Diagnosis of soil-transmitted helminths in the era of preventive chemotherapy: effect of multiple stool sampling and use of different diagnostic techniques

Abstract

Background: Soil-transmitted helminth infections are common throughout the tropics and subtropics and they disproportionately affect the poorest of the poor. In view of a growing global commitment to control soil-transmitted helminthiasis, there is a need to elucidate the effect of repeated stool sampling and the use of different diagnostic methods in areas targeted for preventive chemotherapy that are characterized by low-infection intensities. In this study, we focused on schoolchildren on Unguja Island, Zanzibar, an area where anthelminthic drugs have been repeatedly administered over the past decade.

Methodology/principal findings: Three serial stool samples from each of 342 schoolchildren were examined using the Kato-Katz (K-K), Koga agar plate (KAP), and Baermann (BM) techniques. These methods were used individually or in combination for the diagnosis of Ascaris lumbricoides (K-K), Trichuris trichiura (K-K), hookworm (K-K and KAP), and Strongyloides stercoralis (KAP and BM). The examination of multiple stool samples instead of a single one resulted in an increase of the observed prevalence; e.g., an increase of 161% for hookworm using the K-K method. The diagnostic sensitivity of single stool sampling ranged between 20.7% for BM to detect S. stercoralis and 84.2% for K-K to diagnose A. lumbricoides. Highest sensitivities were observed when different diagnostic approaches were combined. The observed prevalences for T. trichiura, hookworm, A. lumbricoides, and S. stercoralis were 47.9%, 22.5%, 16.5%, and 10.8% after examining 3 stool samples. These values are close to the 'true' prevalences predicted by a mathematical model.

Conclusion/significance: Rigorous epidemiologic surveillance of soil-transmitted helminthiasis in the era of preventive chemotherapy is facilitated by multiple stool sampling bolstered by different diagnostic techniques.

Figure 1. Flow chart detailing the study participation and compliance of randomly selected children from Chaani and Kinyasini schools, Zanzibar.

Those children who provided 3 stool samples were included in the final analysis. The final cohort comprised those children who had complete data records, i.e., 3 stool samples examined with 3 different diagnostic methods.

Figure 2. Diagrams detailing the differences in the observed and estimated ‘true’ prevalence of soil-transmitted helminth infections employing different diagnostic methods in relation to the number of stool samples from children from Chaani and Kinyasini schools, Zanzibar.