Public Health Insider:

We’ve recently received multiple reports of people getting sick with norovirus-like illness (nausea, vomiting, and diarrhea) after eating raw oysters in restaurants. In just March alone, we’ve received reports of 13 people whose illnesses we believe are linked to eating raw oysters. An additional 14 people became sick after being in close contact with the ill people who ate oysters. Most of the additional illnesses are among family members.

Eating raw oysters can make you sick because they can be contaminated with norovirus and other germs.  While norovirus is not typically a serious illness for healthy people, it can be very unpleasant. In addition, it’s highly contagious, meaning someone who has been infected with norovirus after eating raw oysters can easily pass the virus to their family and friends.

We talked with Elysia Gonzales, Medical Epidemiologist with Public Health, to better understand the risks of norovirus associated with oysters and how to prevent illness.

Let’s start with the basics. What is norovirus?

Norovirus is a very common, highly contagious virus that causes diarrhea, vomiting, nausea, stomach pain and can also cause body aches, fever, and headache. While most people recover after 1-3 days of these symptoms, the repeated bouts of throwing up can cause dehydration, particularly in children, older people, and people with underlying illnesses.

Norovirus spreads very easily when you have close contact with an infected person, such as sharing utensils or taking care of someone who is sick with norovirus, touching surfaces and objects that have been contaminated with norovirus, or, as in the case of this oyster related outbreak, eating foods that have been contaminated with the virus.

How do you know this was norovirus that made people sick?

One person associated with the recent norovirus cases tested positive for norovirus. We don’t have laboratory confirmation for the remaining cases, but symptoms and timing are suggestive of norovirus. Often in norovirus outbreaks no laboratory testing is done because people usually recover quickly.

Where are the oysters coming from that have made people sick?

Some oysters linked to the illnesses reported in King County now appear to have come from a batch of oysters harvested in British Columbia, which have since been recalled due to norovirus contamination. The origin of other oysters linked to recent norovirus cases in King County is unknown at this time.

Regardless of where an oyster was harvested it poses a high risk of carrying norovirus.

What can be done to reduce my risk from eating oysters?

Oysters have long been recognized as a source of norovirus. Raw or undercooked oysters are a particular problem. Protect yourself and reduce your risk by choosing fully cooked oysters that have been thoroughly fried, baked, or made into a stew that has reached 145°F. Use a thermometer to check.

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 so cooking to 145°F provides a margin for safety.

Avoid eating raw oysters like oyster shooters and oysters on the half shell. Adding hot sauce or lemon to oysters does not kill the virus.

What other steps can be taken to reduce my risk from norovirus?

Wash hands, cutting boards, and counters used for shellfish preparation immediately after use to avoid cross contaminating other foods. And, as general advice to prevent the spread of norovirus, wash hands thoroughly with soap after using the bathroom or changing diapers, and before preparing any food or eating.

If you’ve been sick with norovirus, wait at least 48 hours after the last episode of vomiting and/or diarrhea before preparing any food for others.

Are consumers warned of the risk of eating uncooked oysters?

When eating out, pay attention to any consumer advisories on the menu.  The advisory is there to let you know which animal foods are served raw or undercooked. There is an increased risk of becoming ill from consuming any raw or undercooked meat or seafood.

Why are oysters a particular risk?

Shellfish such as oysters, clams, and mussels are filter feeders. They ingest norovirus if it is present in the water. Because they are filter feeders, these shellfish may concentrate the virus to much higher levels than might be found in the surrounding 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.

What does Public Health do when someone reports illness from oysters or other shellfish?

Public Health investigates every report of illness after consumption of shellfish whether we believe the illness to be norovirus, Vibrio, or some other germ. Our Environmental Health investigators visit the food establishment where the person got the oysters.  We collect shellfish tags to determine the source of the oysters, and we review oyster handling procedures with food establishment staff.  We provide the shellfish tag information to the Washington state Department of Health Shellfish Program.

Oysters are typically contaminated in the water before harvesting but they can also be contaminated later from improper handling by an infected person. Or the contaminated oysters can cross contaminate other foods and surfaces within the food establishment. For these reasons, our investigators also evaluate other risk factors that may contribute to the spread of norovirus in the establishment, such as the presence of any ill food workers, ill family members, poor handwashing practices, or touching ready-to-eat foods with bare hands.  If any risk factors are identified, we make sure the establishment manager takes appropriate interventions to prevent further spread of norovirus.

What should I do if I or someone I live with gets norovirus?

If a person gets norovirus from eating a contaminated oyster (or from any other source) it is possible to spread the virus to people who didn’t eat any oysters. Norovirus is highly contagious, and infected people are most likely to spread it from the moment they feel ill until at least three days after they no longer have symptoms. It’s very important that people who are sick with norovirus stay home for 2-3 days after symptoms have ended to keep from infecting other people. In addition:

  • Wash your hands carefully with soap and warm water frequently, particularly after using the bathroom, changing diapers, before eating, and before preparing food.
  • Wash fruits and vegetables thoroughly before eating them, and avoid undercooked foods.
  • If someone gets sick put on gloves and promptly clean toilets, sinks, and other areas that may be contaminated with vomit or stool, including removing and washing clothing and linens. After cleaning, disinfect with a solution of 1/3 cup household bleach mixed with one gallon of water.
  • Remove and wash clothing or linens with hot water and detergent immediately if they become contaminated.
  • Avoid preparing food for others for at least two to three days after symptoms have ended.