Pacific Northwest heat wave causes vibrio bacteria outbreak in oysters

Fresh Pacific oysters on ice.

A heat wave that sent temperatures into the triple digits for three days in the U.S. Pacific Northwest in late June and early July drove up levels of the vibrio bacteria in area oysters, causing record numbers of illnesses from the bacteria and prompting oyster recalls.

The Washington State Department of Health (DOH) reported 75 lab-confirmed cases of vibriosis as of Wednesday, 29 July, and said there are likely many unreported cases. According to figures provided by DOH, the previous record number of vibriosos cases through 28 July was 48 in 2018. 

The outbreak occurred over the holiday weekend of 4 July and coincided with the reopening of many of Washington’s businesses and restaurants. DOH said that several people visiting the area had fallen ill, including tourists from California, Canada, and Mexico. Officials had traced 48 of the cases back to commercial growers, while five of the cases were from recreational catch. The other cases with either unknown or still under investigation.

Vibriosis cases have been traced back to 24 different growing in areas in Washington state, including Samish Bay and Hood Canal, both of which have large tracts of oysters. A recall was issued for all oysters harvested in Samish Bay from 29 June onward, and the area was closed until at least 6 August, pending the results of ongoing tests. Officials were working to gather harvest from the area, while the Interstate Shellfish Sanitation Conference reported that some oysters from the affected spot had been exported to China, Cambodia, and the Philippines.

The DOH reported that water temperatures had dropped, but they were still finding high levels of vibrio bacteria in the water and they expected the number of vibriosis cases to continue to rise through the end of summer.

A news release from the DOH recommended that people consult its Shellfish Safety Map and said all shellfish should be harvested as soon as possible after the tide has receded, since oysters flush vibrio bacteria when they are submerged in water. DOH also said shellfish should be cooked to 145 degrees Fahrenheit for at least 15 seconds.

The high levels of vibrio bacteria are another challenge for area shellfish growers that saw major losses from pandemic restaurant shutdowns and are faced with new permitting regulations.

Photo courtesy of hlphoto/Shutterstock


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