I hate to show my ignorance here, but… Are there any or many North American fresh water gastropod or bivalve species that could potentially transmit disease to human by skin (hand) contact? I am asked this question all the time and am not confident in my stock answer, which is, “no but wash your hands anyway.” We are in north central Texas. Thanks.Here, for the general entertainment of our group, is my reply:
No, you're not "showing your ignorance!" That's such an interesting question, I'm going to answer it four times.
1) Nah, don't worry about it. I've hand-collected freshwater mollusks all over North America for 40 years, and never given it a second thought.
2) Well, actually, some very widespread freshwater mollusk species can thrive in polluted environments. I've seen strikingly high densities of Physa acuta, Lymnaea humilis, and the pisidiid clam Musculium transversum downstream from sewage treatment plants. (Unionids, not so much!) It might be a good idea to wash your hands after pulling anything from such waters - rocks, bottles, beer cans, snails or clams included. There was a little scare in the Myrtle Beach area a summer ago when state wildlife officials advised residents of a trailer park not to handle Pomacea insularum introduced into a nasty drainage pond in their neighborhood. See my blog post of 14Aug08* to read more.
3) Several years ago I visited Northern Michigan to sample Physa parkeri** from the lovely waters of Douglas Lake, at the University of Michigan Biological Station. I made my collection in a one-gallon thermos jug, planning to transport the sample back to Charleston alive. The next morning I was disappointed (but perhaps not terribly surprised) to discover that quite a few individuals had expired. So I reached into the jug (repeatedly) to remove the dead individuals for preservation, leaving the live ones undisturbed. Here's the case of schistosome dermatitis ("swimmers itch") that resulted.
4) Nah, don't worry about it. I never do.
*Two dispatches from the Pomacea front [14Aug08]
**Physa parkeri turned out to be an ecophenotypic variant of Physa gyrina. See Dillon & Wethington (2006) The Michigan Physidae revisited: A population genetic study. Malacologia 48: 133-142. [PDF]