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Probable Biological Control of Schistosomiasis Mansoni in a Puerto Rican Watershed

The Los Peña community, now emerging from a suburban slum status to a stabilized, small, housing settlement, has been the object of various schistosomiasis studies since about 1933. Trials at stopping transmission among the dense creekside population have included stibophen chemotherapy, health education, improved domestic water supplies, installation of sanitary privies, and a single chemical mollusciciding test. Although those controls were partially effective, the disease remained significantly endemic through 1952. Since this time, surveys of the snail intermediate host species, Australorbis glabratus, in the main stream showed it to be in precipitous population decline, faced with a phenomenally large colony of an ampullarid snail, Marisa cornuarietis, first seen in the stream in 1952. During March, 1956, this highly competitive species was successfully transplanted to all upstream stations in the watershed, and during a period of 1.5 years has almost eliminated the host species in these areas.

Two recent successive annual surveys of the same creekside families indicate that no new infections are appearing in the statistically sensitive preschool age group. The community prevalence in all ages and both sexes has levelled off at a rate of about 6 or 7 per cent. This first instance of probable biological control of schistosomiasis transmission should be repeated in other endemic environments capable of supporting the remarkable snail Marisa. This field test is a step toward the solution of certain control problems in Puerto Rican schistosomiasis on a permanent and economically feasible basis.