Observational epidemiologic studies of endemic waterborne risks: cohort, case-control, time-series, and ecologic studies.

Observational studies have assessed endemic waterborne risks in a number of countries. Time-series analyses associated increased water turbidity with increased gastroenteritis risks in several public water systems. Several cohort studies reported an increased risk of gastroenteritis in populations using certain public or individual water systems. Although several case-control studies found increased waterborne risks, they also found increased risks associated with other exposures. An increased risk of campylobacteriosis was associated with drinking untreated water from non-urban areas and some tap waters; other significant risks included contaminated poultry and foreign travel. Increased risks of cryptosporidiosis and giardiasis were associated with drinking water in some populations; other risk factors included foreign travel, day care exposures, and swimming. These observational studies provide evidence that some populations may be at an increased risk of endemic or sporadic illness from waterborne exposures, but not all studies found an increased risk. Differences in waterborne risks may be due to differences in water quality. System vulnerabilities and contamination likely differed in the areas that were studied. The information from these studies may help inform estimates of waterborne illness for the US population but is inadequate to estimate a population attributable risk.