Evaluation of diagnostic methods for the detection of intestinal schistosomiasis in endemic areas with low parasite loads: Saline gradient, Helmintex, Kato-Katz and rapid urine test.

Warllem Junio Oliveira,Carlos Graeff-Teixeira,Áureo Almeida Oliveira,Fernando Sergio Barbosa,Paulo Marcos Zech Coelho,Vanessa Normandio De Castro,Andressa Mariana Saldanha Elias,Frederico Gil,Maria Aparecida Gomes,Fernanda Do Carmo Magalhães,Catieli Gobetti Lindholz,Deborah Aparecida Negrão-Corrêa,Vivian Favero,Martin Johannes Enk,Stefan Michael Geiger,Mariângela Carneiro,Lisette Van Lieshout

doi:10.1371/journal.pntd.0006232

Abstract

BackgroundIn some tropical countries, such as Brazil, schistosomiasis control programs have led to a significant reduction in the prevalence and parasite burden of endemic populations. In this setting, the Kato-Katz technique, as the standard diagnostic method for the diagnosis of Schistosoma mansoni infections, which involves the analysis of two slides from one fecal sample, loses its sensitivity. As a result, a significant number of infected individuals are not detected. The objective of this study was to perform extensive parasitological testing of up to three fecal samples and include a rapid urine test (POC-CCA) in a moderate prevalence area in Northern Minas Gerais, Brazil, and evaluate the performance of each test separately and in combination.Methods and findingsA total of 254 individuals were examined with variants of the standard Kato-Katz technique (up to18 Kato-Katz slides prepared from three fecal samples), a modified Helmintex (30 g of feces), the saline gradient (500 mg of feces), and the POC-CCA methods. We established a reference standard taking into consideration all the positive results in any of the parasitological exams. Evaluation of the parasite burden by two Kato-Katz slides confirmed that most of the individuals harbored a light infection. When additional slides and different parasitological methods were included, the estimated prevalence rose 2.3 times, from 20.4% to 45.9%. The best sensitivity was obtained with the Helmintex method (84%). All parasitological methods readily detected a high or moderate intensity of infection; however, all lost their high sensitivity in the case of low or very low intensity infections. The overall sensitivity of POC-CCA (64.9%) was similar to the six Kato-Katz slides from three fecal samples. However, POC-CCA showed low concordance (κ = 0.34) when compared with the reference standard.ConclusionsThe recommended Kato-Katz method largely underestimated the prevalence of S. mansoni infection. Because the best performance was achieved with a modified Helmintex method, this technique might serve as a more precise reference standard. An extended number of Kato-Katz slides in combination with other parasitological methods or with POC-CCA was able to detect more than 80% of egg-positive individuals; however, the rapid urine test (POC-CCA) produced a considerable percentage of false positive results.

Highlights

Recent estimates of helminth infections indicate the existence of more than one billion infected individuals in underdeveloped areas of Africa, Asia, and in Central and South America [1]
The recommended Kato-Katz method largely underestimated the prevalence of S. mansoni infection
An extended number of Kato-Katz slides in combination with other parasitological methods or with POC-cathodic antigens (CCA) was able to detect more than 80% of egg-positive individuals; the rapid urine test (POC-CCA) produced a considerable percentage of false positive results

Summary

Introduction

Recent estimates of helminth infections indicate the existence of more than one billion infected individuals in underdeveloped areas of Africa, Asia, and in Central and South America [1]. Since the implementation of the National Schistosomiasis Control Program (NSCP) in the 1970s and decades of consequent chemotherapeutic interventions, the Brazilian health authorities reported significant improvements in terms of transmission, prevalence, and parasite load in the country’s endemic regions, especially in the states of Minas Gerais and Bahia [10]. In this new epidemiological scenario, most of the infected individuals in endemic areas harbor low parasite loads and are very unlikely to be detected with the commonly used parasitological methods [11,12]. The objective of this study was to perform extensive parasitological testing of up to three fecal samples and include a rapid urine test (POC-CCA) in a moderate prevalence area in Northern Minas Gerais, Brazil, and evaluate the performance of each test separately and in combination

Objectives

Methods

Results

Conclusion