Waterborne Outbreak of Shigella Sonnei Associated with a State Park--New Jersey 2012

BACKGROUND: On June 24, 2012 an emergency department physician called the New Jersey Department of Health (NJDOH) to report two unrelated families being evaluated for diarrheal illnesses; both families mentioned swimming in a lake at a NJ state park a few days prior. Stool specimens from these initially reported patients tested positive for Shigella sonnei.  Local health department (LHD) and NJDOH staff launched an investigation to identify additional cases and to determine possible infection sources.  

METHODS: We reviewed reported gastrointestinal illnesses to identify additional associated cases. We defined (1) confirmed cases as gastrointestinal illnesses with positive cultures for S. sonnei and lake exposure, (2) probable cases as gastrointestinal illnesses and lake exposure, and (3) secondary cases as gastrointestinal illnesses epidemiologically linked to a confirmed case. A supplemental questionnaire was developed and provided to the LHD to interview case-patients who had lake exposures.  A team of NJDOH epidemiologists and a registered environmental health specialist assessed the lake’s facilities (including a concession stand, septic system, and potable water system) and swimming area.

RESULTS: We identified 12 confirmed, 13 probable and 3 secondary cases with illness onset dates from June 21-24 and lake exposure from June 20-22.   There was no evidence of septic system failure or leakage or contamination of the potable water.  Per case-patient interviews, the vast majority of people brought food from home to the park and did not purchase concession items.  However, during the lake exposure timeframe, the weather was very warm, and the number of park attendees dramatically increased; almost 400 people swam in the lake on June 20 at 2 PM with a high temperature of 95°F. The lifeguard log stated that the park was short-staffed and therefore did not open additional areas for swimmers to spread out. On June 21, the lake was closed due to a thunderstorm; no case-patients reported illness onsets after this closure, coinciding with case count drop-off.

CONCLUSIONS: This Shigella outbreak was likely caused by fecal contamination of a lake.  A high volume of visitors who were confined to a small area of the lake likely facilitated disease transmission which was interrupted by the lake’s weather-related closure.  Investigation findings ruled out septic system breaches and contamination of potable water/concessions.  This outbreak underscores the importance of recognizing recreational waterborne illnesses and promoting appropriate measures to limit and prevent such illnesses and promoting appropriate measures to limit and prevent such illnesses.