Drinking Water Quality and Health Care Utilization for Gastrointestinal Illness in Greater Vancouver

J. Aramini¹, M. McLean², J. Wilson¹, J. Holt³, R. Copes⁴, B. Allen³, W. Sears⁵

Executive Summary

The risk of microbial disease associated with drinking water is presently a priority concern among North American water jurisdictions. Numerous past outbreaks, together with recent studies suggesting that drinking water may be a substantial contributor to endemic (non-outbreak related) gastroenteritis, demonstrate the vulnerability of many North American cities to waterborne diseases and have fuelled ongoing debates in Canada and the United States concerning the need for stricter water quality guidelines, changes in watershed management policies, and the need for additional water treatment. The Greater Vancouver Regional District (GVRD) water supply system serves approximately two million consumers from a system consisting of three unfiltered surface water supplies. Although strict GVRD policies minimize the potential for human fecal contamination of the source water supplies, the GVRD watersheds support many wildlife species that can potentially shed organisms pathogenic to humans. Because Greater Vancouver's water treatment strategy relies principally on watershed protection and chlorination^b, and these two strategies together do not eliminate all risk of waterborne disease transmission, it is possible that some disease-causing organisms reach the consumer.

In this study, a Generalized Additive Modeling (GAM) approach was used to investigate and quantify the associations between gastrointestinal-related health outcomes in Greater Vancouver on a specific day (as assessed by hospital admissions, physician visits, and visits to B.C. Children's Hospital emergency room) and GVRD water quality parameters (primarily turbidity) 1 to 39 days earlier, from 1992 to 1998. To accomplish this, other significant and confounding variables were controlled, including seasonal and long-term effects, and day-of-the-week effects. In this study, both seasonal and long-term trends and turbidity were modeled nonparametrically, using regression smoothers. Two different, though related, modeling approaches were used in this study: a Poisson regression approach and a Binomial (case-control) regression approach. For the Poisson modeling approach, the relationship between the daily number of health outcomes as measured by hospital admissions and emergency care visits, and GVRD water quality was assessed. For the Binomial (case-control) modeling approach, the relationship between case status (gastrointestinal vs. respiratory conditions) and GVRD water quality was investigated for each health outcome group.

Evidence supporting a turbidity-gastroenteritis relationship was identified. Statistically significant turbidity-gastroenteritis relationships were found among multiple age groups, among all three administrative health care data sets, and among the three water source distribution populations. Furthermore, consistencies among the associations were observed between the two modeling approaches (Poisson and Binomial regression). In general, the probability of gastrointestinal disease (as assessed by relative rates and odds ratios) increased as turbidity increased, and among several health outcome groups, relative rates/odds ratios reached values of greater than two. Among the four age groups evaluated, the apparent turbidity-gastroenteritis relationships were strongest among 2-18-year-olds and 18-65-year-olds, and four prominent lag-times (the period from the turbidity event to the time of the measured health outcome) were observed: 3-6 days, 6-9 days, 12-16, and 21-29 days. These lag-times are consistent with the incubation periods of common waterborne bacterial and protozoal gastroenteritis-causing organisms, or multiples of these incubation periods, and support the findings of related endemic and epidemic waterborne disease research. Additional research focussing on pathogen-specific outcomes is needed to confirm the etiological nature of the observed associations.

Given the potential public health implications of the relationships identified, attempts were made to estimate the relative and absolute gastrointestinal health impacts attributable to Greater Vancouver drinking water. To favor conservative estimates (underestimates), it was assumed that no gastrointestinal events resulted from water with a turbidity of less than or equal to 1 nephelometric turbidity unit (NTU). Comparing estimated relative health impacts among the three water supplies, over the entire study period, variations in GVRD drinking water quality explained 2.1%, 0.8%, and 0.9% of emergency-associated, gastroenteritis-related physician visits and 1.3%, 0.2%, and 0.3% of gastroenteritis-related hospitalizations among individuals supplied by the Capilano, Seymour, and Coquitlam water sources respectively. Extrapolating from the above relative impact figures, over a 6-year period, variations in GVRD drinking water quality explained approximately 17,500 physician visits (1.6% of all gastroenteritis-related physician visits), 85 hospital admissions (0.6% of all gastroenteritis-related hospital admissions) and 138 pediatric hospital emergency room visits (1.6% of all gastroenteritis-related pediatric hospital emergency room visits). Given inherent limitations in both the data sources and the available analytic methodologies, the estimates provided above must be interpreted cautiously. The true health impact estimates may have been substantially less, or greater than those calculated. Efforts continue to assess the variability and to refine the above estimates.

The results of this study are consistent with the findings of a number of epidemiological and microbiological studies carried out across North America and support several public health and water supply management beliefs: (1) significant levels of endemic (day-to-day) gastroenteritis events are potentially waterborne in nature, (2) watershed protection together with chlorination may not adequately protect against the waterborne transmission of enteric pathogens, and (3) turbidity appears to be a valuable water quality indicator. Although additional research is warranted to more accurately estimate the health impacts associated with Greater Vancouver drinking water, to validate the associations observed, and to clarify the roles of specific waterborne pathogens, this study provides evidence to support the hypothesis that during the study period, enteric waterborne pathogens present in each of Greater Vancouver's three drinking water supplies contributed to endemic gastroenteritis among Greater Vancouver residents.

b In July 2000, a new ozone primary disinfection facility began treating Coquitlam source water.