Health effects of marine bathing in New Zealand

Prospective epidemiological studies on the possible health effects from sea bathing were carried out at seven popular New Zealand bathing beaches over the 1995 summer. The association of gastrointestinal/respiratory symptoms or other infections with human or animal faecal contamination of the beach water was examined. Beach users were interviewed at the beach and then followed up within 5 days to ascertain any illness symptoms. On each of the 107 interview days multiple samples of the beach water were examined for three faecal indicators (faecal coliforms, E. coli, enterococci). Of the usable responses, 2307 users did not enter the water; 1577 did. Log-linear modelling showed that enterococci was most strongly and consistently associated with illness risk for the exposed groups, particularly for respiratory illness among paddlers and long-duration swimmers. Crude risk differences for these two groups were 7 and 33 per 1000 individuals, rising to 62 and 87 per 1000 individuals for the highest enterococci quartile. No substantial differences in illness risks were found between the human and animal waste impacted beaches, though both were markedly different from the control beaches. The results are being used to develop recommendations for sampling design and use of single-sample maxima in new bathing-water guidelines.