The measurement of bilharziasis prevalence and schistosomal egg output. Aims and techniques, with an account of a field method

Abstract

For bilharziasis prevalence surveys based on excretal examination the two primary requirements are precision and reproducibility. For a reproducible prevalence survey a subsample of stool or urine must be exhaustively examined for eggs. This requirement is identical with that for egg-counting. Adequate methods for both purposes are therefore the same. For urine studies methods involving concentration and further subsampling involve more possible sources of error than do recent methods using filtration, followed by staining of eggs with ninhydrin. A method is described for use in the field whereby the urine is sucked into a plastic filter-holder by hypodermic syringe. For stool samples concentration techniques involve a variable extraction fraction. Stoll-counting uses a very small sample for bilharziasis work and possibly new methods, again using filtration and ninhydrin staining, will prove of greater value. Field testing of methods has not yet reached the position where any one technique can be said to be ideal.