Diagnosis of Soil-Transmitted Helminths in the Era of Preventive Chemotherapy: Effect of Multiple Stool Sampling and Use of Different Diagnostic Techniques

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Abstract
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. 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. Rigorous epidemiologic surveillance of soil-transmitted helminthiasis in the era of preventive chemotherapy is facilitated by multiple stool sampling bolstered by different diagnostic techniques. Diseases caused by parasitic worms inflict an enormous public health burden in developing countries. There is a growing effort to control worms with drugs. The success of repeated drug administrations can be assessed by measuring the decline in the prevalence and intensity of worm infections. Accurate diagnosis is a challenge, especially in areas with low infection intensities. We studied the effect of stool sampling efforts and the use of different diagnostic techniques on the measured prevalence of worms, including hookworm, large intestinal roundworm (Ascaris lumbricoides), whipworm (Trichuris trichiura), and dwarf threadworm (Strongyloides stercoralis) in Zanzibar, where worm control has been implemented over the past decade. Three early morning stool samples were collected from each of 342 schoolchildren on 3 consecutive days and analyzed with different techniques. The observed prevalence of the different worms increased with an enhanced sampling effort and when different diagnostic methods were combined. Examination of 3 stool samples per individual resulted in prevalences of T. trichiura, hookworm, A. lumbricoides, and S. stercoralis of 47.9%, 22.5%, 16.5%, and 10.8%, respectively. To conclude, the examination of multiple stool samples and the use of different techniques are recommended for accurate diagnosis of worms in areas undergoing repeated mass drug administration.