Original Article
Shigellosis Among Swimmers in a Freshwater Lake
Abstract
Objective: Shigella infection is highly communicable; however, outbreaks associated with swimming in recreational fresh water are rarely identified.
Materials and Methods: A cohort study of lake visitors was performed.
Results: Seventeen (24.6%) case patients among 69 persons who visited the lake over the holiday weekend were identified. Attack rates increased with increasing exposure to lake water; the risk of illness was greatest among swimmers who reported getting lake water in their mouths (relative risk = 5.37, 95% confidence interval = 2.2, 13.3). Shigella sonnei was isolated from stool samples of four of eight swimmers tested.
Conclusions: The outbreak likely was caused by fecal contamination of lake water by an infected swimmer; there was no evidence of sewage contamination into the lake. Fresh water is a potential source of infection in patients with acute gastroenteritis and recent exposure. Since testing and chlorination of lake water is impractical, prevention relies on avoidance of fecal contamination and/or minimizing ingestion of the water.
Key Points
* Lake water was the vehicle of transmission for Shigella sonnei.
* The most likely source of Shigella in the lake water was transient fecal contamination by an infected swimmer.
* Since there are no reliable methods for determining if fresh water is contaminated, prevention relies on avoidance of contamination and minimizing ingestion of lake water.
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