Norovirus Outbreak Associated With a Natural Lake Used for Recreation — Oregon, 2014

Amy Zlot, MPH; Maayan Simckes, MPH; Jennifer Vines, MD; Laura Reynolds, MPH; Amy Sullivan PhD; Magdalena Kendall Scott, MPH; J. Michael McLuckie; Dan Kromer, MPA; Vincent R. Hill, PhD; Jonathan S. Yoder, MPH; Michele C. Hlavsa, MPH

Disclosures

Morbidity and Mortality Weekly Report. 2015;64(18):485-490.

Introduction

In July 2014, Multnomah County public health officials investigated a norovirus outbreak among persons visiting Blue Lake Regional Park in Oregon. During the weekend of the reported illnesses (Friday, July 11–Sunday, July 13) approximately 15,400 persons visited the park. The investigation identified 65 probable and five laboratory-confirmed cases of norovirus infection (70 total cases). No hospitalizations or deaths were reported. Analyses from a retrospective cohort study revealed that swimming at Blue Lake during July 12–13 was significantly associated with illness during July 13–14 (adjusted relative risk = 2.3; 95% confidence interval [CI] = 1.1–64.9). Persons who swam were more than twice as likely to become ill compared with those who did not swim in the lake. To control the outbreak, Blue Lake was closed for 10 days to prevent further illness. This investigation underscores the need for guidance for determining when to reopen untreated recreational water venues (e.g., lakes) associated with outbreaks, and communication tools to inform the public about the risks associated with swimming in untreated recreational water venues and measures that can prevent illness.

On July 14, Multnomah County Health Department (MCHD) was notified of 13 cases of acute gastrointestinal illness among members of three separate groups who had visited Blue Lake Regional Park over the previous weekend, July 11–13. MCHD began to investigate the potential outbreak to identify risk factors for illness, and develop and implement control measures to prevent additional illness.

The park, located just outside of Portland (Multnomah County), is a popular destination for city residents during the summer. The park has a lake for swimming, picnic grounds, paddleboats, and a splash pad (a chlorinated spray ground with features that shower and pour water). During the weekend of the reported illnesses, approximately 1,700 persons visited the park on Friday, July 11; 7,700 on Saturday, July 12; and 6,000 on Sunday, July 13, thousands more visitors compared with an average summer weekend.

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Table 1. Demographic characteristics of cases and non-cases,* outbreak of norovirus at Blue Lake Recreational Area — Oregon, 2014
Chacteristic	Age^† (yrs)		Gender^§ (%)
Chacteristic	Median	Range	Female	Male
Cases	10	4–27	72.2	27.8
Non-cases	31	1–68	62.9	37.1

* N = 109 persons interviewed; cases are defined as persons reporting onset of illness (vomiting or diarrhea) and non-cases are defined as those persons not reporting illness.
^†p-value = 0.0002.
^§p-value = 0.45.

Table 2. Relative risk and rates of illness from norovirus, by exposure, in persons* during outbreak of norovirus at Blue Lake Recreational Area — Oregon, 2014
Characteristic	Exposed to risk factor				Not exposed to risk factor				Relative risk model			Adjusted relative risk model			% ill persons exposed to risk factor
	Ill		Not ill		Ill		Not ill		Relative risk model			Adjusted relative risk model
	No.	(%)	No.	(%)	No.	(%)	No.	(%)	Risk	(95% CI)	p-value	Risk	(95% CI)	p-value
Swam	13	(54.2)	11	(45.8)	5	(5.9)	80	(94.1)	9.2	(3.6–23.4)	<0.001	2.3	(1.1–4.9)^†	0.02	72.2
Used the splash pad	11	(31.4)	24	(68.6)	7	(9.5)	67	(90.5)	3.3	(1.4–7.8)	0.0039	1.4	(0.6–3.0)^§	0.51	61.1
Boated	10	(71.4)	4	(28.6)	8	(8.4)	87	(91.6)	8.5	(4.0–17.8)	<0.001	1.8	(0.7–4.7)^¶	0.22	55.6
Aged 4–10 years versus 11+ years	14	(28)	36	(72)	4	(6.8)	55	(93.2)	1.3	(1.1–1.6)	0.0039	1.5	(0.7–3.3)**	0.3	77.8
Drank from drinking fountain	16	(17.2)	77	(82.8)	2	(12.5)	14 5	(87.5)	1.4	(0.3– 5.4)	0.27	NA	NA	NA	88.9
Ate any food	16	(16.2)	83	(83.8)	2	(25)	6	(75)	0.6	(0.2– 2.3)	0.52	NA	NA	NA	88.9
Used the restroom	13	(16.7)	65	(83.3)	2	(8.3)	22	(91.7)	2.0	(0.5–8.2)	0.17	NA	NA	NA	86.7
Played on play structure	5	(20.8)	19	(79.2)	13	(15.3)	72	(84.7)	1.4	(0.5–3.4)	0.52	NA	NA	NA	27.8

Abbreviation: NA = not applicable; 95% CI = 95% confidence interval.
* N = 109; however, counts used to calculate measures might not add up to 109 because of missing data.
^†Controlled for boating, splash pad, and age.
^§Controlled for swimming, boating, and age.
^¶Controlled for swimming, splash pad, and age.
**Controlled for swimming, splash pad, and boating.