Testing for soil-transmitted helminth transmission elimination: Analysing the impact of the sensitivity of different diagnostic tools

Abstract

In recent years, an increased focus has been placed upon the possibility of the elimination of soil-transmitted helminth (STH) transmission using various interventions including mass drug administration. The primary diagnostic tool recommended by the WHO is the detection of STH eggs in stool using the Kato-Katz (KK) method. However, detecting infected individuals using this method becomes increasingly difficult as the intensity of infection decreases. Newer techniques, such as qPCR, have been shown to have greater sensitivity than KK, especially at low prevalence. However, the impact of using qPCR on elimination thresholds is yet to be investigated. In this paper, we aim to quantify how the sensitivity of these two diagnostic tools affects the optimal prevalence threshold at which to declare the interruption of transmission with a defined level of confidence. A stochastic, individual-based STH transmission model was used in this study to simulate the transmission dynamics of Ascaris and hookworm. Data from a Kenyan deworming study were used to parameterize the diagnostic model which was based on egg detection probabilities. The positive and negative predictive values (PPV and NPV) were calculated to assess the quality of any given threshold, with the optimal threshold value taken to be that at which both were maximised. The threshold prevalence of infection values for declaring elimination of Ascaris transmission were 6% and 12% for KK and qPCR respectively. For hookworm, these threshold values are lower at 0.5% and 2% respectively. Diagnostic tests with greater sensitivity are becoming increasingly important as we approach the elimination of STH transmission in some regions of the world. For declaring the elimination of transmission, using qPCR to diagnose STH infection results in the definition of a higher prevalence, than when KK is used.

Defining stopping criteria for ending randomized clinical trials that investigate the interruption of transmission of soil-transmitted helminths employing mass drug administration

“…In this study, we set k to 0.285 for A . lumbricoides [ 23 ] and to 0.35 for hookworm [ 24 ], these parameter estimates were derived from field studies in Kenya and Zimbabwe. The other parameters are listed in Table 1 .…”

“…We assumed 94% sensitivity for qPCR assays for both A . lumbricoides and hookworm species [ 23 , 25 ]. Most diagnostic tools for STH species are based on the presence or absence of eggs in a host stool sample.…”

What does soil-transmitted helminth elimination look like? Results from a targeted molecular detection survey in Japan

“…Additionally, the sample size of the included individuals was low (n = 682), and the 95% confidence interval included a fairly wide range of potential prevalence estimates (0-0.48%) [47]. This sample size allowed us to reasonably conclude that the prevalence was not above the elimination threshold at which transmission is likely to be sustained (1-2% prevalence by PCR) [69][70][71].…”

The impact of community-wide, mass drug administration on aggregation of soil-transmitted helminth infection in human host populations

“…The impact of successive rounds of MDA on STH prevalence is the primary focus of the Bill & Melinda Gates Foundation funded DeWorm3 project in India, Malawi and Benin [18]. This project is investigating whether a communitywide MDA-only approach to STH treatment, would be sufficient to reach a prevalence threshold below which helminth parasite populations cannot sustain transmission, thus leading to transmission elimination [19][20][21]. All STH species reproduce sexually within a human host to produce fertile eggs.…”