Water samples from three pristine rivers in the Pacific Northwest were analyzed for Giardiu cyst presence and concentration using a membrane-filtration-immunofluorescence-assay procedure. A total of 222 samples were collected either monthly or bimonthly over a ninemonth period from 17 sampling stations on three rivers and 12 tributaries. Cyst recovery efficiency was monitored using samples seeded with cysts at levels ranging from 0.5 to 50 cysts/L. The recovery efficiency of the procedure averaged 21.8 percent + 6 percent through 26 sets of samples. Gimfiu cysts were found in 94 (43 percent) of the samples. The corresponding cyst concentrations calculated from the recovery efficiency and the sample volume ranged from 0.1 to 5.2 cysts/L. The distribution of cyst concentrations in positive samples was lognormal. Both the magnitude of cyst concentrations (as indicated by the mean value) and the variability (as indicated by the slope of the distributions) differed among the three rivers. No statistically supportable seasonal variations were found. The principal conclusion was that Giurdiu cysts appear to be continuously present, though at low concentrations, even in relatively pristine rivers. The presence of Giardia lamblia cysts in sources of public water supply has been documented in many areas of the United States and Canada.’ It has been described as one of the most common etiological agents contributing to outbreaks of waterborne gastroenteritis.2 Control of Giardia cysts in water treatment has been challenging because their removal by filtration requires consistently efficient operation and specialized analytical techniques,” and because they are relatively resistant to conventional disinfectants.’ Knowledge of Giardia cyst concentrations in water is relevant to public health protection for several reasons, yet no data on cyst concentrations in water have been reported. First, the risk of infection through consuming cystcontaminated water must be directly proportional to cyst concentration, assuming uniform cyst viability and ‘infectivity characteristics. Second, the concentration of cysts in untreated sources of water supply undoubtedly depends on the nature and magnitude of sources (and sinks) throughout the watershed. Examination of surface streams in remote areas has shown that the frequency of cyst discovery is directly related to the intensity of human activity in the watershed.: High levels of Giardia
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