Background: In order to facilitate the setting of guidelines, this review article evaluates the health risks caused by poor microbiological quality of recreational natural water.
Methods: Studies on uncontrolled waters, such as sea, lakes and rivers were considered in this review through MEDLINE and WHO resources. Out of the 37 studies identified, 22 were reviewed because they addressed associations of interest and fulfilled the validity criteria.
Results: Most studies reported a dose-related increase of health risk in swimmers with an increase in the indicator-bacteria count in recreational waters. Relative risk (RR) values for swimming in polluted water versus clean water were often significant (usually 1.0 < RR < 3.0). The indicator microorganisms that correlate best with health outcomes were enterococci/faecal streptococci for both marine and freshwater, and Escherichia coli for freshwater. In both marine and freshwater, increased risk of gastro-intestinal symptoms was reported for water quality values ranging from only a few indicator counts/100 ml to about 30 indicator counts/100 ml. These values are low compared with the water qualities frequently encountered in coastal recreational waters. Studies which showed a higher threshold for increased risk and case-rate values in some countries may suggest immunity due to endemicity or a lower pathogen-to-indicator ratio in the natural waters.
Conclusions: The review strongly suggests a causal dose-related relationship between gastrointestinal symptoms and recreational water quality measured by bacterial indicator counts.