Killer Snails vs. River Prawns

Restoring an ecosystem may protect millions of people from a deadly (yet neglected) tropical disease.

Photo: Upstream Alliance

Even with their creepy eyestalks and fang-studded tongues, snails hardly seem threatening—even to those with a strong aversion to slime. But in tropical locales, there is good reason to fear these slow-moving mollusks.

Within snails of the genus Biomphalaria and Bulinus lurk thousands of parasitic worms, each waiting for the right moment to burst forth to the nearest human. Once they find their quarry, they drill through the skin and make their way to the blood vessels of the intestines and bladder.

Abdominal pain, diarrhea, and bloody urine are often what’s in store for the unlucky person who plays host to this worm party. The resulting disease is called schistosomiasis. Left untreated, the parasites can bring on liver damage, kidney failure, infertility, and bladder cancer. Infection in women can lead to a heightened chance of contracting HIV. In kids, the little nasties can stunt growth and cause learning disabilities.

Schistosomiasis affects 220 million people worldwide, 114 million of them children. It’s present in 74 countries and second only to malaria in its prevalence. According to the Centers for Disease Control and Prevention, these parasites will kill around 280,000 people this year.

Photo: Upstream Alliance

Now, what if I told you we could fight this deadly scourge with a beady-eyed predator known as the prawn?

Think of prawns like the beefed up cousins of shrimp. These clawed crustaceans are generalist predators, meaning they’ll eat whatever they come across—and they positively love escargot.

Prawns are also native to many of the places stricken with schistosomiasis. But the thing is, as development projects have dammed up rivers across the developing world in recent decades to provide irrigation and agriculture, they have also disrupted the prawn’s life cycle and blocked them off from much of their habitat. And where the prawns disappear, snails go buck-nutty, and their accompanying worms, the ones that cause schistosomiasis, surge.

Now before you go all Hayduke and start screaming about dam demolition, here’s a gentle reminder that some dams do provide all kinds of crucial services for humans.

In Senegal, for instance, the Diama Dam prevents back-rushing seawater from poisoning water and irrigation wells. It also raises water levels for the Senegal River upstream, providing 3.5 million people in four countries across the Sahel Desert with much-needed freshwater supplies. The dam also helps stabilize communities (and their economies) that would otherwise have to deal with long cycles of flooding and extreme drought.

“I think the dam needs to stay,” says Susanne Sokolow, a research biologist at Stanford University and an author of a recent study on schistosomiasis. “It’s just that some of the unintended consequences of the dam are now wreaking havoc on the people there, and we need to find more clever solutions that balance those benefits with the drawbacks.”

OK, so how’s this for clever? Sokolow and a cross-disciplinary team of scientists want to reintroduce prawns to their native habitats in Senegal, where locals have dealt with what the researchers describe in the study as a “massive outbreak and persistent epidemic” of schistosomiasis since the construction of the Diama Dam in 1986. Prawn ladders will enable the crustaceans to bypass the dam and complete their natural life cycle, all the while gobbling up snails upstream and downstream year after year.

But that’s not all. Prawns are freaking delicious—just substitute “prawn” each time Bubba says “shrimp.” So Sokolow hopes that while staving off disease, the prawn resurgence could also help establish a local aquaculture market.

“There are wider socioeconomic variables in place here,” says Sokolow. “Some people may know they have schistosomiasis but not care as much as the fact that they can’t put food on the table for their kids.”

OK. So, for those of you keeping track, this means that the prawn solution could simultaneously:

  1. Partially restore the environment to pre-dam biodiversity
  2. Reduce potentially fatal schistosomiasis infections by naturally controlling the snail population and disrupting the parasite’s life cycle
  3. Create a protein-rich (and yummy) source of food
  4. Generate revenue for some of the poorest people on the planet

That’s a win-win-win-win, if it works.

And there’s already evidence that it might. In their study, which was published last week in the journal PNAS, Sokolow and her fellow scientists showed that when prawns were introduced into a small section of river upstream from the Diama Dam, the abundance of infected snails went down by 80 percent. Over a period of 18 months, the number of people afflicted with schistosomiasis also went down by 13 percent to 23 percent.

Photo: Upstream Alliance

Also hopeful is a mathematical model designed by Giulio Alessandro De Leo, Sokolow’s colleague at Stanford, which predicts that prawns coupled with anti-parasite medications (administered every once in a while) could eradicate the disease from high-transmission sites altogether.

We’ve focused on “preventive chemotherapy” or anti-worm drugs for the past two decades, says De Leo. But there’s promise in looking to the environment for help.

You see, you can’t just throw money or even vaccines at schistosomiasis to make it go away. Even if you were to eradicate the pest in 99 percent of the people in a village, all it would take is one little kid with active parasites in his body taking a whiz in the river and the whole cycle can begin again. (The eggs are present in both human urine and feces.) Therefore, a self-sustaining solution—with the added economic incentives to encourage local support—may well be the best possible way to combat this disease.

So, when does the prawn revolution begin?

First, the team wants to replicate its findings on a larger scale and get some serious numbers on what it would cost to construct a prawn ladder. And still, many unknowns lie ahead. Chief among them is whether a wild, native species like prawn can be farmed in a way that creates both biological protection and profit.

Research grants from the Gates Foundation, the National Science Foundation, and the National Institutes of Health are rolling in to help them figure it out. That’s good news, because Sokolow’s project, officially called the Upstream Alliance, could potentially boost the health and wealth for millions of infected, often hungry, people.

So, go tell all the snails and schistosomiasis worms out there that the prawns are coming. And hell’s coming with them.


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