Climate change is a major factor affecting trematode communities. Reproduction rates of trematodes within their snail hosts increases substantially with temperature. Because there are many more cercariae produced in warming climates, it is assumed that infection will increase where climate change induces warming trends. Climate change is expected to have different effects at high elevation sites versus lower elevation areas. One example of this ``elevational mismatch'' involves waterfowl migrations. When temperatures become warm enough at lower elevations waterfowl will begin to migrate. However, in much of the central Rockies climate change is expected to bring heavier snow that lingers longer into the spring. As the waterfowl fly over the Rockies, forage areas that used to be snow-free during their migration are now still snow covered due to a combination of earlier migration times and longer snow coverage. As a result, these waterfowl are forced to fly over their old foraging grounds to reach open waters. These waterfowl are the definitive hosts of many trematode species. If these birds are forced to bypass these high elevation ponds, they will not disburse eggs into the water, and trematode numbers will be much lower and trematode communities will be very different. Altering trematode populations and communities can have large effects on ecosystem structure and function. I have been following these interannual fluctuations at Crested Butte in the central Rockies of Colorado. I have been paying particularly close attention to the fluctuations in populations of avian schistosomes. In this are, avian schistosomes have been causing cercarial dermatitis (a painful rash) since at least 1957.

Cercarial dermatitis (commonly referred to in the US as `swimmer's itch') is an intense rash that forms in response to trematode parasites (schistosomes) penetrating the human skin. These parasites (1)mature within aquatic snails, (2)are shed into the water column, and (3)burrow into the host's skin. This burrowing induces a severe rash referred to as cercarial dermatitis. While most schistosome species are confined to tropical areas, avian schistosomes, (those that infect birds as their terminal hosts), are distributed globally. Avian schistosomes cannot distinguish between a wading bird and a wading human and therefore routinely burrow into human skin, thereby inducing the rash known as cercarial dermatitis. It has been assumed that once avian schistosomes penetrate the skin of an incompatible host (i.e. a human), they die within the skin, but avian schistosomes are indeed capable of penetrating mammalian skin and migrating to the lungs, causing pulmonary haemorrhages in rabbits, guinea-pigs, hamsters, and monkeys or partial paralysis in mice. Climate change has been suspected of driving a world-wide intensification of cercarial dermatitis transmission in tropical and temperate areas.