Water recreation and illness severity.

The health endpoint of prior studies of water recreation has been the occurrence of gastrointestinal (GI) illness. This dichotomous measure fails to take into account the range of symptom severity among those with GI illness, and those who develop GI symptoms but who do not satisfy the definition of GI illness. Data from two US cohort studies were used to assess use of ordinal and semi-continuous measures of GI symptoms, such as duration of GI symptoms and responses to those symptoms such as medication use, interference with daily activities, and utilization of healthcare service. Zero-inflated negative binomial and logistic regression models were used to assess associations between severity and either the degree of water exposure or water quality. Among 37,404 water recreators without baseline GI symptoms, we observed individuals with relatively low severity satisfying the case definition of GI illness, while others with high severity not satisfying that definition. Severity metrics were associated with water exposure. The dichotomous GI illness outcome could be improved by considering symptom severity in future studies. Modeling ordinal and semi-continuous outcomes may improve our understanding of determinants of the burden of illness rather than simply the number of cases of illness attributable to environmental exposures.