Comparison of filtration staining (Bell) and thick smear (Kato) for the detection of quantitation of Schistosoma mansoni eggs in faeces.

We compare results of one Bell and one Kato-Katz examination performed on each of 315 stool specimens from residents in an area in north-eastern Brazil endemic for schistosomiasis mansoni. The prevalence of Schistosome infection detected by the Bell technique was 76% and by the Kato-Katz technique was 63%. 81% (44/54) of the infections missed by a Kato-Katz smear were light infections (one of 50 epg range by Bell examination). Over, all, 55% (44/80) of stools in this egg count range by the Bell technique were negative on a single Kato-Katz smear. This implies that five Kato-Katz smears per stool would ensure a 95% probability (0.55(5) X 100) of detecting such light infections. However, a single Kato-Katz smear detected eggs in 97% (124/128) of stools with a Bell count greater than 100 epg. For stools positive by both methods the egg counts per gram of stool were higher (p less than 0.001) by Kato-Katz examination. Geometric mean egg counts for the infected population were 199 epg by the Kato-Katz and 92 epg by the Bell methods. 64% (59 v. 36) more persons were classified as heavily infected (greater than 400 epg) by the Kato-Katz method than by the Bell method. The differing measurements of schistosome infection obtained with the Bell and Kato-Katz methods must be considered when comparing data on morbidity-infection relationships.