An analysis of age-prevalence data by catalytic models. A contribution to the study of bilharziasis.

The analysis of age-prevalence curves obtained from bilharziasis surveys is not usually carried out to the maximum depth the data allow for. With a view to obtaining greater value from the available information, the author analyses age-prevalence data for three common human schistosomes by Muench's catalytic models. The two-stage model is found the most appropriate. Use of this model assumes constant rates of becoming positive and of becoming negative. Two separate analyses are required, one for the first few years of life of the human host and one for the remainder. Satisfactory fits to the data are obtained only when the proportion of older people who remain positive is determined. This proportion varies with the intensity of transmission.The calculated rates of becoming positive agree very well with rates obtained by observing originally negative children for 12 months or longer.The parameters in the model are used in interpreting data from skin-test surveys. It is concluded tentatively that in order to be positive by the skin test, a person must have had both sexes of worms at some time in the past and to be carrying living female worms or to have been exposed recently to female cercariae.It may become possible to use the model to predict the entire age-prevalence curve from data on children, thus saving much effort in parasitological surveys.