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ROBERT SIEGEL, HOST:

It's ALL THINGS CONSIDERED from NPR News. I'm Robert Siegel.

MELISSA BLOCK, HOST:

And I'm Melissa Block. Now we're going to hear how a rise in a debilitating disease is being fueled in part by people trying to improve their lives.

In Central Africa, a boom in agriculture and fishing on the shores of Lake Malawi is changing the environment and in the process creating more favorable conditions for a parasitic worm. NPR's Jason Beaubien reports on research that drew the link.

JASON BEAUBIEN, BYLINE: Schistosomiasis usually won't kill you, but it can make you pretty sick. The intestinal disease is also known as bilharzia. It comes from flatworm parasites that burrow into people's skin, usually when they're bathing in warm, shallow water. In many lakes in Central Africa, including Lake Malawi, it's a big problem.

BERT VAN BOCXLAER: In some villages around Lake Malawi, up to around 70 percent of the people and 95 percent of schools' children are infected.

BEAUBIEN: Bert Van Bocxlaer is a postdoctoral researcher with the Smithsonian and Ghent University in Belgium. Most of his research is on the evolutionary patterns of snails. Van Bocxlaer was studying snails in Lake Malawi, including the snails that carry schistosomiasis, when he realized that the increase in disease was being driven by people. Over the last 30 years, the number of people living around the lake has more than doubled. Agricultural output has risen dramatically. But Van Bocxlaer found that many of the things that people have done to try to make their own lives better have also made things better for the parasites and worse for human health.

BOCXLAER: There has been overfishing of fish that usually eat these snails.

BEAUBIEN: Additional farming on the shores of the lake has pushed more sediment into the water, which has improved the habitat for the problematic snails.

BOCXLAER: The second aspect - there has been another snail that has been introduced from Asia that is occurring there now and that may be favored by the fish that, before, ate the intermediate host of this disease.

BEAUBIEN: The parasite cycles between the snails and people. In the water, the tiny worms penetrate people's skin. The parasites mate inside people's intestines. Human excrement then carries the parasite's eggs back into the lake, where they hatch and go on to live inside the snails.

Once they're large enough, they swim away from the snails and go out in search of another human host. This cycle has gone into overdrive recently, as the number of predatory fish has gone down and the number of people in the area has gone up.

BOCXLAER: People are doing more agriculture, more fishing because they need food, right? But on the other hand, intensified use of these natural resources really causes an effect in the aquatic ecosystem that increased the likelihood of transmission of this disease.

BEAUBIEN: Then Bocxlaer himself came down with schistosomiasis while doing this research, which has been just published in Trends In Parasitology. He says, this made him think even more deeply about the problem. He says, understanding the ways in which humans are helping to fuel the disease is an important step in figuring out, eventually, how to stop it. Jason Beaubien, NPR News.

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