Swimmers itch is caused by the larval stage of a flatworm and not a bacteria. The larval stage that infects human, the cercariae, is the stage being released by snails (not by birds and mammals). Although ducks are a preferred host for the cercarial larval stage, so are gulls, muskrats, and beavers. These aquatic mammals and birds serves as the preferred host for the cercarial larval stage and are necessary to complete the life cycle.

C. William Kilpatrick
Howard Professor of Zoology
and Natural History