Just Add Prawns

Ecologists deploy a small weapon in a big disease battle.

May 2023

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At the Senegal River, villagers washing their clothing.

WATER WOES: The Senegal River is an important resource for local residents, but it is contaminated with the flatworms that cause schistosomiasis, the world’s second most devastating parasitic disease after malaria. Photo: Hilary Duff

Giulio De Leo tries never to touch the water in the lower basin of the Senegal River. Even when the weather tops 100 degrees Fahrenheit, the Stanford ecologist and his team of researchers slide into thick plastic waders that seal them off from toe to chest. They collect data with sweat dripping down their necks and pooling in their boots while, all around them, children wearing nothing but shorts and T-shirts swim and splash to cool off. Chances are high the children already have the region’s endemic parasitic disease, schistosomiasis. In 2014, when De Leo’s team first traveled there, up to 99 percent of children in some villages were infected.  

The disease, colloquially called schisto, is effortless to contract: As soon as your skin touches contaminated water, you can pick up parasitic worms that later cause problems ranging from diarrhea to bladder cancer. Smaller than dust mites, the parasites replicate in freshwater snails, then swim out into the water looking for human hosts. The Centers for Disease Control deems schisto the second most devastating parasitic disease after malaria, and more than 200 million people are currently infected. “We’re talking about a disease of poverty,” says De Leo. “It’s in rural areas mostly, where there’s no easy access to piped, clean water.” About 90 percent of cases are in sub-Saharan Africa, but the disease is on the move. It recently reached the Mediterranean, where—because of climate change—waters are now warm enough for the parasites to survive.

Guilio De Leo‘We’re talking about a disease of poverty. It’s in rural areas mostly, where there’s no easy access to piped, clean water.’

De Leo

For the past decade, De Leo, a professor of oceans in the Stanford Doerr School of Sustainability, and his collaborators have been testing out an unconventional solution in Senegal: controlling the disease by controlling the ecosystem. In 2012, researchers began adding snail-eating African river prawns to contaminated water, trying to prove that an animal predator could remove a key part of the parasites’ life cycle, stop them from proliferating, and prevent people from becoming infected. But environmental research on schisto was so nascent that the team had to invent their way through the most basic processes, and by the end of the prawn project in 2018, their approach was far from proven. Still, the group’s foundational discoveries have cut a path toward better, faster solutions. That’s important, De Leo says, because when it comes to diseases heavily mediated by the environment, the best medicine might not be medicine at all. It might be a prawn. 

Force of Nature 

By the time De Leo and his team set foot on the dry, red soil of northern Senegal, many of the region’s residents had been living with schisto for more than a generation. Andrea Lund, PhD ’20, conducted focus groups and collected surveys from approximately 600 households in more than a dozen villages to understand local perceptions of the water and the disease. Touching the water is “not good for you,” said one local gardener. “But we have no choice.” Some residents used grasses from the water to build the roofs of their homes. Many of the men spent their days fishing to support their families. The water was where people bathed, did laundry, even drank when wells ran dry. “We cannot protect ourselves,” said the gardener. “We are not easy to protect.” 

ECO-HEROISM: A research team (from left: Raphaël Ndione, then a PhD student at the University of Dakar, field project manager Andrew Chamberlin, and Sokolow) aimed to demonstrate that a natural predator—prawns—would reduce the snails that host the parasite that spreads schistosomiasis. (Photo: Hilary Duff)ECO-HEROISM: A research team (from left: Raphaël Ndione, then a PhD student at the University of Dakar, field project manager Andrew Chamberlin, and Sokolow) aimed to demonstrate that a natural predator—prawns—would reduce the snails that host the parasite that spreads schistosomiasis. (Photo: Hilary Duff)

The schisto problem in Senegal began in 1986, when construction of the Diama Dam blocked the migration of African river prawns. In the absence of a key predator, snail populations upstream exploded, and parasites flourished as minuscule masters of misfortune. Each parasite that enters a snail can lead to 100,000 exiting. Once they burrow through the skin and reach the blood vessels of a human, they can live for more than 30 years, laying up to 2,200 eggs per day. Some eggs are excreted in stool or urine, and then embed themselves in snails, where they can restart their life cycle. The rest lodge in their human host’s organs, causing inflammation. Early on, this can manifest as skin rashes and abdominal pain. In some parts of Africa, blood in the urine has become so common that it is “sort of viewed as a rite of passage for kids,” Lund says. “It’s likened to menstruation, in a way.” Years of chronic infection have led 20 million people worldwide to experience severe health issues, such as liver failure, infertility, and cognitive impairment.  

A cure does exist. For decades, a pill called praziquantel has been used to immediately kill parasites in the body, and it’s conventionally administered in high-risk villages once per year. But it has no preventive effect. In places like Senegal, where repeated exposure to parasites is inevitable, people can become reinfected the same day. A cure isn’t enough. 

Fantastic Beasts 

The prawn emerged as a potential hero in 2009. Fully grown males are magnificent creatures, so large that if you hold one in your hand, you have to extend your arm so its pencil-thin, lobster-like claws don’t poke you in the eye. To maintain their exoskeletons, prawns voraciously hunt one of the few freshwater sources of calcium: snails. 

