Shigellosis from swimming in a park pond in Michigan.

In July 1989 an outbreak of shigellosis occurred among visitors to a recreational park in Oakland County, MI. An epidemiologic investigation discovered an association between illness and swimming in a pond at the park, especially for those who had put their head underwater. No other factors were epidemiologically incriminated. A total of 65 cases were identified; nine were culture confirmed, all Shigella sonnei. Several water samples evaluated for fecal coliform counts shortly after the outbreak were found satisfactory. Cultures of water samples were negative for Shigella species. Inspection of the park's sewage disposal and toilet facilities found all equipment in proper working condition and no evidence of a sewage contamination event from these potential sources. No other commercial or residential sources of potential sewage contamination existed near the pond. Investigators concluded that Shigella contamination of the pond by a swimmer or swimmers on one or more occasions was a strong possibility. Factors supporting this conclusion included elevated incidence of S. sonnei in the community during the 2 months prior to the outbreak, greater use of the pond, warm water and air temperatures, and inadequate water exchange in the pond. This report adds one of the few documented outbreaks of shigellosis implicating bather contamination to the literature on the growing number of incidents that have been associated with recreational use of water.