7 results found
Wright JE, Werkman M, Dunn J, et al., 2018, Current epidemiological evidence for predisposition to high or low intensity human helminth infection: a systematic review, Parasites & Vectors, Vol: 11, ISSN: 1756-3305
Background: The human helminth infections include ascariasis, trichuriasis, hookworm infections, schistosomiasis,lymphatic filariasis (LF) and onchocerciasis. It is estimated that almost 2 billion people worldwide are infected withhelminths. Whilst the WHO treatment guidelines for helminth infections are mostly aimed at controlling morbidity,there has been a recent shift with some countries moving towards goals of disease elimination through mass drugadministration, especially for LF and onchocerciasis. However, as prevalence is driven lower, treating entire populationsmay no longer be the most efficient or cost-effective strategy. Instead, it may be beneficial to identify individuals ordemographic groups who are persistently infected, often termed as being “predisposed” to infection, and targettreatment at them.Methods: The authors searched Embase, MEDLINE, Global Health, and Web of Science for all English language, humanbasedpapers investigating predisposition to helminth infections published up to October 31st, 2017. Thevarying definitions used to describe predisposition, and the statistical tests used to determine its presence, are summarised.Evidence for predisposition is presented, stratified by helminth species, and risk factors for predisposition to infection areidentified and discussed.Results: In total, 43 papers were identified, summarising results from 34 different studies in 23 countries. Consistentevidence of predisposition to infection with certain species of human helminth was identified. Children were regularlyfound to experience greater predisposition to Ascaris lumbricoides, Schistosoma mansoni and S. haematobium than adults.Females were found to be more predisposed to A. lumbricoides infection than were males. Household clustering ofinfection was identified for A. lumbricoides, T. trichiura and S. japonicum. Ascaris lumbricoides and T. trichiura also showedevidence of familial predisposition. Whilst strong evidence for predisposition
Werkman M, Wright JE, Truscott JE, et al., 2018, Testing for soil-transmitted helminth transmission elimination: Analysing the impact of the sensitivity of different diagnostic tools., PLoS Neglected Tropical Diseases, Vol: 12, ISSN: 1935-2727
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.
Truscott JE, Werkman M, Wright JE, et al., 2017, Identifying optimal threshold statistics for elimination of hookworm using a stochastic simulation model, Parasites & Vectors, Vol: 10, ISSN: 1756-3305
BackgroundThere is an increased focus on whether mass drug administration (MDA) programmes alone can interrupt the transmission of soil-transmitted helminths (STH). Mathematical models can be used to model these interventions and are increasingly being implemented to inform investigators about expected trial outcome and the choice of optimum study design. One key factor is the choice of threshold for detecting elimination. However, there are currently no thresholds defined for STH regarding breaking transmission.MethodsWe develop a simulation of an elimination study, based on the DeWorm3 project, using an individual-based stochastic disease transmission model in conjunction with models of MDA, sampling, diagnostics and the construction of study clusters. The simulation is then used to analyse the relationship between the study end-point elimination threshold and whether elimination is achieved in the long term within the model. We analyse the quality of a range of statistics in terms of the positive predictive values (PPV) and how they depend on a range of covariates, including threshold values, baseline prevalence, measurement time point and how clusters are constructed.ResultsEnd-point infection prevalence performs well in discriminating between villages that achieve interruption of transmission and those that do not, although the quality of the threshold is sensitive to baseline prevalence and threshold value. Optimal post-treatment prevalence threshold value for determining elimination is in the range 2% or less when the baseline prevalence range is broad. For multiple clusters of communities, both the probability of elimination and the ability of thresholds to detect it are strongly dependent on the size of the cluster and the size distribution of the constituent communities. Number of communities in a cluster is a key indicator of probability of elimination and PPV. Extending the time, post-study endpoint, at which the threshold statistic is measured improves
Werkman M, Truscott JE, Toor J, et al., 2017, The past matters: estimating intrinsic hookworm transmission intensity inareas with past mass drug administration to control lymphatic filariasis, Parasites & Vectors, Vol: 10, ISSN: 1756-3305
BackgroundCurrent WHO guidelines for soil-transmitted helminth (STH) control focus on mass drug administration (MDA) targeting preschool-aged (pre-SAC) and school-aged children (SAC), with the goal of eliminating STH as a public health problem amongst children. Recently, attention and funding has turned towards the question whether MDA alone can result in the interruption of transmission for STH. The lymphatic filariasis (LF) elimination programme, have been successful in reaching whole communities. There is the possibility of building upon the infrastructure created for these LF-programmes to enhance the control of STH. Using hookworm as an example, we explore what further MDA coverage might be required to induce interruption of transmission for hookworm in the wake of a successful LF programme.ResultsAnalyses based on the model of STH transmission and MDA impact predict the effects of previous LF control by MDA over five years, on a defined baseline prevalence of STH in an area with a defined transmission intensity (the basic reproductive number R0). If the LF MDA programme achieved a high coverage (70, 70 and 60% for pre-SAC, SAC and adults, respectively) we expect that in communities with a hookworm prevalence of 15%, after 5 years of LF control, the intrinsic R0 value in that setting is 2.47. By contrast, if lower LF coverages were achieved (40, 40 and 30% for pre-SAC, SAC and adults, respectively), with the same prevalence of 15% at baseline (after 5 years of LF MDA), the intrinsic hookworm R0 value is predicted to be 1.67. The intrinsic R0 value has a large effect on the expected successes of follow-up STH programmes post LF MDA. Consequently, the outcomes of identical programmes may differ between these communities.ConclusionTo design the optimal MDA intervention to eliminate STH infections, it is vital to have information on historical MDA programmes and baseline prevalence to estimate the intrinsic transmission intensity for the defined setting (R0). The b
Werkman M, Truscott JE, Wright JE, et al., 2017, ARE WE ON THE RIGHT TRACK? STOPPING CRITERIA FOR ENDING SOIL-TRANSMITTED HELMINTHS RANDOMIZED CLINICAL TRIALS, 66th Annual Meeting of the American-Society-of-Tropical-Medicine-and-Hygiene (ASTMH)
Wright JE, Truscott J, Werkman M, et al., 2017, ASSESSING BETWEEN-VILLAGE HETEROGENEITY OF HOOKWORM TRANSMISSION IN A LOW-INTENSITY SETTING, 66th Annual Meeting of the American-Society-of-Tropical-Medicine-and-Hygiene (ASTMH)
Toor J, Truscott JE, Alsallaq R, et al., 2017, ARE WE ON OUR WAY TO ACHIEVING THE 2020 GOALS FOR SCHISTOSOMIASIS MORBIDITY CONTROL USING CURRENT WHO GUIDELINES?, 66th Annual Meeting of the American-Society-of-Tropical-Medicine-and-Hygiene (ASTMH)
This data is extracted from the Web of Science and reproduced under a licence from Thomson Reuters. You may not copy or re-distribute this data in whole or in part without the written consent of the Science business of Thomson Reuters.