THE INTERPRETATION OF INTENSITY AND AGGREGATION DATA FOR INFECTIONS OF SCHISTOSOMA-HAEMATOBIUM

The relationships between mean intensities of infection, the aggregati on 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 aggregatio n and age, and an age-dependent relationship between the mean and aggr egation of infection intensity when levels of infection are high. Thes e patterns are qualitatively compared with the output of mathematical models based on an immigration-death process (model I), and incorporat ing age-dependent changes in the distribution of exposure to infection as suggested by field data (model II), or reductions in the rate of i nfection as a function of either current (model III) or cumulative (mo del IV) parasite burdens, that is, density-dependent processes without or with memory, respectively. Models II and III were able to reproduc e observed patterns, but model IV, which is a possible representation of acquired immunity, was not. These results are consistent with the f ollowing assumptions: (i) age-related patterns of aggregation can be g enerated without recourse to density-dependent processes; (ii) the epi demiological impact of density-dependent processes depends on whether these act with or without memory; and (iii) any acquired immunity to h uman S. haematobium infection may be significantly less than life-long .