In August 1974, 31 of 45 cases of Shigella sonnei infection in Dubuque, Iowa, were traced to swimming in an 8-km stretch of the Mississippi River. Comparison of the first case in each affected family with neighborhood controls showed a significant correlation between swimming and illness (P <.0001). A significant association between diarrheal illness and swimming (P <.0001) was also demonstrated by a retrospective survey of 60 families who had camped at a park beside the river; the attack rate for swimmers who got water in their mouths while swimming was 18%. They had been swimming in water where the mean fecal coliform count was 17,500 organisms per 100 ml; the federal recommended upper limit for swimming water is 200 per 100 ml. A water sample obtained at the park swimming area one month after authorities had banned swimming in the area yielded S sonnei with the same antibiogram, colicin type, and phage type as the isolates from six swimmers.
This study demonstrates for the first time that shigellosis—a disease that can be caused by the ingestion of only 10 to 100 organisms—can be contracted by swimming in polluted water.
(JAMA 236:1849-1852, 1976)