For the second time this week Seattle health officials reported that people who recently ate raw oysters there became sick.

Six people in two dinner parties who ate raw oysters at the White Swan Public House restaurant on June 30 and July 3 became sick with gastric symptoms consistent with vibriosis, according to a Thursday notice posted by Seattle-King County Public Health Department.

On Wednesday the department reported three people who ate raw oysters at the Salted Sea restaurant in Seattle on two occasions earlier in June became sick with similar symptoms. One of those victims tested positive for infection from Vibrio parahaemolyticus bacteria.

The cases associated with the White Swan Public House are less clear, although those victims also reported watery diarrhea, abdominal cramping, and vomiting.

“No laboratory testing was done. Symptoms are suggestive of vibriosis; however without testing we cannot rule out norovirus,” the health department reported Thursday regarding the White Swam patrons.

Regardless of the pathogen involved, the health department reported inspections of both restaurants showed no factors that would have contributed to the spread of the Vibrio bacteria, such as insufficient refrigeration or evidence of cross-contamination.

However, because Vibrio is naturally occurring, it can make people sick even if proper handling and storage procedures are followed for seafood.

“We have recently shared information about people who have gotten sick after eating raw oysters in restaurants. Raw oysters can be contaminated by bacteria that occur naturally in our marine waters, and the bacteria can only be killed by properly cooking,” the health department’s medical epidemiologist Dr. Meagan Kay told Food Safety News Thursday afternoon.

The raw oysters served at White Swan Public House were harvested from multiple growing areas and bays in Washington state and New Zealand, according to Seattle-King County officials. Officials cannot further pinpoint the source of the oysters, so they have not ordered harvest bans or issues recalls.

The oysters served at the Salted Sea restaurant came from multiple growing areas and bays in Washington state, according to the health department.

Scientists not surprised
For years public health officials have seen increasing numbers of vibriosis infections among people who eat raw oysters from coastal waters during warmer months. But scientists say it’s not just a seasonal problem at this point in history.

Researchers have been tracking rising temperatures of water in the North Atlantic and the Pacific Northwest along with the number of people infected by Vibrio bacteria.

A professor of marine microbiology at the University of Southern Mississippi, D. Jay Grimes, said it’s been documented that there is a link between the warming of the oceans and the number of annual infections. He cited the Prince William Sound in Alaska as an example.

Since 1997 the temperature of the water in the sound made famous by the oil spill of the Exxon Valdez has been increasing by .21 degrees C annually, according to research cited by Grimes. In 2004, the daily mean water temperature there did not drop below 59 degrees F.

That same year a vibriosis outbreak among 62 out of 189 passengers on a cruise ship that served fresh, raw oysters harvested from the sound “pushed the (vibriosis) range 1,000 kilometers” further north than ever before.

“An oyster fisherman in that bay had been supplying cruise ships for years with no problems,” Grimes told Food Safety News on Thursday.

The scientist said a direct correlation has been documented between vibriosis cases stemming from oysters harvested in the Prince William Sound and the rising temperature of the water there. Similarly, the waters of the Puget Sound around Seattle have been increasing, as has the number of Vibrio infections in that area.

“You don’t want to panic the public,” Grimes said. “For the average person who is healthy, the infection is usually self-limiting and resolves in a few days. But for high-risk groups like diabetics and people who are immune-compromised, it can be fatal.”

The situation in the North Atlantic is even more severe, according to research published in the summer of 2016. It’s a relatively simple equation: warmer water = more marine picoplankto, including Vibrio bacteria.

A research team including Rita R. Colwell examined data about water temperatures in the North Atlantic and bacteria levels from 1958 to 2011. Of the nine areas they reviewed, water temperatures increased in all of them and Vibrio bacteria levels increase in eight of them.

Advice to consumers
Anyone who has eaten raw oysters in the Seattle area recently and developed symptoms of vibriosis should seek medical attention and tell their doctors of the possible exposure to the bacteria so the proper diagnostic tests can be performed.

Symptoms include watery diarrhea, bloody diarrhea, abdominal cramping, nausea, vomiting, headache and fever. People are usually sick for one to seven days, according to the U.S. Centers for Disease Control and Prevention. People usually become sick within 24 hours of eating contaminated seafood.

Annually, vibriosis causes an estimated 80,000 illnesses and 100 deaths in the United States, CDC reports.

Seattle-King County has seen between 20 and 90 confirmed cases of vibriosis reported annually for the past 10 years. The five-year average is 30 cases per year, but higher numbers in recent years spurred action by health officials.

In 2015 there were 32 laboratory-confirmed cases of vibriosis in the state, with 34 cases and 46 cases confirmed in 2014 and 2013, respectively.

To prevent infection by Vibrio bacteria, the Seattle-King County Public Health Department offered these tips:

• Always cook shellfish and other seafood thoroughly before eating;
• Wash cutting boards and counters used for shellfish preparation immediately after use to avoid cross contaminating other foods;
• Wash hands thoroughly with soap after handling raw shellfish;
• Stay out of brackish or salt water if you have any wounds — including minor scrapes and cuts — or cover wounds with waterproof bandages to prevent a skin infection; and
• Wash wounds and cuts with soap and water if they have been exposed to seawater, raw seafood or raw seafood juices, to prevent skin infections from Vibrio bacteria.

(To sign up for a free subscription to Food Safety News, click here.)