Water and non-water-related risk factors for gastroenteritis among bathers exposed to sewage-contaminated marine waters.

All previously published epidemiological studies of the health effects of bathing in marine waters contaminated with domestic sewage contain three major methodological weaknesses in study design: (1) failure to control for the substantial amount of temporal and spatial variation in indicator organism densities shown to occur within just a few hours at marine water bathing locations; (2) failure to relate indicator organism density directly to the individual bather; and (3) failure to rigorously control for non-water-related risk factors on previously reported associations between bathing in marine waters and illness among such bathers. We report the results of two intervention follow-up studies specifically designed to address these methodological weaknesses. We restricted study outcome to bathing-associated gastroenteritis since this is the illness most consistently reported to be associated with bathing in marine waters, and upon which both current US Marine Water Quality Criteria and other standards used worldwide are based. Our results show that faecal streptococci was the only indicator organism to predict the occurrence of gastroenteritis among bathers, and this occurred at only one of the three water quality sampling depths used in our study. The consumption of three different foods known or suspected to act as vectors in the transmission of gastroenteritis, as well as one non-food, non-water-related risk factor for gastroenteritis were found to significantly increase the risk of gastroenteritis among bathers. Multiple logistic regression modelling showed that these non-water-related risk factors confounded the relationship between exposure to marine waters of varying faecal streptococci densities and the occurrence of gastroenteritis among bathers to a moderate degree. Moreover, these analyses showed that the risk of gastroenteritis to the individual bather caused by these non-water-related risk factors, approximated the risk of gastroenteritis among bathers exposed to waters containing relatively high faecal streptococci densities. The implications of these findings with regard to the validity of present marine water quality criteria and on the need for, and design of, future epidemiological studies of bathing water associated illness are discussed.