Monterey-based researchers use nature to fight skin-burrowing parasites.

Sitting at her desk in Pacific Grove, rocking her new baby back and forth with one arm, Susanne Sokolow explains how her current project is a dream come true. As a research biologist at Stanford University’s Hopkins Marine Station, Sokolow is striving to reduce and eliminate a deadly human disease using ecology.

Schistosomiasis is a sometimes fatal illness that plagues about 200 million people worldwide. It is caused by a parasitic worm, which reproduces in the blood stream and damages the kidneys, bladder, intestines and liver. The drug Praziquantel was brought to market 30 years ago to combat schistosomiasis, but there was a problem: The people it cured were getting reinfected.

“The drug kills 99 percent of the worms in your body,” Sokolow explains, “but the schistosomiasis burden throughout the world remains high.”

“But there was another, missing actor on the stage,” adds Stanford Biology Professor Giulio De Leo, who is working on the issue with Sokolow and almost 30 other scientists around the globe. He’s Skyped into the conversation from his native Italy. At 1am his time, his eyelids are heavy, and fly-away hair forms a halo around his head.

The worms that cause schistosomiasis must live in more than one kind of host in order to survive. These worms reproduce sexually within their human hosts, but once the infected person excretes their eggs, the newborn worms must find and invade freshwater ramshorn snails native to Africa and the Middle East. There, they reproduce asexually; a single larva can produce hundreds of copies of itself.

“THE SNAIL IS A LITTLE FACTORY FOR PARASITES.”

“The snail is essentially a little factory for parasites,” says Sokolow.

The snail then releases these hundreds of larvae into a river, where they again search for a human host and enter the blood by burrowing into the skin.

And that’s why Praziquantel wasn’t keeping the infections down. People could be cured, but they’d be reinfected unless the snails were treated, too.

So Sokolow and her colleagues introduced a third player. The native river prawns Macrobrachium vollenhoveni were the natural predator of the snails before dams throughout East and South Africa cut off migration routes. When these dams were built in the 1980s, cases of schistosomiasis spiked.

“Our group is working on coupling a medical intervention with an environmental intervention,” Sokolow says.

Sokolow, in concert with De Leo, Kuris and other researchers, stocked prawns at a field site in Senegal, and observed a subsequent decline in the local parasite burden.

“The prawns were close to extinct in this part of Senegal,” Sokolow recalls, “so we had to fly them in from Cameroon. They were carried in boxes on people’s arms and on their heads, and then they took a motorcycle, which took them to a bus, which took them to the airport.”

According to Sokolow’s study, published this month in the Proceedings of the National Academy of Sciences, the prawns decreased the number of infected snails by 80 percent. They also provided an added benefit.

“The prawns are delicious,” De Leo says, “and they used to be part of the diets of the local fishermen. What became clear is this restoration of the prawns might be used to facilitate the development of small-scale agriculture.”

De Leo developed mathematical models that set a sustainable prawn harvest limit. So now, the prawns eat the snails, keeping schistosomiasis in check, and the people eat the prawns, which don’t transmit the disease. The concept has been proven, at least in one small Senegalese community. But Sokolow and De Leo agree that there’s still a lot of work to be done.

“It’s all fine and good to say that this is an innovative way in which we might intervene, and that it might be sustainable. But it’s a much bigger problem to figure out how to do it,” says Sokolow.

To that end, Sokolow and De Leo have coalesced a group of scientists and medical specialists to form the Upstream Alliance, which is planning to tackle schistosomiasis on a larger scale. Stanford is opening a new Center for Disease Ecology, Health and Development to explore how similar ecological principles might be used to reduce other kinds of disease. And Sokolow and De Leo will continue networking, searching for funding, and figuring out how to maximize both the public health and economic benefits of prawn reintroductions in West Africa.

“It’s a life-long pursuit,” says Sokolow, boosting her fussy baby boy up on her hip. I ask her for her favorite freshwater prawn recipe. “Their flavor is delicate and sweet, and lobster-like,” she says. “I’ve just done them with butter and herbs, and it was divine.”

Learn more at www.theupstreamalliance.org