The increased sensitivity of qPCR in comparison to Kato-Katz is required for the accurate assessment of the prevalence of soil-transmitted helminth infection in settings that have received multiple rounds of mass drug administration

Julia C Dunn,D T J Littlewood,Nay Soe Maung,Raju Misra,Roy M Anderson,Marina Papaiakovou,Nay Yee Wyine,Kay Thwe Han,Alison A Bettis,Aye Moe Moe Lwin,Darren Chooneea

doi:10.1186/s13071-020-04197-w

Abstract

BackgroundThe most commonly used diagnostic tool for soil-transmitted helminths (STH) is the Kato-Katz (KK) thick smear technique. However, numerous studies have suggested that the sensitivity of KK can be problematic, especially in low prevalence and low intensity settings. An emerging alternative is quantitative polymerase chain reaction (qPCR).MethodsIn this study, both KK and qPCR were conducted on stool samples from 648 participants in an STH epidemiology study conducted in the delta region of Myanmar in June 2016.ResultsPrevalence of any STH was 20.68% by KK and 45.06% by qPCR. Prevalence of each individual STH was also higher by qPCR than KK, the biggest difference was for hookworm with an approximately 4-fold increase between the two diagnostic techniques. Prevalence of Ancylostoma ceylanicum, a parasite predominately found in dogs, was 4.63%, indicating that there is the possibility of zoonotic transmission in the study setting. In individuals with moderate to high intensity infections there is evidence for a linear relationship between eggs per gram (EPG) of faeces, derived from KK, and DNA copy number, derived from qPCR which is particularly strong for Ascaris lumbricoides.ConclusionsThe use of qPCR in low prevalence settings is important to accurately assess the epidemiological situation and plan control strategies for the ‘end game’. However, more work is required to accurately assess STH intensity from qPCR results and to reduce the cost of qPCR so that is widely accessible in STH endemic countries.

Highlights

The most commonly used diagnostic tool for soil-transmitted helminths (STH) is the Kato-Katz (KK) thick smear technique
Analyses based on stochastic models of parasite transmission and controls have calculated that if the true prevalence is reduced below a threshold of 2%, there is an 80% positive predictive value (PPV) that transmission of STH has been interrupted and infection will not bounce back to endemicity, this conclusion is sensitive to the baseline prevalence prior to the use of Mass drug administration (MDA) [3, 5]
Combining the three hookworm species, prevalence of hookworm was 25.62%

Summary

Introduction

The most commonly used diagnostic tool for soil-transmitted helminths (STH) is the Kato-Katz (KK) thick smear technique. STH prevalence and intensity of infection in many settings to the point that the interruption of transmission and parasite elimination are feasible goals in countries with consistently high MDA coverage over many rounds [2,3,4]. The World Health Organization (WHO) defines EPG thresholds to group infection intensity in a person into low, medium or high [8]. These vary between species as different species produce varying mean number of eggs per fertile female worm [9]. A further complication is density dependence, when female worms produce fewer eggs as the worm burden within an individual increases (i.e. there is more competition for resources within the host) [10, 11]

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