To control the outbreak, Blue Lake was closed for 10 days to prevent other illness. Telephone interviews of both ill and non-ill persons were conducted as part of a retrospective cohort study using contact information for persons on the reservations list for the park picnic grounds during the weekend of interest, July 11–July 13. Persons who had made the reservations were contacted by MCHD and asked to provide contact information for up to eight persons in their group; 139 persons were identified. State and Portland metro-area local health departments (Clackamas, Clark, Multnomah, and Washington counties) interviewed 109 (78%) of the 139 persons. A probable case was defined as any vomiting or diarrhea with onsets 7–45 hours after visiting the park, in a person who visited the park on July 11, 12, or 13. A confirmed case was defined as meeting the probable case definition and having laboratory-confirmed norovirus infection. Because this was a high profile outbreak and was heavily covered in the media, MCHD received 52 additional reports of illness from persons who contacted MCHD and other local health departments with symptoms consistent with norovirus infection but they were not included in the retrospective cohort study because they were not identified through the reservation list. The investigation identified 65 probable and five laboratory-confirmed cases of norovirus infection (70 total cases).
In the cohort study, approximately 17% (18 of 109) of participants met the case definition; 10 (56%) reported having any diarrhea and 14 (78%) reported vomiting. The median incubation period was 31 hours (range = 7–45 hours), and the median duration of illness was 10 hours (range = 4–24 hours). However, at the time of their interview, three persons reported that their symptoms had not yet resolved. No hospitalizations or deaths were reported.
The percentage of visitors who were at the lake on Saturday, July 12 and became ill was 25% (17 of 68 persons) (Figure). Only one person who visited the lake on Sunday became ill, and no Friday visitors became ill. Those who became ill were significantly younger than those who were not ill (Table 1). Swimming (including immersion under water or wading in the lake), using the splash pad, boating, and younger age (aged 4–10 years) were each significantly associated with becoming ill in bivariate analyses (Table 2). However, when all of these risk factors were assessed simultaneously in one logistic regression model to calculate adjusted relative risk estimates, only swimming remained significantly associated with illness. Persons who swam were 2.3 times (95% CI = 1.1–4.9) more likely to become ill compared with those who did not swim in the lake (Table 2). The attributable risk for swimming in the lake was 91.3% (95% CI = 87.9%–93.2%).
Cases of norovirus infection* associated with recreational activities at Blue Lake Regional Park, Multnomah County, by date and time of onset of symptoms — Oregon, July 11–14, 2014
* N = 18.
†On Saturday, July 7, there were thousands more visitors compared to an average summer Saturday.
The mechanism by which the lake became contaminated is unknown; however, a swimmer's vomit or fecal incident in the lake over the weekend, probably on Saturday, July 12, could explain the point source outbreak pattern (Figure).
Morbidity and Mortality Weekly Report. 2015;64(18):485-490. © 2015 Centers for Disease Control and Prevention (CDC)