Norovirus is spiking in the US: What do we need to know?

Norovirus is spiking across the United States this winter. Also referred to as the “winter vomiting bug”, norovirus is a highly contagious virus that spreads via contaminated surfaces, food and water, and close contact with infected individuals.

Here’s what to know about the virus, how it spreads and the current outbreak in the US.

What is norovirus?

Norovirus is the cause of 58 percent of food-borne illness in the US.

It is sometimes known as the Norwalk virus, named after the first strain of norovirus identified in Norwalk, Ohio in the 1970s.

The 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 prevalent between November and April, a period often referred to as “norovirus season”.

How does norovirus spread?

Norovirus is highly contagious. It spreads through direct contact with virus particles on surfaces and people, by consuming contaminated liquids and food, and via aerosols and lack of proper hand washing.

Transmission occurs in several ways. Infection can happen when people touch surfaces containing norovirus particles or individuals 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 typically spread when an infected person touches food, beverages or ice before serving it, or when people share food or eating utensils or have close personal contact.

The virus targets the small intestine where it replicates and causes inflammation, resulting in gastroenteritis, or disrupted gut function, which leads to diarrhoea and vomiting.

The virus is remarkably resilient and can remain on both hard or soft surfaces – such as sheets – and in various environments for up to two weeks, which makes it particularly challenging to contain.

Is norovirus airborne?

Norovirus is not an airborne virus like influenza or COVID-19. However, it can become aerosolised if an infected person vomits and small droplets containing the virus are released into the air. When inhaled, these droplets can lead to infection.

Norovirus tends to spread quickly in confined places like cruise ships, where many passengers gather in close quarters or enclosed spaces, and dine together.

Earlier this month, three separate cruise ship outbreaks of norovirus around Hawaii and the Caribbean sickened 301 passengers. It was the highest number of cruise ship outbreaks ever recorded in a single month.

Can you get norovirus from oysters?

Raw or undercooked shellfish, particularly oysters, are well-documented carriers of norovirus.

Oysters constantly feed themselves by straining their food from large quantities of water in a process known as “filter feeding”. During this process, they can pick up contaminants like norovirus from polluted waters. Because norovirus is so contagious, even small particles in the water can carry enough virus to cause illness.

The US Food and Drug Administration (FDA) has issued an advisory against consuming or selling oysters from specific harvest periods and companies in British Columbia, Canada due to potential norovirus contamination. This advisory applies to restaurants, retailers, and consumers.

In 2022, a multistate outbreak in the US was linked to raw oysters harvested from Texas waters. In 2023, several norovirus outbreaks in Europe were traced to oysters.

A senior scientist at at the US Centers for Disease Control and Prevention said that while news reports often centre around infections from oysters, at least 80 percent of cases are transmitted via person-to-person contact.

“Eating raw seafood always is a risk for enteric [intestine-related] infections such as norovirus, but compared to the total number of food borne outbreaks, oyster-related outbreaks make up a small proportion”, he told Al Jazeera via email, requesting anonymity because he was not authorised to speak to the media.

An oyster farmer opens an oyster shell in southern Croatia [File: Eldar Emric/AP Photo]

How big is the current US outbreak?

This winter, the US is experiencing a significant surge in norovirus cases.

According to the data from the CDC in the US, the number of outbreaks spiked 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 linked cases of illness traced to a common source, such as contaminated food or close contact in a specific setting. An outbreak refers to clusters of infections, not individual cases.

In December, Minnesota’s Department of Health has logged 40 norovirus cases – twice the typical number for that month.

The CDC scientist said that while norovirus cases typically rise in the US during this time of year, a relatively new strain, GII.17, has become the dominant type this season.

A lower population immunity to GII.17 compared to the previously dominant strain, GII.4 Sydney, may have contributed to an earlier surge in cases this December, rather than the usual January increase, he explained.

What are the symptoms of norovirus?

The typical incubation period of norovirus is 12-24 hours, however, it can happen as quickly as 10 hours after exposure. The hallmark symptoms are sudden-onset and severe vomiting and diarrhoea. These symptoms can last 24 to 36 hours.

The resulting dehydration is a serious concern, particularly for young children, older adults and individuals with weakened immune systems.

Young children have smaller fluid reserves, and older adults or those with chronic illnesses can find it more challenging to tolerate rapid fluid loss.

Signs of dehydration include dry mouth, decreased urination and dizziness, as well as atypical sleepiness in children.

Other symptoms include:

• Nausea
• Abdominal pain
• Body aches, headaches
• Low-grade fever

Most people recover fully within a few days.

Norovirus is particularly hard to manage in closed quarters like cruise ships [File: Lynne Sladky/AP Photo]

How is norovirus treated?

Norovirus cannot be treated with medication – it needs to run its course. The primary focus is on managing symptoms, particularly dehydration by drinking plenty of fluids, such as water or oral rehydration solutions.

Beverages such as coffee, tea and alcohol can exacerbate dehydration, and should be avoided.

In the event of severe dehydration, medical intervention, such as intravenous fluids, may be required. Some doctors may also prescribe over-the-counter anti-diarrhoeal medications.

How to prevent norovirus infections?

The CDC recommends several measures to prevent norovirus infections:

• Practice rigorous hygiene: Wash 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 close attention to high-touch areas like door handles and countertops.
• Stay home: While symptomatic, avoid preparing food for others. Stay home for at least two days after symptoms resolve.
• Practice food safety: Cook food thoroughly and wash hands frequently. Avoid consuming raw shellfish, which can sometimes harbour the virus.
• Avoid relying on hand sanitisers: Alcohol-based hand sanitisers are less effective against norovirus and should not be used as a substitute for proper handwashing.