Macrobrachium vollenhovenii is a big name for a little creature, a freshwater prawn found throughout West Africa. Despite its size, the shrimp can have a big impact and could help control a parasitic disease.
In 1986, Senegal finished building the Diama Dam on the Senegal River, creating a reservoir of fresh water for nearby cities and villages and the surrounding farmland. People living near the dam and the river didn’t have much reason to celebrate, though. Soon after the dam went up, a disease called schistosomiasis swept through villages along the river and its tributaries. The epidemic, says disease ecologist Susanne Sokolow, was “unprecedented, massive, and persistent.”
Schistosomiasis is caused by parasitic worms that move between human and snail hosts via fresh water. Their eggs are released when an infected person uses a water source like a river or pond as a toilet. The eggs hatch, and the larvae infect snails, living inside them for a while before emerging and seeking out human hosts. When people use the contaminated water for swimming, bathing, or even doing laundry, the worms penetrate their skin, enter the blood vessels, and begin producing new eggs that will then be released and start the cycle all over again.
Not all of the eggs make their way out of their human hosts, though. Sometimes they become lodged in a blood vessel or an organ, causing inflammation and complications that can lead to liver failure, cognitive impairment, stunted growth, and bladder cancer. The disease is awful, and it isn’t easy to shake. Neither the infection nor the drugs that clear it out create a long-lasting immune response in the body, so reinfection is common and treatment is only a temporary fix.
The epidemic that spread along the Senegal River was a horrific and unintended consequence of the new dam. The project created a perfect home for the worm-carrying snails by keeping them safe from their main predator, the prawn. The dam was a road block on the prawns’ migration route up and down the river and kept them from the waters upstream from it.
Now, Sokolow and her research team have shown that throwing the prawns back into the mix could make the river less welcoming to the snails and help control schistosomiasis. At the village of Lampsar, the scientists blocked off a section of the river with netting and stocked it with prawns. Then they distributed schistosomiasis treatments to the villagers there and at another, prawn-less village nearby. For the next 18 months, they kept track of the snails and schistosomiasis infections in snails and people at both villages.
At the end of the study, the prawn zone at Lampsar had half as many snails—and around 80 percent fewer worm-infected snails—as the water at the other village. Schistosomiasis infections in Lampsar’s villagers also dropped after the prawns were brought in and the disease became less prevalent there than in the other village, even though Lampsar had more infections than its neighbor before the study started.
The prawn is a double whammy against the parasites because it both preys on the larval worms when it eats infected snails and takes away potential hosts when it eats snails that haven’t been infected yet. Making them at home again in the upper stretches of the river and restoring their populations there will require working around the dam, though, which blocks the route they use to travel back and forth from their breeding sites in the river’s estuary. One idea the scientists suggest is a “prawn ladder” built into the dam that would allow them to get past it and move about the river as they please.
To supplement that and reach the population levels needed to control the snails and schistosomiasis, they also suggest starting shrimp farming operations in the riverside villages. Small, fast-growing prawns tend to eat the most snails, so they could be left to do their business while the largest prawns—which don’t hunt as many snails and often cannibalize other prawns—could be caught and eaten or sold.
“Biological conservation does not always benefit human health, but where it does, it provides a win-win outcome for humans and nature,” the researchers say. “Add the economic benefit of aquaculture to the equation, and river prawn restoration might become a win-win-win-win: for disease control, biodiversity restoration, poverty alleviation, and improved nutrition.”