Swimming-associated gastroenteritis and water quality.

A direct, linear relationship between swimming-associated gastrointestinal illness and the quality of the bathing water was obtained from a multi-year, multiple-location prospective epidemiologic-microbiologic research program conducted in New York City, 1973-1975, Lake Pontchartrain, Louisiana, 1977-1978, and Boston, Massachusetts, 1978. Several microbial indicators were used in attempting to define the quality of the water; and, of those examined, enterococci showed the best correlation to total and "highly credible" gastrointestinal symptoms. The frequency of gastrointestinal symptoms also had a high degree of association with distance from known sources of municipal wastewater. A striking feature of the relationship was the very low enterococcus and Escherichia coli densities in the water (10/100 ml) associated with appreciable attack rates (about 10/1000 persons) for "highly credible" gastrointestinal symptoms. Moreover, the ratio of the swimmer to nonswimmer symptom rates indicated that swimming in even marginally polluted marine bathing water is a significant route of transmission for the observed gastroenteritis.