In July 2019, the California Department of Public Health investigated an outbreak of Shigella sonnei infections among persons who reported playing in the Santa Ana River, a waterway spanning 100 miles through southern California. A total of 24 cases with closely related isolates (within 0–2 alleles by core-genome multilocus sequence typing) of Shigella sonnei. Among 19 ill persons for whom epidemiologic data were available, 16 reported that during July 6–August 5 they played in a swim area in a shallow portion of the river where water quality was not regularly monitored. Two of the 16 ill persons also reported swallowing river water. No other common risk factors were identified. Two of 15 ill persons for whom clinical data were available were hospitalized; none died. Date of symptom onset ranged from July 6 through August 7.

In response to the outbreak, local public health officials closed public access to the swim area during August 8–15. Surface water samples were collected upstream, downstream, and at the swim area and tested for E. coli, a bacterial indicator of fecal contamination. The concentration of E. coli ranged from 350 through 1,600 most probable number/100 mL at these sites. Investigation into possible sources of fecal contamination upstream and at the swim area did not definitively identify an outbreak source.