By Donna Campisano, specialist, Communications, APHL

Each year about 2,500 outbreaks of norovirus—a highly contagious group of viruses that inflame the lining of the stomach and intestines causing nausea, vomiting and diarrhea—are reported, says the US Centers for Disease Control and Prevention (CDC).

But this year may be worse.

According to CDC, 91 confirmed or suspected outbreaks (defined as two or more cases) were reported during the week ending December 5, exceeding any other week in 2024 and easily surpassing the number of outbreaks for the same period in previous years.

What’s behind the surge? Inside Minnesota and Wisconsin outbreaks

Norovirus infections are highly contagious, so it’s no surprise that cases tend to peak in cooler months when more people congregate indoors. (Most norovirus outbreaks occur between November and April, although cases appear at all times of the year.)

But experts say there’s more at play than just the weather.

In Wisconsin, for the week ending December 28, 2024, norovirus was detected in 41.4% of the reported 606 samples tested by the state laboratory’s clinical partners, said Kyley Anhalt, senior microbiologist at the Wisconsin State Laboratory of Hygiene (WSLH). At the same time last year, there was a 16.6% positivity rate.

One reason for the increase, Anhalt said, is the emergence of a new dominant norovirus strain.

“For over a decade, the leading strain of norovirus in this country was GII.4 Sydney [P16],” Anhalt commented. “This year, GII.17 [P17] is the dominant strain, and fewer people have immunity to it.”

Amy Saupe, senior epidemiologist with the Minnesota Department of Health (MDH), said her state is seeing a similar surge—and for a similar reason.

“We had 76 outbreaks reported in December, comprising more than 1,500 individual cases of norovirus infection,” she said. “That’s about three times the number of outbreaks we typically have during that time of year. We can’t point to one particular reason for the increase,” Saupe added. “Some norovirus years are bigger than others. And things like what norovirus strains are circulating and what population immunity looks like all play a role.”

In addition, Minnesota and Wisconsin may be seeing particularly high norovirus numbers because of their states’ robust reporting systems.

In Wisconsin, for example, over 130 clinical laboratories provide test result data to the WSLH on various pathogens, including norovirus. A select handful of these laboratories also send in sporadic positive norovirus samples that are not associated with an outbreak.

“We genotype these specimens and send the data to CDC via CaliciNet [a national norovirus surveillance system comprised of federal, state and local public health laboratories],” Anhalt explained. “Since the norovirus season started on August 1, WSLH has genotyped over 200 norovirus positive sporadic samples throughout the state. This number is quite robust compared to other states and may be part of the narrative of why Wisconsin is seeing higher rates.”

Like most states, Minnesota only monitors norovirus outbreaks and not individual cases. Schools and long-term care facilities in the state are required to report outbreaks. Some outbreaks also get reported to Minnesota’s Foodborne and Waterborne Illness Hotline. Once an outbreak is reported, the MDH Public Health Laboratory tests stool specimens looking for norovirus genetic material. If the lab finds norovirus, it sequences the genetic material to determine the strain and then uploads that information to CaliciNet.

The importance of surveillance

Even though individual cases of norovirus may never be reported—meaning significantly more people may be infected with this group of viruses than the data suggests—surveillance is tantamount.

“Laboratory testing is one of the major pillars of surveillance,” Anhalt commented. “Laboratory diagnosis of a GI Illness can help in deciding what an individual patient may need, even though, for norovirus, care tends to be palliative. However, the data from testing can identify when new genotypes, new clusters and new outbreaks arise. This information can then go to the public health departments or facilities seeing outbreaks. From there, it can be decided if an alert needs to be issued to the public or new protocols for sanitation need to be put in place, etc.”

Stopping the spread

Most people with norovirus (also sometimes called the “stomach flu,” although norovirus infections are different from influenza infections) get better in one to three days. But they can still spread the virus, usually via contaminated surfaces and sharing the food or drink of an infected person (or eating food handled by them), for two weeks or more after symptoms improve, says CDC.

Key to stopping the spread of norovirus is proper handwashing. Don’t rely on hand sanitizer—it’s ineffective against norovirus. Instead, use soap and water and rub your hands together for 20 seconds. Wash your hands often, and especially:

• Before and after eating or handling food
• After using the bathroom
• After tending to someone who is sick
• After changing diapers
• After handling soiled laundry

More tips:

1. Wash the laundry of a sick person in hot water.
2. Frequently disinfect surfaces.
3. Cook foods to proper temperatures and wash fruits and vegetables.
4. Do not cook food for others if you’ve been sick with vomiting or diarrhea and do not resume cooking until at least 48 hours after symptoms resolve.
5. Stay home when you don’t feel well and for up to 48 hours after norovirus symptoms stop.

The post Norovirus Is Spiking: Why the Surge and How to Protect Yourself appeared first on APHL Blog.

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