Shigella Sonnei Outbreak Associated with a City-Run Recreational Water Source with Secondary Childcare Cases in 2012 in Clark County, Washington

BACKGROUND:  Shigella subspecies account for nearly 14,000 cases of acute gastroenteritis in the US usually associated with poor hygiene and contaminated food and water. Although usually self-limiting, secondary cases are common due to a low infective dose. In mid-July 2012, 3 laboratory-confirmed cases of Shigella sonnei were reported to Clark County Public Health (CCPH). All cases reported swimming in Klineline Pond, a city-operated freshwater lake, and one reported returning to childcare despite testing positive. CCPH investigated to determine the outbreak’s scope and source.

METHODS:  CCPH defined confirmed cases as having a positive lab test for Shigella organisms after being exposed to Klineline Pond or exposed to a childcare facility attended by an ill child exposed to Klineline Pond. Probable cases were defined as being exposed to these sources and reporting acute diarrhea after exposure. Suspect cases were defined as sharing these exposures and reporting other gastro-intestinal symptoms after exposure. Enhanced surveillance was performed through a provider advisory, a press release, and interviews of suspected cases and families of childcare attendees.

RESULTS:  24 cases of Shigella sonnei were identified through active case finding. 19 of 24 reported direct exposure to Klineline Pond while 5 reported exposure only to the childcare facility. No confirmed cases were reported among those with exposure only to the childcare. CCPH closed Klineline Pond to swimming with the assistance of the Vancouver Parks Department and inspected the childcare alongside the Washington Department of Early Learning (DEL). Water samples from Klineline Pond found levels of fecal coliforms above EPA recommended action levels although Shigella sonnei could not be isolated. Klineline Pond remained closed for 10 days before 2 consecutive water samples contained fecal coliform counts below EPA recommended action levels.

CONCLUSIONS:  Despite not isolating Shigella sonnei from water samples, the epidemiologic evidence suggests that Klineline Pond was the source of illness. Secondary cases may have occurred at the childcare but this could not be confirmed with laboratory evidence. This investigation highlights the importance of indentifying contacts of primary cases, particularly in high risk settings such as daycares, testing for indicator species when testing the environment for Shigella, and collaborations between local public health agencies and other local or state agencies.