Deworming Programs in Schools Have Limited Efficacy

Joe Barber Jr, PhD

February 28, 2013

School-based deworming programs may have limited efficacy on soil-transmitted helminths at the community level, particularly in areas where hookworms predominate, according to the findings of a recent analysis.

Roy M. Anderson, PhD, from Imperial College London in the United Kingdom, and colleagues published their findings online February 28 in PLoS Neglected Tropical Diseases.

The authors note that although soil-transmitted helminths are easily treated, reinfection is common. "[Soil-transmitted helminths] are easily treated with one of four drugs: albendazole and mebendazole, and to a lesser extent, levamisole, and pyrantel pamoate," the authors write. "However, reinfection commonly occurs due to the inability of the human host to mount protective immunity to reinfection by intestinal helminths, combined with inadequate hygiene and sanitation to restrict or eliminate reexposure in environments continuously contaminated with the egg or larval free-living transmission stages of these parasitic worms."

In the study, the authors analyzed the proportion of parasites targeted by school-based deworming, using data on demographics, enrollment, and age-specific intensity of infection. They also used mathematical models to estimate the potential effect of deworming on transmission in different situations.

Using previously published data, the authors determined that the percentage of the total population of parasites carried by school-aged (age, 5 - 14 years) children in different areas varied from 9.4% for Necator americanus to 49.4% for Ascaris lumbricoides, indicating that less than half of the worm population would be targeted in a single round of deworming.

In addition, the researchers determined that the percentage of girls enrolled in school ranged from 45.5% in Niger to 71.3% in the Cook Islands. Using mathematical modeling, the authors further reveal that annual deworming programs targeting only children would have only modest effects on transmission in the general population, and increasing the frequency of treatment did not substantially alter the mean worm burden.

"The benefits of deworming for the affected children are many, but if we are to plan for long-term control and, in the longer term, elimination of these pathogens we need to consider strategies that will reduce transmission from year to year," the authors write. "As this paper shows, there are many outstanding data gaps and needs for new modelling studies both to understand the dynamics of transmission under such programmes, and to design optimal treatment strategies for the future."

In a related commentary, David Addiss, MD, MPH, director of Children Without Worms in Decatur, Georgia, remarked that the current study forms the basis for a fundamental conversation about current global soil-transmitted helminthiasis control. "Anderson and colleagues demonstrate the power of epidemiologic modeling to raise critical questions and sharpen our thinking," he writes. "The key [challenge] that they reveal is the need for clarity of purpose and renewed commitment to the promise [to reduce soil-transmitted helminthiasis morbidity]. From a clear, shared vision of elimination of [soil-transmitted helminthiasis] as a public health problem, our strategies, objectives, and programs will naturally flow."

"Following the London Declaration on Neglected Tropical Diseases, pharmaceutical companies are making large donations of treatments for soil-transmitted helminths, which is great news for reducing the burden of these parasites," Dr. Anderson said in a journal news release. "To make best use of donations, we need more calculation based on sound understanding of disease transmission, as opposed to just relying on opinion.

"Since there are very few studies which look at the indirect effect of school-based treatment, we need to do more studies to estimate mixing patterns and the impact on transmission in order to design effective programmes in the future."

Dr. Anderson holds a nonexecutive board membership with GlaxoSmithKline. The other authors and the commentator have disclosed no relevant financial relationships.

PLoS Negl Trop Dis. Published online February 28, 2013.