The raw deal: Why under-cooked oysters make you sick



In the last few weeks we have seen a spike in illness associated with consumption of raw oysters, and as the summer heats up, we anticipate more. We sat down with Jenny Lloyd, one of the epidemiologists investigating these illnesses, to learn more about what’s making people sick.

Why do raw oysters make people sick?
The majority of reports that Public Health has been investigating recently have been for Vibrio-like illness (see the outbreak disclosure page for more details) associated with raw oysters. Vibrio species are bacteria that occur naturally in marine waters, where oysters live and are harvested. Eating undercooked or raw shellfish, especially raw oysters in warm-weather months, is the main risk for acquiring vibriosis from infection with Vibrio parahaemolyticus.

How do you minimize the risk of Vibrio infection?
To minimize the risk of Vibrio infection, practice safe shellfish preparation (such as keeping shellfish cold (less than 41°F) and cook shellfish thoroughly, following these guidelines. People who take antacids, have suppressed immune systems, or some other chronic health conditions are more likely to get sick and should consider avoiding raw oyster consumption.

Less commonly, Vibrio bacteria can also cause a skin infection when an open wound is exposed to brackish  water (where seawater and fresh water mix). This type of infection is more common for travelers with recreational water exposure outside of Washington State.

What are the symptoms of Vibrio infection?
When ingested, Vibrio bacteria can cause watery diarrhea, often accompanied by abdominal cramping, nausea, vomiting, fever, and chills. Usually these symptoms occur within 24 hours of ingestion (but can range from four hours to three days) and last three or more days. Severe illness is rare and typically occurs in people with a weakened immune system.

If you think you’ve been infected with Vibrio, contact your healthcare provider.

Why are we seeing more Vibrio right now?
Growth of Vibrio species in seawater increases in warmer water, and so, Vibrio levels in shellfish increase during the summer months.

What do commercial harvesters do to protect consumers from Vibrio?
The Washington State Department of Health licenses commercial shellfish harvesting. Commercial harvesters follow strict refrigeration and handling requirements during warm summer months and shellfish growing areas are regularly monitored for illness-causing pathogens.

I like to harvest oysters myself! What precautions should I take?
Recreational harvesters should also take steps to prevent illness by:

  • Checking the biotoxin status of the beach you plan to harvest
  • Harvest as soon as possible after the tide goes out
  • Do not harvest oysters exposed to direct sunlight
  • Refrigerate or ice oysters immediately

For information on our current Vibrio outbreak, visit our website.

Originally posted on July 18, 2017.