On the calculation of intestinal schistosome infection intensity

The effects of using different methods to calculate individual infection intensities on the age-infection distribution of Schistosoma mansoni field data are demonstrated. Methods are tested on a maximum of three stool samples per person collected on three consecutive days; the methods considered for the calculation of individual infection intensities are the geometric mean (GM), arithmetic mean (AM) and pseudo geometric mean (GM of stool samples instead of replicates). In addition, the effects of calculating the infection intensity for each age group using either AMs or GMs are compared. Differences occur in the shape of the age-infection profiles obtained by using either the arithmetic or geometric group mean. When using the AM, peak infection intensity occurs in a younger age group compared to using the GM, and all three methods of calculating individual infection intensity give the first peak of infection in the same age group. However, differences occur in the position of the second peak which occurs earlier with the two GMs than with the AM. Bootstrapping procedures show that the individual AM, gives a different age group for the first peak of infection at least 25% of the time when compared to either of the GMs, and 31% of the time for the second peak, while the two GMs give the same peak age groups around 90–92% of the time for both peaks. When using the GM, to calculate infection intensity for each age group, there are no differences between the three methods used to calculate individual infection intensity. This is confirmed by bootstrapping procedures. The results are discussed in relation to the distribution of parasites and levels of parasite aggregation. The implications of the results for field studies are also discussed.