The Application of Catalytic Models to Schistosomiasis in Snails

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Abstract
1. With the aid of field growth curves, age-prevalence curves can be derived from field collections of snails, using cercarial shedding as proof of infection by schistosomes.2. Such curves were obtained from eight collections of field snails: one sample each of Bulinus (Physopsis) nasutus productus and Biomphalaria pfeifferi, intermediate hosts respectively of Schistosoma haematobium and S. mansoni in Tanzania, and six samples of Biomphalaria glabrata, intermediate host of S. mansoni on St. Lucia, West Indies.3. Catalytic curves were fitted to the data and in each case the two-stage curve gave the most satisfactory fit.4. This curve is the resultant of two opposing forces: one force being the rate of infection and the other the rate of loss of infection in a snail population.5. The rate of infection may be defined as the number of successful snail-miracidium contacts per 1,000 snails per week, while the rate of loss of infection is taken to represent the number of deaths per 1,000 infected snails per week.6. The underlying assumptions of this form of analysis are discussed and, in view of the independent verification of some of the calculated rates, the values are considered realistic.7. Besides providing quantitative data for inclusion in mathematical models of schistosome transmission, the technique offers an additional means of assessing the efficacy of certain methods of controlling transmission.