Illnesses associated with the consumption of raw oysters

Without laboratory testing, these two illnesses can sometimes be difficult to tell apart, making investigations somewhat more complex.

• Vibrio is much more likely to occur in the warm summer months.
• Norovirus tends to be more common during the winter months, but can occur at any time of the year.

About norovirus

Norovirus is a highly contagious virus that is frequently spread person-to-person and is often associated with food. Shellfish such as oysters, clams, and mussels are filter feeders which could ingest norovirus if it is present in the water. Though all shellfish can be a source of norovirus infection if consumed raw or undercooked, oysters are much more commonly consumed raw than other shellfish.

Norovirus illness often has a sudden onset of nausea and vomiting and/or watery diarrhea with cramps. A low grade fever, chills, and body aches sometimes occur. Norovirus rarely causes severe complications. Dehydration is the most common complication, particularly among young children and the elderly. No vaccine is available for norovirus.

To prevent norovirus infection:

1. While cooking shellfish can reduce the risk of norovirus illness, quick steaming or cooking until the shells just open may not be enough to protect against norovirus illness. Norovirus can survive cooking temperatures up to 140°F.
2. Wash hands, cutting boards, and counters used for shellfish preparation immediately after use to avoid cross contaminating other foods.
3. Wash hands thoroughly with soap after using the bathroom or changing diapers, and before preparing any food or eating.
4. Wait at least 48 hours after the last episode of vomiting and/or diarrhea before preparing any food for others.

For more information on norovirus, see:

1. Norovirus facts, Public Health – Seattle & King County
2. Norovirus facts, Foodsafety.gov

About Vibrio

Vibrio is a bacteria consisting of multiple species, including Vibrio parahaemolyticus. The bacteria are naturally occurring in marine waters. Eating undercooked or raw shellfish, especially raw oysters, is the main risk factor for acquiring an infection with Vibrio parahaemolyticus. Growth of Vibrio species is amplified during the warmer months and Vibrio levels in shellfish increase during the summer.

To prevent Vibrio infection:

1. Because raw seafood can be contaminated with Vibrio, always cook shellfish and other seafood thoroughly before eating.
2. Wash cutting boards and counters used for shellfish preparation immediately after use to avoid cross contaminating other foods.
3. Wash hands thoroughly with soap after handling raw shellfish.
4. Stay out of brackish or salt water if you have any wounds (including scrapes and cuts), or cover your wound with waterproof bandage to prevent a skin infection.
5. Wash wounds and cuts with soap and water if they have been exposed to seawater or raw seafood and raw seafood juices, to prevent a skin infection from Vibrio.

For more information on vibriosis, see:

• Vibrio facts, CDC

* Disclosures posted previously online at: kingcounty.gov/outbreak
^ Includes at least one ill person with laboratory-confirmed Vibrio parahaemolyticus or norovirus

* Disclosures posted previously online at: kingcounty.gov/outbreak
^ Includes at least one ill person with laboratory-confirmed Vibrio parahaemolyticus or norovirus

* Disclosures posted previously online at: www.kingcounty.gov/outbreak
^ Includes at least one ill person with laboratory-confirmed Vibrio parahaemolyticus or norovirus