Control of Schistosoma Haematobium Morbidity on Pemba Island: Validity and Efficiency of Indirect Screening Tests

Article excerpt

Introduction

The validity and efficiency of different measures of haematuria in detecting individuals with Schistosoma haematobium infection have been compared by Lwambo [1]. Microhaematuria elicited by reagent strips at the 1+ positivity limit showed the best diagnostic performance in detecting S. haematobium infection in both children and adults throughout the intervention phase of the schistosomiasis control programme on Pemba Island, United Republic of Tanzania.

In this article, we report the validity and efficiency of such reagent strips to detect haematuria in individuals at risk of S. haematobium morbidity. Schistosomiasis haematobia morbidity has been reviewed by Chen & Mott [2], Hatz et al. [3] and Lwambo [1]. Egg excretion intensity and haematuria correlate positively with morbidity due to S. haematobium infection at the individual level.

In 1985 WHO set a cut-off count of [is greater than or equal to] 50 eggs per 10ml of urine as the threshold for risk of S. haematobium morbidity [4]. Lwambo showed that on Pemba Island the prevalence of [is greater than or equal to] 50 eggs per 10ml urine is linearly related to, and positively correlated with, the prevalence of visual haematuria and of microhaematuria at the 2+ positivity limit among school-age children [1]. However, the prevalence of such heavy infection and of visual haematuria are nonlinearly related to the prevalence of infection. The prevalence of visual haematuria may therefore approximate to that of heavy morbidity in the community. Nevertheless, the diagnostic performance of these haematuria techniques in detecting individuals with [is greater than or equal to] 50 eggs per 10ml urine (at risk of morbidity) has not been adequately studied.

In this article we report the results of a study to determine the validity and efficiency of different measures of haematuria to detect individuals at risk of S. haematobium morbidity and the effect of repeated selective population chemotherapy on the validity and efficiency of these methods using data from the first three evaluation surveys in the Pemba Island schistosomiasis control programme [5, 6].

Methods

The study area and data collection techniques have been described by Savioli et al. [5, 6], while the methods for determining the validity and efficiency of screening techniques have been described by Lwambo [1]. A case at risk of S. haematobium morbidity is defined as any individual excreting [is greater than or equal to] 50 eggs per 10ml of urine. The validity and efficiency of haematuria as an indirect screening method for S. haematobium morbidity were evaluated against a single urine filtration technique as the "gold standard".

Data for all schools and villages for each evaluation study were pooled in a single measure. For each of the indirect screening methods (history of haematuria, microhaematuria 1+ and 2+, visual haematuria) data were collated as shown in Table 1.

Table 1: Collation of data used for each of the indirect
screening method(a)

              True disease status
              (filtration results)

Test status   +ve(b)   -ve(b)   Total(b)
+ve             a         b      a + b
-ve             c         d      c + d
Total         a + c    a +  c    b + d

(a) History of haematuria, microhaematuria 1+ and 2+, and visual haematuria.

(b) a = No. of individuals whose screening test is positive and the individual is at risk of disease (i.e. those who excrete [is greater than or equal to] 50 eggs per 10ml of urine) (true positives); b = No. of individuals whose screening tests is positive but are not at risk of disease (i.e. those who excrete [is less than] 50 eggs per 10ml urine) (false positives); c = No. of individuals whose screening test is negative but are at risk of disease (false negatives); and d = No. of individuals whose screening test is negative but are not at risk of disease (i. …