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While the parasite hypothesis may conflict with conventional ideas about infectious disease and human health (malaria, for example, is caused by a parasite), the worms the scientists are investigating are not just any kind of parasite.

For one, these worms, known as trematodes, must sequentially infect certain hosts to complete their lifecycle. Snails to crabs to birds might be a typical sequence for one species, snails to fish to birds for another. These trematodes also stand apart from other parasites in that they cause negligible disease for their highest trophic level hosts, usually birds. The worms' lifecycle thus typically begins in a snail and ends in a bird, with the intermediate host animals being primary variables among worm species.

Intrigued by the prospects of developing a new tool for monitoring changes in wetland ecology, NOAA's California Sea Grant recently awarded support to parasite mavens Armand Kuris of the Department of Ecology, Evolution and Marine Biology at the University of California, Santa Barbara, and Kevin Lafferty of the Biological Resources Discipline at the U.S. Geological Survey to collect California horn snails from 30 coastal salt marshes between Marin County and Imperial Beach at the U.S.-Mexico border.

"The horn snail is a mobile data recorder," Lafferty said. "It is a hub for more than 20 trematode species." If any one requisite intermediate host is missing, the parasite cannot reproduce and hence will be underrepresented in the resident snail population, he explained.

A survey of the trematode population in resident snails thus becomes a clever means of reconstructing the food web in the area, as the trematodes reflect the predator-prey relationships that must be occurring to support their reproductive
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Source:Society for Academic Emergency Medicine