Identification and quantification of pathogenic helminth eggs using a digital image system

B Jiménez,C Maya,G Velásquez,F Torner,F Arambula,J.A Barrios,M Velasco

doi:10.1016/j.exppara.2016.04.016

Abstract

A system was developed to identify and quantify up to seven species of helminth eggs (Ascaris lumbricoides -fertile and unfertile eggs-, Trichuris trichiura, Toxocara canis, Taenia saginata, Hymenolepis nana, Hymenolepis diminuta, and Schistosoma mansoni) in wastewater using different image processing tools and pattern recognition algorithms. The system was developed in three stages. Version one was used to explore the viability of the concept of identifying helminth eggs through an image processing system, while versions 2 and 3 were used to improve its efficiency. The system development was based on the analysis of different properties of helminth eggs in order to discriminate them from other objects in samples processed using the conventional United States Environmental Protection Agency (US EPA) technique to quantify helminth eggs. The system was tested, in its three stages, considering two parameters: specificity (capacity to discriminate between species of helminth eggs and other objects) and sensitivity (capacity to correctly classify and identify the different species of helminth eggs). The final version showed a specificity of 99% while the sensitivity varied between 80 and 90%, depending on the total suspended solids content of the wastewater samples. To achieve such values in samples with total suspended solids (TSS) above 150 mg/L, it is recommended to dilute the concentrated sediment just before taking the images under the microscope. The system allows the helminth eggs most commonly found in wastewater to be reliably and uniformly detected and quantified. In addition, it provides the total number of eggs as well as the individual number by species, and for Ascaris lumbricoides it differentiates whether or not the egg is fertile. The system only requires basically trained technicians to prepare the samples, as for visual identification there is no need for highly trained personnel. The time required to analyze each image is less than a minute. This system could be used in central analytical laboratories providing a remote analysis service.

Highlights

Wastewater reuse in agriculture has a long history
The lowest sensitivity obtained was for fertile Ascaris lumbricoides eggs (66%), followed by Taenia saginata eggs (80%), while for unfertile Ascaris lumbricoides, Toxocara canis, and Trichuris trichiura eggs sensitivity was equal and slightly higher (86%)
Even though the system was capable of successfully discriminating the four most common species of helminth eggs from other objects, the results were not as good as desired. This is because an accurate identification of Ascaris lumbricoides is crucial since it is the most common species (60%e80% of eggs identified; USEPA, 1992; Capizzi-Banas et al, 2004; Lustigman et al, 2012; WHO, 2012a)

Summary

Introduction

Wastewater reuse in agriculture has a long history. In some countries, it is practiced with highly treated wastewater, while in many developing countries, where sanitation coverage is still poor, low quality water is used. Water reuse for irrigation saves significant volumes of fresh water, provides nutrients to soil, reducing or eliminating the need for chemical fertilizers, contributes to the expansion of agricultural land in arid and semi-arid areas, increases income for farmers, and is a relatively cheap disposal method for wastewater avoiding the pollution of other surface water bodies. Both the availability of water and the nutrients contained in the wastewater used for irrigation increase soil fertility and crop yield and enable the cultivation of produce with higher profitability

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Conclusion