Use of indicator microbes to assess water quality

Temporal and spatial indicator variation is substantial and difficult to relate to individual bathers (Fleisher, 1990) unless study design is experimental (Kay et al., 1994; Fleisher et al., 1996); limited precision of methods for counting indicator organisms, causing measurement error (Fleisher, 1990; Fleisher et al., 1993); bacterial indicators may not be representative of viruses, which may be important etiological agents

Use of seasonal means to assess water quality

Some studies use seasonal means and not daily measurements of indicator organisms to characterize individual exposure, thus adding substantial inaccuracy

Assessment of exposure pathway

Certain studies do not account for the potential infection pathway to definite exposure (e.g., mainly head immersion or ingestion of water for gastrointestinal symptoms).

Difficulties in exposure recall further increase inaccuracy of individual exposure

Non-control for confounders

Non-control for confounders (e.g., food and drink intake, age, sex, history of certain diseases, drug use, personal contact, additional bathing, sun, socioeconomic factors) may influence the observed association

Selection of unrepresentative study population

Results reported for certain study populations (e.g., limited age groups regions with certain endemicities) are a priori not directly transferable to populations with other characteristics

Self-reporting of symptoms

Most observational studies relied on self-reporting of symptoms by study populations. Validation of symptoms by medical examination (Fleisher et al., 1996; Kay et al., 1994) would reduce potential bias. External factors, such as media or publicity, may have influenced self-reporting

Response rate

Response rates were >70% in all, and >80% in most studies. Differential reporting (e.g., higher response among participants experiencing symptoms) would probably not have major consequences

Recruitment method

Recruitment methods were to approach persons on beaches in almost all observational studies and by advertisement for randomized controlled studies

Interviewer effect

Differences in methodology of data collection among interviewers may influence study results