Advancing Towards the Elimination of Intestinal Parasites Requires More Sensitive Diagnostic Tools

Quantitative PCR is much more sensitive than the current standard technique for detecting intestinal helminths, says a study led by the Barcelona Institute for Global Health (ISGlobal), an institution supported by “la Caixa” Foundation, in collaboration with the Manhiça Health Research Center (CISM). This molecular technique, combined with a statistical analysis, can help to detect transmission hotspots in regions that are advancing towards the elimination of these parasites.

Soil-transmitted helminths (STH) are intestinal parasites that affect over 1 billion people worldwide; most of them are children who live under poor hygiene conditions in tropical and subtropical countries. Although infections 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 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, which aims to improve the treatment of STH.

In this study, the research team compared the validity of different diagnostic methods to estimate the prevalence of infected people 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.

During 2018, the researchers collected two consecutive stool samples from almost 800 participants living in different district neighbourhoods, and analysed the presence of eggs from different helminths by looking for them under the microscope (the standard technique) or by amplifying specific genetic sequences by PCR. With the conventional method, only 10% of the neighbourhoods had more than 20% of people infected by at least one type of helminth, while with the PCR approach, 86% of neighbourhoods had a prevalence above 20%.

“This means that if we establish transmission maps based on the standard 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, who coordinated the study.

The authors point out that further work is needed to reduce costs and harmonise how samples are collected and analysed, but quantitative PCR represents the best available tool to evaluate the progress towards interrupting the transmission of this neglected disease.

Reference

Grau-Pujol B, Marti-Soler H, Escola V et al.  Towards soil-transmitted helminths transmission interruption: The impact of diagnostic tools on infection prediction in a low intensity setting in Southern Mozambique. PLoS Negl Trop Dis. 2021. doi: 10.1371/journal.pntd.0009803