The number of people infected by the intestinal worm Strongyloides stercoralis could be four times higher than current estimations, according to a study led by ISGlobal, an institution supported by “la Caixa”. The study shows there is a correlation between Strongyloides prevalence and that of another parasitic worm (hookworm). These new estimates will contribute to meet the growing demand for ivermectin, a drug used to treat several neglected tropical diseases.
Intestinal worms are a major public health problem, which is why the WHO called for their control in 2001. Currently, the soil-transmitted helminths that are target of massive control campaigns are roundworm ( Ascaris lumbricoides), flatworm (Trichuris trichiura) and hookworm (Necator americanus and Ancylostoma duodenale). More recently, the effective control of Strongyloides in school-aged children was included in the 2030 health targets.
To achieve this target, better estimates of the prevalence and geographical distribution of Strongyloides are necessary for ensuring an adequate production of ivermectin, the drug used to treat the disease. “The problem is that data on Strongyloides prevalence and morbidity are scarce, and this is due to the difficulty in diagnosing the infection” explains study coordinator, Alejandro Krolewiecki. “The current estimates of 30 to 100 million people infected are based on reports that lack scientific evidence,” he adds.
In this study, the research team built on the observation that hookworm and Strongyloides share biological and epidemiological features, such as infection route, risk factors and host age distribution, as well as an overlapping geographical distribution. The authors performed a systematic review to establish a correlation coefficient between both infections. The analysis included 119 articles published over the last twenty years which reported on prevalence for both infections, in school-aged children or in the community. The reviewed articles included a total of almost 100,000 individuals from five geographic regions.
Based on the correlation coefficients obtained in the study, the team estimates that around 386 million people are infected with S. stercoralis worldwide, of which 22 million are school-aged children. The lack of adequate sanitation is a common risk factor for both species, which would explain the association between both infections.
“These results indicate that we can use hookworm prevalence as a proxy to estimate the global burden of S. stercoralis infections,” says co-author and ISGlobal researcher Helena Martí, who participates with Kroleweicki in a project led by ISGlobal to study the efficacy of combining ivermectin and albendazole for the treatment of soil-transmitted-helminths, including Strongyloides.
Pedro E. Fleitas, Marina Travacio, Helena Martí-Soler, M. Eugenia Socías, Walter R. Lopez, Alejandro J. Krolewiecki. The Strongyloides stercoralis-hookworms association as a path to the estimation of the global burden of strongyloidiasis: A systematic review. PLoS Negl Trop Dis. 2020. https://doi.org/10.1371/journal.pntd.0008184