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Raw-oyster enthusiasts, beware. This summer, King County has seen twice the usual number of reported infections caused by the bacteria Vibrio parahaemolyticus.

Vibrio lives in seawater and multiplies in warm summer temperatures. It can cause severe diarrhea in people who eat raw and undercooked shellfish, or skin infections in people who swim with open wounds.

King County reported 21 confirmed or probable cases in July and August so far, while the typical total for the two months is 10.

“This is the tip of the iceberg that we hear about,” said Megan Kay, epidemiologist at Public Health — Seattle and King County. Because cases are underreported, Kay estimates there are 142 unreported cases of Vibrio infection for every confirmed or probable one.

Almost all the known cases resulted from eating raw oysters in restaurants, even though warnings are printed on every menu. Vibrio can be present in all shellfish, but oysters are the variety most likely to be eaten raw.

To entirely kill the bacteria, shellfish have to be cooked to an internal temperature of 145 degrees Fahrenheit for at least 15 seconds, which takes longer than just waiting for the shells to open in an oven. Officials also advised keeping shellfish refrigerated before serving and not to rinse cooked shellfish in possibly contaminated seawater.

People who take antacids are at especially high risk for Vibrio infection because stomach acids normally help kill the bacteria.

The infection runs its course over several days, and treatment is usually not necessary. Symptoms, which include moderate to severe diarrhea, stomach cramps, vomiting, fever and headache, usually appear 12 to 24 hours after exposure.

Skin infections are less common, but people with open wounds should avoid swimming in warm seawater.

The amount of Vibrio bacteria in seawater declines as temperatures cool in the fall.

Health officials said residents should weigh the risks of eating raw shellfish in the summer. Kay said they wanted to make sure everybody was aware of the risks.

“It’s not just a mild tummy ache. It’s pretty severe cramping and diarrhea.”

Sarah Zhang: 206-464-2195 or On twitter @sarahzhang