Quality control in laboratory investigations on Schistosoma mansoni in St. Lucia, West Indies. Maintenance of artificial prevalence as a laboratory aid.

A method is described of maintaining an apparent S. mansoni prevalence rate of 10% in an area estimated to have a natural rate of below 5% following two chemotherapy campaigns. To sustain interest and therefore accuracy among microscopists engaged on screening, known S. mansoni-positive stools were seeded, without their knowledge, among those collected in the field. The problems of achieving a desired level of seeding, and of a possible further dimension of the scheme are discussed.