There have been an unusually high number of norovirus outbreaks sickening people across the country this month, according to data from the Centers for Disease Control and Prevention.
There were 91 suspected or confirmed norovirus outbreaks in the first week of December, the most recent data available. This is a higher number than the C.D.C. has recorded during the same week in recent years. It also exceeds the number of outbreaks recorded in early December in the years leading up to the Covid-19 pandemic.
Outbreaks can occur throughout the year, but they are most common between November and April. The highly contagious virus, which sickens an estimated 19 to 21 million people nationwide each year, can cause sudden and unpleasant gastrointestinal symptoms.
This month, at least 80 people fell ill from norovirus linked to raw oysters served at a restaurant event in Los Angeles. The Food and Drug Administration has advised people not to eat the oysters implicated in that outbreak, which came from British Columbia, Canada, and had been sold in 14 states and the District of Columbia before being recalled. Other recent recalls linked to norovirus have involved fresh and frozen berries, according to an F.D.A. database.
How does norovirus spread?
Dr. William Schaffner, a professor of infectious diseases at the Vanderbilt University School of Medicine, said there are two characteristics that make norovirus “extraordinarily” contagious: It’s easily transmissible, and it’s hardy enough to survive on surfaces for days or even weeks. A person can contract norovirus from contact with an infected person or from contaminated food, water or surfaces.
Dr. Schaffner recalled a case in Tennessee in which a member of a bridge club became sick with norovirus and vomited in the parking lot. Everyone else at the table then became infected. He said that public health investigators later found norovirus on the playing cards.
A person sick with norovirus can shed significant amounts of the virus, and it doesn’t take much to infect someone else. Because of this, the virus thrives in crowded, indoor spaces like cruise ships, nursing homes, schools and day care centers, experts said.
Contaminated water also poses a threat: Untreated sewage can pollute water, which then spreads norovirus into the food supply through irrigation, said Donald W. Schaffner, a food microbiologist at Rutgers University who is not related to Dr. Schaffner of Vanderbilt.
Norovirus outbreaks are also frequently linked to shellfish like oysters and mussels, which eat by filtering water and can end up carrying concentrated amounts of bacteria or viruses in their flesh, Dr. Schaffner of Rutgers added.
How can you avoid it?
Good hand hygiene may be the best defense.
But don’t expect hand sanitizer alone to protect you: The structure of the virus makes it less susceptible to alcohol-based sanitizers and wipes. Soap and water do a better job of rinsing the virus away, Dr. Schaffner of Vanderbilt said. The C.D.C. recommends washing hands often and for at least 20 seconds, especially after using the bathroom or changing a diaper and before giving a person medicine or preparing food.
Norovirus is fairly resistant to heat. The C.D.C. recommends cooking oysters and other shellfish until their internal temperature reaches 145 degrees, and thoroughly washing fresh fruits and vegetables. You should also wash food preparation surfaces like cutting boards well.
Take care to limit contact with infected people or surfaces that might be contaminated with the virus. Soiled linens should be washed with detergent on the longest available cycle and tossed into the dryer on a high-heat setting. A person infected with norovirus should not prepare or handle food for other people until at least two days after symptoms have gone away, according to the C.D.C.
Norovirus can remain in feces for up to two weeks and a person can remain contagious throughout that period, even after they have started feeling better.
“It’s a persistent little critter,” said Dr. Schaffner of Rutgers.
How do you know if you have norovirus?
Some of the symptoms of norovirus, including diarrhea and fever, can make it feel similar to other stomach bugs. What typically sets it apart is the rapid start of symptoms.
“You’ve been feeling fine, but suddenly, out of nowhere, you have this explosive vomiting episode,” Dr. Schaffner of Vanderbilt said.
There is no antiviral treatment for norovirus infections. Doctors generally recommend staying home from work or school and drinking plenty of fluids, he said. A norovirus infection typically clears up within a few days without other treatment.
Young children, older adults and immunocompromised people are most at risk of becoming very sick from norovirus because they are more likely to experience severe dehydration, which can cause fatigue and dizziness, as well as fussiness in children. Very sick patients may need to go to a hospital to receive IV fluids.
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