In 1986, after almost five years of construction, the Diama Dam was finally completed along the mouth of the Senegal River. The dam stopped saltwater from intruding upstream, thus creating a stable reservoir of freshwater for farmers and for Senegal’s capital city of Dakar.
But it also had unintended consequences. By restraining the saltwater, the dam favoured the growth of freshwater algae and plants, which in turn fed large numbers of snails. The snails are hosts for parasitic flatworms that cause schistosomiasis—a horrible water-borne disease that damages the kidneys, bladder, intestines, and liver. As the snail population boomed, they triggered a huge outbreak of schistosomiasis, which spread with unprecedented speed and still persists today. In some places, more than 90 percent of villagers are infected. In damming the river, Senegal also damned the people along it.
But help is at hand. A team of scientists led by Susanne Sokolow from Stanford University has been working on a way of stopping the outbreak by bringing the snails—and their parasites—under control. Their plan? Add prawns.
The lower Senegal River used to be home to a hand-sized, long-clawed prawn called Macrobrachium vollenhovenii, that would devour the parasite-carrying snails. Every year, the female prawns would walk downstream to the estuary to lay their eggs; later, the larvae would swim back upstream. The Diama Dam cut off both routes and exterminated the prawns. By reintroducing them, Sokolow hopes to control the rampant snails and bring schistosomiasis to heel.
“It’s not a new idea,” she says. In 1999, a team of scientists successfully used crayfish to reduce both snail populations and schistosomiasis infections in a couple of Kenyan villages. Unfortunately, they used an American crayfish that had been introduced to Africa several decades before, and was considered an invasive species. “There was a strong backlash from the environmental community, so nothing ever came of it,” says Sokolow.
To avoid a similar backlash, her team—known as The Upstream Alliance—used M.vollenhovenii, an indigenous West African species, rather than the foreign crayfish. Working with parasitologists Armand Kuris and Kevin Lafferty, Sokolow ran several lab tests to confirm that the prawn would actually eat the infected snails. (It did, and voraciously so.)
Buoyed by that success, the team staged a pilot experiment in June 2011. At a village called Lampsar, they netted off a part of the river that villagers frequent, and stocked it with prawns. At a nearby village, slightly upstream but otherwise as similar to Lampsar as possible, they did nothing.
Before the prawns arrived, the Lampsar villagers were five times more likely to carry schistosomiasis parasites than their upstream peers. After they added the prawns, Sokolow’s team treated anyone who was infected with drugs. Eighteen months later, they checked for re-infections. This time, they found the opposite ratio: the upstream village had four times as much schistosomiasis as Lampsar, whose waters contained half as many snails and a fifth as many parasite-shedding ones. The village, which had been written about since World War II as a hotspot for schistosomiasis, had become almost free of disease.
In other words: the prawns had worked.
Sokolow would be the first to admit that with just two villages, it is impossible to draw any broader conclusions about how effective the prawns might be. “We know that it was just a demonstration,” she says. The promising results are consistent with ecological theory, the lab experiments, and the Kenyan trial, but “we need to do a wide-scale replicated study and really nail the proof of concept.”
Sokolow’s colleague Giulio de Leo created a mathematical model to predict what would happen. He found that at a certain density—0.3 prawns per square metre—the prawns will completely eliminate schistosomiasis by eating any newly infected snails before they can release their own parasites. This will take at least 20 years, but not if infected villagers are also treated with the drug praziquantel. Then, schistosomiasis ought to decline and disappear within just five years.
De Leo says that this combined approach has many advantages over praziquantel alone, effective though the latter is. “After praziquantel administration, villagers in rural areas of Senegal have no other option than go back to river and step into schistosome-contaminated waters for their daily chores, thus getting re-infected over and over again,” says de Leo. In other words, the villagers might briefly shake off the disease, but the river will never let them forget it.
By contrast, the prawns should prevent infections in the first place, by decimating the snails that harbour and transmit the parasites. Others have tried to do this by attacking the snails with toxic “molluscicides”. Like praziquantel, that was just a temporary measure, and one heavy in collateral damage: the chemicals killed off many fish and crustaceans, too. “Re-introduction of native prawns may offer an ecologically friendly and much more lasting solution to snail control,” says de Leo. “We believe that, when coupled with praziquantel administration, it may be the game changer in the fight against schistosomiasis.”
“We have too long been enamored of the idea that pills alone could solve the problem,” says Eric Loker from the University of New Mexico, who led the Kenyan study in 1999. “I commend the authors for putting a needed spotlight back on what happens in the water. The snails are abundant, often resilient, and impart stability to the transmission cycle. We don’t have a bed-net like option for snail control like we do for the mosquitoes that transmit malaria.”
But “a dose of reality is also in order,” he says. The snails often live in “complex, heavily vegetated, small bodies of water”, many of which were created by the construction of the Diama dam. Whether the prawns can even reach their pre-dam levels, let alone penetrate these new habitats, is unclear.
Meanwhile, the dam still prevents the prawns from travelling to and from their breeding grounds. If the team wants a new population to establish itself in the river, they’ll have to create some kind of bypass around the dam—a “prawn passage”—that will allow the animals to traverse their old migration routes.
Even that might not be enough. De Leo’s model shows that to eradicate schistosomiasis entirely, the team will need densities of prawns 2.5 times higher than what they actually managed in their small pilot study. That’s not unachievable, but it might be higher than what artificial prawn corridors can maintain. So, in areas where schistosomiasis is especially common, the team might have to regularly supplement the waters with prawns raised in aquaculture.
This isn’t a problem, though. It might even be a good thing, because the prawns have two important benefits beyond their hunger for snails: they are tasty, and valuable. They can sell for three to five times the price of local fish. The Upstream Alliance team believes villagers should be able to rear them in small aquaculture facilities to get both food and money. This idea is especially feasible because it’s the small, fast-growing prawns that kill the most snails, leaving villagers to harvest the larger and more valuable individuals with impunity. (It’s also okay to eat prawns that have dined on infected snails because their digestive systems kill the parasites.)
These economic benefits are crucial. Schistosomiasis affects more than 260 million people around the world, including some 114 million children. These huge numbers make it unfeasibly expensive to treat people with even a really cheap drug. But if countermeasures can actually return money to a community, they stand an even greater chance of succeeding. It’s a rare win-win-win scenario, where conserving a displaced animal can benefit human health and alleviate poverty. “We’re now working with geographers, ecologists, economists, and government ministries to find out how we can optimise a sustainable, long-term strategy,” says Sokolow.
Reference: Sokolow, Huttinger, Jouanard, Hsieh, Lafferty, Kuris, Riveau, Senghor, Thiam, N’Diaye, Faye & de Leo. 2015. Reduced transmission of human schistosomiasis after restoration of a native river prawn that preys on the snail intermediate host. PNAS http://dx.doi.org/10.1073/pnas.1502651112