schistosomiasis in India at present is primarily a disease of livestock. The status of the schistosome problem in India may change with the recent discovery of an endemic focus of urinary schistosomiasis in the Bombay State.

In the Orient, S. japonicum is transmitted by several species of Oncomelania and infects man and a wide range of mammalian hosts over a large geographical area. Recent studies indicate the existence of geographical and physiologic strains of the parasite and its intermediate host. On Formosa, contrary to the situation in other areas of the Orient, S. japonicum is a parasite of lower mammals, man playing only a questionable role in its propagation. The Formosan schistosome has been designated as a “non-human” strain.

Geographical and physiologic strains of S. mansoni and its intermediate hosts in the Americas have been adequately demonstrated. Discovery of wild rodents infected with S. mansoni has altered the parasite-host relationships and epidemiological aspects of schistosomiasis in South America.

Schistosome dermatitis has not been differentiated from other types of dermal irritation in countries where the population is concerned with other more serious parasitic infections. In the Americas, however, schistosome dermatitis has received considerable recognition since it is widespread and occurs as a result of dermal invasion of man by animal schistosomes whose cercariae are present in fresh as well as marine waters.

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