A study performed in Mozambique, now published, provides valuable information for the STOP project, and shows that combining quantitative PCR with geostatistical methods is the best strategy for estimating the prevalence of intestinal worm infections in the community
Although infections by soil-transmitted helminths are frequently asymptomatic, those with a high parasite load (high intensity infections) can lead to anaemia, malnutrition and delays in physical and cognitive development. To control the transmission of these intestinal parasites, the WHO recommends mass drug administration once or twice a year (if the prevalence is higher than 20% or 40%, respectively), as well as improving access to safe water, sanitation and hygiene (WASH) to avoid reinfections. The ambitious goal set by the WHO is that, by 2030, 96% of endemic countries have less than 2% of moderate to high intensity infections.
“As regions approach the WHO targets, it will be important to have diagnostic tools that are sensitive enough to detect low intensity infections and identify transmission hotspots in the community,” explains Jose Muñoz, ISGlobal researcher and leader of the STOP project.
A study performed in Mozambique by Muñoz and colleagues a couple of years ago has now been published, and provides valuable information for the STOP project. The aim of the research team was to compare the validity of different diagnostic methods to estimate the prevalence of STH in the district of Manhiça, considered a zone of low intensity infections. The team also used the detailed database of the district to create prevalence maps in the different neighbourhoods and detect possible transmission hotspots.
With the conventional microscopy method, only 10% of the neighbourhoods had more than 20% of people infected by at least one type of helminth, while with the quantitative PCR method, 86% of neighbourhoods had a prevalence above 20%.
“This means that if we establish transmission maps based on the standard microscopy technique, we are considerably subestimating the prevalence of infections and risk missing transmission hotspots,” explains Berta Grau, first author of the study.
“Using the conventional method, the district reached the WHO targets in 2017: a prevalence under 20% and less than 2% of high intensity infections. But the PCR results indicate there could be an uptick of cases if mass drug administration is interrupted, given that the district has poor hygiene conditions,” adds Muñoz.
Although further work is needed to reduce costs and harmonise how samples are collected and analysed, the findings show that quantitative PCR represents the best available tool to evaluate the progress towards interrupting the transmission of this neglected disease.