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Controlling Deadly Parasite Takes Coordinated Effort

Schistosomiasis is a parasitic disease endemic to many parts of the world. People infected with the parasite can suffer long-term liver and neurological damage and can die without treatment.

Like all parasitic diseases, schistosomiasis has a complicated life cycle. Infected humans excrete the eggs in their feces. When snails exposed to the waste become infected, the parasite develops inside them. The snails release a free-swimming form of schistosoma into water, where humans are infected when they wade or bathe. The adult schistosoma lays eggs inside its human host and the cycle continues.

Robert Spear, a professor of environmental health at the University of California at Berkeley says there are many ways to interrupt this complicated life cycle. One way, for example, would be to keep people out of the water, but that's not always possible.

Spear says improving sanitation practices help, as does controlling snail habitat. "In some places, you can change crop types from crops that are irrigated and wet all year round to tree crops," he says. "[Tree crops] have a dry part of the season, and you can use this chemical called niclosimide to kill the snails as you do with other pesticides."

One cost effective method includes installing biogas digesters in individual houses. "You collect both the pig waste and the human waste, [and] treat it in a small digester system that's built underneath the house," Spear explains. "You can still use the residual material for fertilization, but the parasite eggs are inactivated." The process generates methane for cooking, he says, making it a "win-win situation."

But Spear says none of these methods works in isolation. If one village eradicates the parasites, but the neighboring one does not, then schistosoma will invariably re-infect the cleaned village.

So, Spear worked with colleagues in Chengdu, China, to come up with a suite of methods that can be applied to control the parasite on a regional basis. "That's a new thought in China, because their entire strategy is village focused."

He says they used mathematical models to forecast disease transmission among villages in a region and along rivers and drainage canals, and plan coordinated eradication strategies.

Spear's results and methods are further detailed in a paper published in the recent issue of the Proceedings of the National Academy of Sciences.