A modest surge in norovirus cases at the end of last year

The New York Times reported that an unusually high number of norovirus outbreaks occurred nationwide at the end of last year (2024). In the first week of December 2024, 91 suspected or confirmed outbreaks were recorded, a significant increase compared to prior years. An estimated 19–21 million people in the U.S. contract norovirus annually. Norovirus is the number one cause of food poisoning in America.

There was a well-publicized incident in which over 80 people fell ill due to raw oysters served at a restaurant in Los Angeles. The oysters, sourced from British Columbia, Canada, were recalled after being sold in 14 states and the District of Columbia.  Other recent recalls linked to norovirus have involved fresh and frozen berries.

Norovirus is a highly contagious virus that causes gastroenteritis, an inflammation of the stomach and intestines. It is often referred to as the "stomach flu," although it is not related to the influenza virus. Norovirus is a leading cause of foodborne illness worldwide and can spread rapidly, especially in close-contact settings like schools, cruise ships, nursing homes, and hospitals.

Norovirus infection can give rise to nausea, forceful vomiting, watery diarrhea, abdominal pain, and loss of taste. Symptoms typically develop 12–48 hours after exposure and last 1–3 days. They include:

• Nausea
• Vomiting
• Diarrhea
• Stomach cramps
• Lethargy and weakness
• Fever, headache, and muscle aches (less common)

The infection course is typically self-limiting because the body's immune system can effectively clear the virus without the need for specific medical intervention. In addition, norovirus primarily affects the gastrointestinal tract and does not typically spread to other parts of the body.

As a result, a norovirus infection usually isn't deadly, but it can cause serious dehydration that requires hospitalization, especially in young children, older adults, and people with underlying illnesses. The CDC estimates it results in about 570-800 deaths annually in the U.S.

No antiviral treatments exist for norovirus. Instead management includes staying home to recover (and not spread the virus), drinking fluids to prevent dehydration, and seeking medical attention for severe dehydration, especially in children, older adults, or immunocompromised individuals.

Norovirus is transmitted through direct contact with an infected person, consuming contaminated food or water, and touching contaminated surfaces. It is usually spread by the fecal–oral route, and high levels of the virus are found in the feces and vomit of an infected person. Touching a surface containing a lot of virus and then touching your mouth can lead to transmission. In addition, polluted water can carry the virus into the food supply through irrigation. Shellfish, such as oysters and mussels, filter water and can concentrate bacteria or viruses, which could help explain the Los Angeles outbreak.  Even small amounts of the virus can cause infection.

A notable feature of norovirus is its persistence. The virus is highly resilient and can survive on surfaces for days. It resists common disinfectants and temperature changes, making outbreaks difficult to control. Individuals can remain contagious even after feeling better. The ability to survive harsh conditions along with its infectiousness make containment challenging.

Most of the food-borne spread of the pathogen occurs in restaurants and via catering, where food workers touch food such as salad vegetables, fruit and food that already has been cooked, immediately before it is consumed.

This persistence combined with the close quarters and mandatory food service of cruise ships make them particularly vulnerable to norovirus outbreaks, although it should be mentioned that they represent only 1% of norovirus cases, which is a lot if you think about it.

The best way to avoid norovirus is by: 

• Hand Hygiene: Wash hands with soap and water for at least 20 seconds, especially after bathroom use, diaper changes, or before handling food (Figure 1).
• Food Safety: Cook shellfish to an internal temperature of 145°F, wash fresh fruits and vegetables thoroughly, and clean food preparation surfaces, including cutting boards, thoroughly.
• Avoid Contamination: Limit contact with infected individuals and contaminated surfaces. Infected individuals should avoid food preparation for others until at least two days after symptoms resolve.

Figure 1. To avoid being infected from or spreading norovirus, practice good hand hygiene. Wash hands with soap and water for at least 20 seconds especially after bathroom use or before handling food.