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Keeping snails in check could significantly reduce parasitic worm infections

Stanford research suggests a new way to significantly curtail cases of schistosomiasis, one of the most common afflictions in the developing world.

At times the snail can be a very effective disease-causing machine. Particularly during the rainy season, waterborne snails that congregate in puddles, marshy areas and lakes provide a comfortable home for parasitic worms that may be very harmful to humans. The worms can infiltrate the human blood system, leading to gastrointestinal, liver and bladder problems, and even death.

This infectious disease, known as schistosomiasis, is one of the most common afflictions in the developing world, affecting more than 240 million people, says Nathan Lo, a Stanford MD/PhD student in the Division of Epidemiology who has been studying disease patterns and possible treatment and control options for several years.

In a paper in the Proceedings of the National Academy of Sciences, he and his colleagues found that by simply reducing snail populations in local water sources, where the worms reproduce, new infections could be significantly curtailed.

“Schistosomiasis will only be transmitted if snails are in the environment,” Lo, the study’s lead author, told me. “If you’re able to interrupt the worm’s life cycle by removing the snail, you can reduce new infections.”

Moreover, if communities combine snail control with mass treatment programs using very low-cost drugs, it’s possible to reduce the incidence of disease and disability by as much as 40 percent, compared to the conventional strategy of mass treatment of school children alone, the researchers found.

“In the vast majority of situations, adding snail control along with mass treatment makes sense and meets conventional definitions for being cost-effective,” Lo told me. “Where it would be especially cost-effective are in areas of high prevalence and regions where portions of the population are repeatedly missed by treatment campaigns. In some areas, mass treatment alone isn’t enough to reduce prevalence. But snail control has that extra effect.”

Lo said chemical-based snail control programs used to be popular until the advent of mass treatment programs made possible by donation of the drug praziquantel in the early 2000s. The drug, which costs about 21 cents a pill, can reduce egg production by 90 percent in schistosomiasis cases. Pharmaceutical company donations of the drug have led to large-scale treatment programs in schools and communities across sub-Saharan Africa, where schistosomiasis is endemic.

But these treatment programs haven’t been entirely effective in managing the disease, leading to a resurgence of interest now in snail management. The World Health Organization recently published new guidelines for field application of the chemical niclosamide to reduce snail populations, and some countries have begun discussion on reviving their snail control activities. China has long used the chemical to curtail snail populations, successfully reducing the incidence of the disease, Lo said.

“Everyone agrees that snail control can be a very effective strategy, but the question is when and where to apply it?” Lo said. The latest study, based on data from rural communities in Kenya, was designed to help answer that question. In the study, the researchers found that snail control was most effective in areas with high infection rates where community-wide treatment of both adults and children, not just school-based treatment, was also applied. It was less cost-effective in areas with lower infection rates, where twice-yearly treatment of school children alone might be sufficient, the researchers report.

Lo said one concern with chemical snail control is the impact on the environment. The chemical can be toxic to fish and frogs, though there is no evidence it is harmful to humans or livestock. There are biologic alternatives to reduce snail populations, including use of farmed shrimp, which eat the snails. A previous Stanford study found that the introduction of shrimp was an effective method for reducing schistosomiasis transmission in Senegal, Lo said.

He said the latest study argues for inclusion of snail control in global guidelines and in national schistosomiasis guidelines, especially for use in high-prevalence areas, “hot spots” of transmission and communities where treatment doesn’t reach everyone. Lo’s collaborators in the research include Stanford colleagues Eran Bendavid, MD, and Jason Andrews, MD, as well as investigators at Case Western Reserve University.

Previously: Using robotics to combat schistosomiasisScientists call for end to devastating worm diseases and Massive campaign against parasitic worm disease is cost-effective, new study shows
Photo by Oregon State University

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