Lake swimming source of Oregon norovirus outbreak

Health officials reported in MMWR that up to 70 people were sickened by norovirus in the summer of 2014 after swimming in a contaminated lake in Oregon.

More than half of those affected by the outbreak were children, they said.

“Children are prime targets for norovirus and other germs that can live in lakes and swimming pools because they’re so much more likely to get the water in their mouths,” Michael J. Beach, PhD, associate director for healthy water at the CDC’s National Center for Emerging and Zoonotic Infectious Diseases, said in a press release. “Keeping germs out of the water in the first place is key to keeping everyone healthy and helping to keep the places we swim open all summer.”

On July 14, Multnomah County health officials were notified of more than a dozen cases of acute gastrointestinal illness among patrons at Blue Lake Regional Park, near Portland, who swam in the park’s recreational lake the previous weekend, between July 11 and July 13. An investigation later identified 65 probable and five laboratory-confirmed cases of norovirus infection. The median age of the cases was 10 years. Five of the ill swimmers provided stool samples, all of which tested positive for norovirus genogroup I by PCR.

Researchers believe the source of the outbreak likely was a swimmer who either had vomited or defecated in the lake.

A retrospective analysis suggests swimmers at Blue Lake were 2.3 times (95% CI, 1.1-4.9) more likely to develop vomiting or diarrhea than park visitors who did not enter the water. There were no hospitalizations or deaths.

To contain the outbreak, health officials closed Blue Lake to visitors for 10 days. More than 15,000 people had visited the park during the weekend of the outbreak.

“This investigation underscores the need for guidance for determining when to reopen untreated recreational water venues (eg, lakes) associated with outbreaks,” the researchers wrote.

This is not the first outbreak of gastrointestinal illness associated with Blue Lake, according to the report. In 1991, a dual pathogen outbreak of Escherichia coli and Shigella sickened up to 80 swimmers, and more than 100 people were infected with norovirus in 2004.

According to the CDC, outbreaks associated with untreated recreational lakes are not uncommon. Between 2009 and 2010, there were 81 disease outbreaks associated with recreational water use, 24 of which were linked with untreated water.

The CDC recommends several basic steps to avoid illness while swimming:

• refrain from swimming when experiencing vomiting or diarrhea;
• shower before entering the water;
• do not urinate or defecate in the water;
• do not swallow lake or pool water;
• take children on bathroom breaks; and
• check diapers regularly and change them in a bathroom or diaper-changing area away from the water.

Raising public awareness and reducing the risks for illness associated with swimming has traditionally focused on treated water, according to the researchers.

“However, as this outbreak highlights, healthy swimming promotion resources for untreated recreational water venues are also needed,” they wrote. – by John Schoen

Disclosure: The researchers report no relevant financial disclosures.