The interpretation of intensity and aggregation data for infections of Schistosoma haematobium.

The relationships between mean intensities of infection, the aggregation of infection among hosts, and host age are analysed using data from 2 large (> 3000 individuals) field studies of Schistosoma haematobium infection. The data show a convex relationship between mean intensity and age, a convex relationship between an inverse index of aggregation and age, and an age-dependent relationship between the mean and aggregation of infection intensity when levels of infection are high. These patterns are qualitatively compared with the output of mathematical models based on an immigration-death process (model I), and incorporating age-dependent changes in the distribution of exposure to infection as suggested by field data (model II), or reductions in the rate of infection as a function of either current (model III) or cumulative (model IV) parasite burdens, that is, density-dependent processes without or with memory, respectively. Models II and III were able to reproduce observed patterns, but model IV, which is a possible representation of acquired immunity, was not. These results are consistent with the following assumptions: (i) age-related patterns of aggregation can be generated without recourse to density-dependent processes; (ii) the epidemiological impact of density-dependent processes depends on whether these act with or without memory; and (iii) any acquired immunity to human S. haematobium infection may be significantly less than life-long.