Controlling Strongyloides stercoralis: WHO’s recommendations for addressing it as a public health issue
Strongyloides stercoralis is one of the most challenging soil-transmitted helminth (STH) species to diagnose and control. Although it affects millions of people worldwide, research on this parasite has been limited. This makes the publication of the first WHO guidelines for its public health control an important milestone.
These new guidelines will inform the strategies that endemic countries follow to address strongyloidiasis as a public health problem. They issue a single recommendation: mass drug administration of single-dose ivermectin, administered to everyone older than 5, in areas where prevalence is above 5%.
The group of experts involved in these guidelines recently published a review of them in Lancet Infectious Diseases. We spoke with Dr. Arancha Amor, researcher at Mundo Sano, co-author of the paper and specialist in STH infections, who has been focused on the challenges surrounding strongyloidiasis in Ethiopia.
The first difficulty with this STH species is diagnosis, she explains, as there is currently no gold-standard method for detecting these intestinal parasites. “The most common diagnosis technique [for STH] involves detecting eggs in stool. But Strongyloides stercoralis appears in larval form, so the egg-detection technique doesn’t work. it might not be that it’s absent—it’s just that you’re not seeing it,” Dr. Amor explains. They need other techniques, like Baermann or PCR, to detect the larvae in stool samples.
The second issue is that the commonly used drug for STH control, albendazole monotherapy, is not very effective against Strongyloides stercoralis—ivermectin is. However, until 2021-2022, it was “not affordable or a viable candidate for use in developing countries,” according to the expert. Since then, WHO has pre-qualified two types of generic ivermectin, “which significantly expands the potential for a control strategy similar to the one used for other species, through large-scale deworming campaigns targeting at-risk populations.”
The STOP2030 project is developing its coformulation of ivermectin and albendazole, at fixed doses so that a single tablet can target the five species present in WHO’s 2030 Roadmap for STH goals, including Strongyloides stercoralis. “With a single pill you target the five species without adding logistical complications. The challenge of a co-formulation is finding a balanced dose that addresses the infection safely and without significantly overdosing or underdosing,” she explains.
Another issue raised in the WHO guidelines for Strongyloides stercoralis public health control is the insufficient study-specific evidence available. WHO recommendations had to take into account studies designed for other diseases, such as lymphatic filariasis and onchocerciasis, where ivermectin is also used, and where a reduction in Strongyloidiasis is a secondary research target.
“That’s why WHO is now calling for well-designed studies specifically aimed at controlling strongyloidiasis,” Dr. Amor says.
“Strongyloides also has some clinically and physiologically unique characteristics,” explains the expert. “It’s a parasitic worm capable of autoinfection, which means that once someone is infected, the parasite can complete its life cycle inside the body.” Some people can remain unknowingly infected for up to 60 years, with the “risk of hyperinfection and disseminated disease during immunosuppression, which has a case fatality rate of over 60%,” as noted in the paper published in Lancet Infectious Diseases.
The challenges of Strongyloidiasis don’t stop there. The main control strategy against STH has been mass drug administration campaigns, usually aimed at school-aged children and women of reproductive age.
“Control strategies would be more effective if we include all the adult population,” concludes Dr. Amor. The next step is to bring all this information to the relevant health authorities, so they can “achieve the goal of ensuring that Strongyloides is not only included in the WHO guidelines but also incorporated into the master plans for the control of neglected diseases.”
