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; 12) failure to relate indicator or ganism density directly to the individual bather; and (3) failure to r igorously control for non-water-related risk factors on previously rep orted associations between bathing in marine waters and illness among such bathers. We report the results of two intervention follow-up stud ies specifically designed to address these methodological weaknesses. We restricted study outcome to bathing-associated gastroenteritis sinc e 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 re sults show that faecal streptococci was the only indicator organism to predict the occurrence of gastroenteritis among bathers, and this occ urred at only one of the three water quality sampling depths used in o ur study. The consumption of three different foods known or suspected to act as vectors in the transmission of gastroenteritis, as well as o ne non-food, non-water-related risk factor for gastroenteritis were fo und to significantly increase the risk of gastroenteritis among bather s. Multiple logistic regression modelling showed that these non-water- related risk factors confounded the relationship between exposure to m arine waters of varying faecal streptococci densities and the occurren ce of gastroenteritis among bathers to a moderate degree. Moreover, th ese analyses showed that the risk of gastroenteritis to the individual bather caused by these non-water-related risk factors, approximated t he risk of gastroenteritis among bathers exposed to waters containing relatively high faecal streptococci densities. The implications of the se findings with regard to the validity of present marine water qualit y criteria and on the need for, and design of, future epidemiological studies of bathing water associated illness are discussed.