The relation between human presence and occurrence of Giardia Cysts in streams in the Sierra Nevada, California

A portable apparatus was used to filter large quantities of water from streams in California's Sierra Nevada. Samples were processed and examined for Giardia spp. Cysts of Giardia were detected in 22 of 49 (44.9 percent) samples collected at sites downstream from areas of high recreational use and in 5 of 29 (17.2 percent) samples collected at sites downstream from areas of low recreational use. These data indicate that intensity of human use may play a significant role or be a useful indicator in Giardia contamination of surface water.