Watershed use and Giardia cyst presence

The presence of Giardia in a heavily visited watershed in the Olympic Mountains was compared to that in an adjacent watershed having much lower human use. Also, statistical relationships between cyst presence and watershed use parameters were examined as indicators of potential causal relationships. The number of Giardia cysts found in water samples ranged from 0.2/100 l to 3/100 l and were highest in areas of high human use. A significantly higher prevalence of Giardia in selected animal species was observed in the heavily visited watershed. Statistically significant relationships were found between aqueous Giardia cyst concentrations and both the prevalence of Giardia in animals and the intensity of human use. Based on the water samples analyzed, a calculated median cyst concentration of 1 cyst per 20 l (0.05 cysts/l) can be expected in relatively pristine rivers.