A saltwater bacteria has sickened more than twice the number of people in King County this summer than typically is reported during this period – leading health officials to warn of the dangers of eating raw or undercooked shellfish.

During July, there were 13 confirmed or probable cases of Vibrio parahaemolyticus infection in King County, compared to an average of four reported in that month in recent years. Since the beginning of August, an additional eight cases have been confirmed, while typically King County would see six for the entire month.

“This is probably the tip of the iceberg. For every case that is reported, an estimated 142 additional cases go unreported,” said Dr. Jeff Duchin, Chief of Communicable Disease for Public Health–Seattle & King County.

People typically get vibriosis from eating raw or undercooked shellfish, particularly oysters, that have Vibrio bacteria in them. Those with pre-existing medical conditions or who take antacids regularly are at higher risk for illness from Vibrio infection. Cooking shellfish until the shells just open is not enough to kill Vibrio bacteria. Shellfish should be cooked to an internal temperature of 145 degrees F for at least 15 seconds.

Symptoms of Vibrio infection can include moderate to severe diarrhea, abdominal cramps, nausea, vomiting, fever, chills and headache. Vibrio bacteria also can cause a skin infection when open wounds are exposed to warm seawater.

“We have warnings on menus about the risks of eating raw shellfish, but people might not always get the message or know that the risks are much higher this time of year,” Duchin said.

Vibrio bacteria occur naturally in marine waters, and they grow more rapidly during the warm months. That’s why Vibrio levels in shellfish increase during the summer, and infections in humans normally peak in late summer. It’s possible that the early warm streak in July has led to a longer period of Vibrio presence in local waters. Once water temperatures begin to cool in October, the bacteria decline.

The worst outbreak in recent years came in 2006, when Washington had 80 lab-confirmed Vibrio cases and King County had 36 confirmed cases. In 2012, King County had 26 cases of vibriosis for the entire year; so far in 2013, 22 confirmed or probable cases have already been reported.

To prevent Vibrio infections:

a. Thoroughly cook shellfish before eating

b. Do not rinse cooked shellfish in seawater, which can re-contaminate them

c. Keep raw or cooked shellfish well-refrigerated before serving

d. Do not harvest shellfish from areas where harvesting has been closed (see, www.doh.wa.gov/CommunityandEnvironment/Shellfish.aspx)

e. Avoid swimming in warm seawater if you have open wounds.

For more information, see: King County’s Vibriosis website: www.kingcounty.gov/healthservices/health/communicable/diseases/vibriosis.aspx

Washington Department of Health website detailing different illnesses related to shellfish: www.doh.wa.gov/CommunityandEnvironment/Shellfish/BiotoxinsIllnessPrevention.aspx