False epidemiological results from the bulk transport of dried filter papers in urinary schistosomiasis

In field studies in Schistosoma haematobium endemic areas, dried filters must be transported in bulk to central laboratories, either for primary screening and counting, or for quality control of field workers. We examined filters before and after bulk transport, from three populations, of high, medium and low prevalence rate. A total of 312 filters were examined before and after transport; 28.6% of all eggs on filters were lost during transport. Negative filters became positive during transport, and hence false prevalence rates resulted from after transport studies. Egg loss did not seem to be related to initial egg density. The operational results of these findings could be serious, in terms of distribution of treatment and evaluation of results and costs.