Norovirus is spreading across the United States this winter. Also called the “winter vomiting bug,” norovirus is a highly contagious virus that spreads through contaminated surfaces, food, and water, and through close contact with infected people.
Here’s what you need to know about the virus, how it spreads and the current outbreak in the US.
What is norovirus?
Norovirus is responsible for 58 percent of foodborne illnesses in the US.
It is sometimes known as Norwalk virus, named after the first strain norovirus identified in Norwalk, Ohio in the 1970s.
Norovirus belongs to a family of small RNA viruses called Caliciviridae, which cause infections in their hosts, including humans and animals such as pigs, cows, rabbits and dogs.
Norovirus outbreaks are most common between November and April, a period often referred to as “norovirus season.”
How does norovirus spread?
Norovirus is highly contagious. It is spread by direct contact with virus particles on surfaces and people, consumption of contaminated liquid and food, aerosols and lack of proper hand washing.
Transmission takes place in several ways. Infection can occur when people touch surfaces that contain norovirus particles or a person infected with the virus and then touch their mouth, nose, or eyes. The virus is also transmitted by drinking liquids or eating food containing norovirus particles.
Norovirus is usually spread when an infected person touches food, drink or ice before serving, or when people share food or cutlery or have close personal contact.
The virus targets the small intestine where it replicates and causes inflammation, resulting in gastroenteritis, or intestinal upset, leading to diarrhea and vomiting.
The virus is extremely resistant and can remain on both hard and soft surfaces – such as sheets – and in different environments for up to two weeks, which makes it particularly challenging to contain.
Is norovirus airborne?
Norovirus is not an airborne virus like the flu or COVID-19. However, it can become an aerosol if an infected person vomits and small droplets containing the virus become airborne. When inhaled, these droplets can lead to infection.
Norovirus tends to spread quickly in enclosed spaces such as cruise ships, where many passengers gather in cramped quarters or closed areas and eat lunch together.
Earlier this month, three separate norovirus outbreaks on cruise ships around Hawaii and the Caribbean sickened 301 passengers. It was the largest number of outbreaks on cruise ships ever recorded in one month.
Can you get norovirus from oysters?
Raw or undercooked shellfish, especially oysters, are well-documented carriers of norovirus.
Oysters feed continuously by squeezing food out of large volumes of water in a process known as “filter feeding”. During this process, they can pick up contaminants such as norovirus from polluted waters. Because norovirus is so contagious, even small particles in water can carry enough virus to cause illness.
The US Food and Drug Administration (FDA) has issued an advisory against the consumption or sale of oysters from certain harvest periods and companies in British Columbia, Canada due to possible contamination with norovirus. This advisory applies to restaurants, retailers and consumers.
In 2022, a multi-state outbreak in the US was linked to raw oysters caught in Texas waters. In 2023, several norovirus outbreaks in Europe have been traced to oysters.
A senior scientist at the US Centers for Disease Control and Prevention said that while news reports often revolve around oyster infections, at least 80 percent of cases are transmitted by person-to-person contact.
“Consumption of raw seafood always poses a risk for intestinal diseases [intestine-related] infections like norovirus, but compared to the total number of foodborne outbreaks, oyster-related outbreaks account for a small proportion,” Al Jazeeri said via email, requesting anonymity because he was not authorized to speak to the media.
How big is the current epidemic in the US?
This winter, the US is experiencing a significant increase in norovirus cases.
According to CDC data in the US, the number of outbreaks jumped from 69 in the last week of November to 91 in the first week of December. Previously, the highest number of outbreaks during a one-week period in the same time frame was 65 in 2019.
A single norovirus “outbreak” is defined as two or more related cases of illness that can be traced to a common source, such as contaminated food or close contact in a particular environment. Outbreaks refer to clusters of infections, not individual cases.
In December, the Minnesota Department of Health reported 40 cases of norovirus — twice the normal number for that month.
A CDC scientist said that while norovirus cases typically spike in the U.S. this time of year, a relatively new strain, GII.17, has become the dominant type this season.
Lower population immunity to GII.17 compared to the previously dominant strain, GII.4 Sydney, may have contributed to the earlier spike in cases this December, rather than the usual increase in January, he explained.
What are the symptoms of norovirus?
The typical incubation period for norovirus is 12-24 hours, however, it can occur as long as 10 hours after exposure. The main symptoms are sudden and severe vomiting and diarrhea. These symptoms can last 24 to 36 hours.
The resulting dehydration is a serious problem, especially for young children, older adults and people with weakened immune systems.
Young children have smaller fluid reserves, and the elderly or those with chronic illnesses may find it more difficult to tolerate rapid fluid loss.
Signs of dehydration are dry mouth, decreased urination and dizziness, as well as atypical sleepiness in children.
Other symptoms include:
- Nausea
- Abdominal pains
- Body aches, headaches
- Increased temperature
Most people make a full recovery within a few days.
How is norovirus treated?
Norovirus cannot be treated with drugs – it must run its course. The primary focus is on managing symptoms, especially dehydration, by drinking plenty of fluids, such as water or oral rehydration solutions.
Beverages such as coffee, tea and alcohol can worsen dehydration and should be avoided.
Severe dehydration may require medical intervention, such as intravenous fluids. Some doctors may also prescribe over-the-counter anti-diarrhea medications.
How to prevent norovirus infections?
The CDC recommends several measures to prevent norovirus infections:
- Observe rigorous hygiene: Wash your hands frequently with soap and warm water for at least 20 seconds, especially before eating and after using the restroom.
- Disinfect surfaces: Use household cleaning solutions specifically designed to kill viruses, paying particular attention to high-touch areas such as doorknobs and countertops.
- Stay at home: While symptomatic, avoid preparing food for others. Stay at home for at least two days after symptoms subside.
- Practice food safety: Cook food thoroughly and wash your hands often. Avoid eating raw shellfish, which can sometimes harbor the virus.
- Avoid relying on hand sanitizers: Alcohol-based hand sanitizers are less effective against norovirus and should not be used as a substitute for proper hand washing.