The crustaceans captured the imagination of Susanne Sokolow, a veterinarian turned ecologist at UC Santa Barbara. She was working in the lab of Armand Kuris, a professor of zoology who had published a landmark study in 1999 showing that invasive crayfish preying on snails reduced rates of schisto in Kenyan villages. Sokolow began developing a project in Senegal based on that work but instead using a native species. This eliminated the invasive potential of a nonnative species and, upriver of the Diama Dam, meant the project could also serve as an ecological restoration effort. 

Her use of living organisms to reduce a pest population—called biological control—was not a new idea. It’s a common practice in agriculture and dates back to the 3rd century. The USDA cites it as easy, safe, and environmentally sound, especially when compared with broad-spectrum pesticides. “If this is true for agriculture,” says De Leo, “why are we not looking at this more for human disease?” 

Just a few months into his faculty appointment at Stanford in 2012, De Leo met Sokolow while she was visiting a friend at Hopkins Marine Station. He needed passionate postdocs to help him launch his lab. Sokolow needed a collaborator with mathematical modeling expertise to turn her theory into reality. Within an hour of knowing each other, they were drawing what Sokolow calls “scary scribbles” on a chalkboard—diagrams, models, and brainstorms of how they might tackle the crisis in Senegal.   

Trials and Vegetation 

Their pilot study tested the idea on a small scale. “You had to think about it really carefully,” says Sokolow. “You’ve got invertebrates like snails carrying a parasite, and people, and then a predator, and everything interacting.” In addition to completing environmental impact reports in Senegal, De Leo modeled the entire ecosystem to predict what would happen when prawns were added and why. 

On left, a researcher's hand holding a handful of snails. On the right, a hand holding one prawn.THE PRAWN BRIGADE: Native African river prawns decreased heavy infections of schisto in a test village by nearly half, De Leo’s team found. In a control village without prawns, infections tripled. (Photos, from left: Guilio De Leo; Hilary Duff) 

The researchers partnered with Biomedical Research Center Espoir Pour la Santé, a Senegalese organization that employs local residents and collects parasitology data from area schools. The research team selected a test village and a control village, then added native African river prawns to the test village’s water source. In 2015, they showed that heavy infections decreased by nearly half where prawns were present but tripled in their absence. Their paper sent a bolt of excitement through the scientific community. “It was transformative,” says Erin Mordecai, an associate professor of biology at Stanford specializing in the ecology of infectious disease. “People do these randomized control trials on drugs, vaccines—biomedical interventions—a lot, but it’s really rare to see a randomized control trial on an environmental manipulation.” 

To test whether their idea was scalable and economically sustainable, De Leo and Sokolow obtained grants for a three-year project. Their hope was to figure out where the snails were, domesticate the wild African river prawn, and build a prawn hatchery to stock them in 10 high-risk villages. The hatchery would provide both a reservoir of predators for snail control and income for local fishermen.  

They quickly found that the most basic tools they needed didn’t yet exist. “Traditionally, when you do schistosomiasis research—I’m not even making this up a little bit, I wish I were—you buy a flour sifter at the kitchen store, and you duct tape it to a stick,” says Chelsea Wood, PhD ’13, an associate professor of biology at the University of Washington who helped lead the research. Instead, the team invented a cube-shaped apparatus made of PVC and aluminum that could consistently and properly sample the murky waters to identify the distribution of snails.   

Snapped into their “sweatbox” waders, the researchers took the device into the water’s tall grasses, where they were bitten by ants, spiders, and mosquitoes. Deeper in, rats, snakes, and dogs emerged from the underbrush. They later used a drone to capture the landscape from above. Over time, they discovered that analyzing vegetation and snails’ preferred habitat via satellite and drone imagery was a better predictor of schisto risk than physically collecting snail samples. The easy, unobtrusive method is now being used in other countries, as well as in research that maps malaria risk. 

Digging In 

With unprecedented knowledge of snail—and thus parasite—distribution, the researchers set out to introduce prawns on a grand scale. But this part of their experiment did not conform to the three-year grant deadline. Their partners in Senegal were unable to produce enough domesticated prawns, and the team discovered that pesticide runoff from nearby farms further reduced the population. Without sufficient predation or money to extend the project, the test villages didn’t receive the full benefit of biological control. De Leo is still looking at the data, but he’s certain it won’t be the straightforward breakthrough he was hoping for. 

Back when Lund began interviewing people for the focus groups, some residents asked her what success would look like. They had watched researchers flock to their waters for decades, testing people for disease, sometimes leaving without telling them what they’d found. De Leo, for one, isn’t leaving. “We identified problems,” De Leo says, “but we think that we also potentially identified solutions.” In 2022, after years of almost exclusively recommending mass drug administration, the WHO added snail control to its plan for the eradication of schisto. While not yet scalable in Senegal, prawns are ideal enemies for infected snails in rice paddies elsewhere. De Leo is now looking into the use of catfish, lungfish, and even ducks (his favorite) as potential predators. He and his collaborators are expanding biological control research in Senegal, Madagascar, and Côte d’Ivoire. And with Sokolow joining as executive director, he has created Stanford’s program for disease ecology, health and the environment to support more interdisciplinary studies like theirs that leverage ecology to benefit human health.  

The disease may be on the move, but thanks to De Leo’s lab, so is the research. 

Kali Shiloh is a staff writer at Stanford. Email her at

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