Stanford researchers announced today that a study in Senegal, West Africa, has found that freshwater prawns can serve as an effective natural solution in the battle against schistosomiasis, a potentially deadly parasitic disease that infects roughly 230 million people. The prawns prey on parasite-infected snails, while providing a marketable, protein-rich food source. Because prawns cannot support schistosomiasis' complex life cycle, they do not transmit the disease themselves.
"The results of our study open the pathway to a novel approach for the control of schistosomiasis," said co-author Giulio De Leo, a biology professor at Stanford's Hopkins Marine Station and a senior fellow at the Stanford Woods Institute for the Environment.
The study, published in the Proceedings of the National Academy of Science, tracked parasite-infected snails and people in two villages. In one village, the international research team and Senegalese partner Biomedical Research Center Espoir pour la Santé stocked a river access point with prawns. Over the course of 18 months, they found 80 percent fewer infected snails and a 50 percent lower disease burden (the mean number of parasite eggs in a person's urine) in people living in the prawn-stocked village.
The study was recognized as the best health project in the Data for Development Challenge Senegal, in which international teams use anonymous mobile phone data to analyze issues ranging from agriculture to urban planning. The competition is organized by telecommunications companies Sonatel and the Orange Group, and supported by Senegal’s Ministry of Higher Education and Research. De Leo and his team used the mobile data to gather intel abot people’s movements among rural areas, with higher potential for schistosomiasis, and urban areas.
“This was an opportunity for us to think at a different scale,” De Leo said. “The mobile data allows us to calibrate the effect of people’s movement on schistosomiasis transmission at the national level in Senegal.”
In a mathematical model of the system, stocking prawns, coupled with infrequent mass drug treatment, eliminated schistosomiasis in high-transmission sites.
"Where drugs, alone, fail to control schistosomiasis due to rapid reinfection, prawns may offer a complementary strategy" for controlling the disease, according to the study.
Local communities could be incentivized to maintain prawn populations in order to market them as a food product, the researchers noted.
"They are delicious,” said lead author Susanne Sokolow, a Woods-affiliated research associate located at Stanford's Hopkins Marine Station. "They can synergize with local efforts in the developing world to fight parasitic disease and to foster new aquaculture-based industries." Thus, the approach could bring four major benefits: disease control, biodiversity restoration, poverty alleviation and improved nutrition.
Worldwide, nearly 800 million people are at risk of getting schistosomiasis — most colloquially known as "blood flukes" — an infestation of parasitic flatworms that can cause anemia, growth stunting, infertility, liver failure, bladder cancer and lasting cognitive impairment.
Currently, the only treatment for the disease is a drug called praziquantel, but insufficient global supplies, cost and other factors limit its effectiveness. Even if it were widely and cheaply available, praziquantel would be an incomplete solution for people who bathe and clean clothing in rivers, for example, and are at high risk of reinfection through frequent contact with schistosome-contaminated waters.
In Africa, where most schistosomiasis cases occur, rates of infection often increased dramatically after construction of dams. De Leo and his fellow researchers speculate this is due not only to the dams' positive impact on snail habitat but to their negative impact on snail predators, including freshwater prawns, that need to travel upstream and downstream to mate and lay eggs.
In addition to stocking river access points, the researchers suggest prawns could be restored to rivers through the use of dam-bypassing passages similar to salmon ladders used in the Western United States.
More natural solutions
Sokolow, De Leo and their colleagues have attracted international attention and more than $6 million in funding from organizations such as the Bill & Melinda Gates Foundation, the National Institutes of Health, the National Science Foundation and Grand Challenges Canada. They say they plan to expand their work to focus on a range of natural solutions to global health and poverty challenges as part of an initiative called the Upstream Alliance. While the prawn research has shown the effectiveness of natural solutions at small scales, the researchers plan to explore whether such approaches can be viable and sustainable on larger scales.
Co-authors on the paper, "Reduced transmission of human schistosomiasis after restoration of a native prawn that preys on the snail intermediate host," include researchers from the 20/20 Initiative; the University of California, Santa Barbara; the Institut Pasteur de Lille, France; and the Biomedical Research Center Espoir pour la Santé, Saint-Louis, Senegal.