Norovirus, the highly contagious illness that causes vomiting and diarrhea, has surged in the Northeast region of the United States in the past few weeks, according to recent data from the Centers for Disease Control and Prevention (CDC). CDC statistics show that food is the most common transmission vehicle for noroviruses.

While surveillance data show an increase in positive test results nationwide—as is typical for norovirus during the winter months—northeastern states have been hit especially hard, with a three-week average of 13.7 percent for positive norovirus tests. Those tests have held above a 10 percent positive rate since December 2023.

Other U.S. regions have also seen an uptick in norovirus illnesses at lower rates. For the past three weeks, southern states have seen a positive test rate of about 9.4 percent, the Midwest is at 10 percent, and western states have seen a 12.6 percent positive rate.

What is norovirus?

The Centers for Disease Control and Prevention (CDC) estimates that noroviruses cause nearly 21 million cases of acute gastroenteritis annually, making noroviruses the leading cause of gastroenteritis in adults in the United States. 

Nature has created an ingenious bug in norovirus. Norovirus’s round blue ball structure is a protein surrounding the virus’s genetic material. The virus attaches to the outside of cells lining the intestine and then transfers its genetic material into those cells. once the genetic material has been transferred, norovirus reproduces, finally killing the human cells and releasing new copies of itself that attach to more cells of the intestine’s lining.

Humans are the only host of norovirus, which has several mechanisms that allow it to spread quickly and easily. Norovirus infects humans in a pathway similar to the influenza virus’ mode of infection. In addition to their similar infective pathways, norovirus and influenza also evolve to avoid the immune system similarly. Both viruses are driven by heavy immune selecion pressure and antigenic drift, allowing evasion of the immune system, which results in outbreaks. Norovirus can survive a wide range of temperatures and in many different environments. Moreover, the viruses can spread quickly, especially in places wher people are close, such as cruise ships and airline flights, even those of short duration. 

How is norovirus transmitted?

Norovirus causes nearly 60 percent of all foodborne illness outbreaks. Norovirus is transmitted primarily through the fecal-oral route, with fewer than 100 microscopic norovirus particles needed to cause infection. Transmission occurs either person-to-person or through contamination of food or water. Of 232 norovirus outbreaks between July 1997 and June 2000, 57 percent were foodborne, 16 percent were spread from person to person, and 3 percent were waterborne. When food is the vehicle of transmission, contamination occurs most often through a food handler improperly handling food directly before it is eaten.

Infected individuals shed the virus in large numbers in their vomit and stool, shedding the most viral particles while ill. Aerosolized vomit has also been implicated as a mode of norovirus transmission. Previously, it was thought that viral shedding ceased approximately 100 hours after infection; however, some individuals continue to shed norovirus long after they have recovered from it, in some cases up to 28 days after experiencing symptoms. Viral shedding can also precede symptoms, which occur in approximately 30 percent of cases. Often, an infected food handler may not even show symptoms.

What are the symptoms and risks of a norovirus infection?

Norovirus illness usually develops 24 to 48 hours after ingesting contaminated food or water. Symptoms typically last a relatively short time, approximately 24 to 48 hours. These symptoms include nausea, vomiting, diarrhea, and abdominal pain.  Headache and low-grade fever may also accompany this illness. People infected with norovirus usually recover in two to three days without serious or long-term health effects. 

Although symptoms usually only last one to two days in healthy individuals, norovirus infection can become serious in children, the elderly, and immune-compromised individuals. In some cases, severe dehydration, malnutrition, and even death can result from norovirus infection, especially among children and among older and immune-compromised adults in hospitals and nursing homes.

How do you diagnose a norovirus Infection?

Diagnosis of norovirus illness is based on the combination of symptoms, particularly the prominence of vomiting, little fever, and the short duration of illness.  If a known norovirus outbreak is in progress, public health officials may obtain specimens from ill individuals for testing in a lab. These lab tests consist of identifying norovirus under an electron microscope. 

How do you treat a norovirus infection?

There is no specific treatment available for norovirus. In most healthy people, the illness is self-limiting and resolves in a few days; however, outbreaks among infants, children, elderly, and immune-compromised populations may result in severe complications among those affected. Death may result without prompt measures. Replacing fluids and minerals such as sodium, potassium, and calcium – otherwise known as electrolytes – lost due to persistent diarrhea is vital. This can be done either by drinking large amounts of liquids or intravenously.

How do you prevent a norovirus infection?

Common settings for norovirus outbreaks include restaurants and events with catered meals (36 percent), nursing homes (23 percent), schools (13 percent), and vacation settings or cruise ships (10 percent). Proper hand washing is the best way to prevent the spread of norovirus. 

Shellfish (oysters, clams, mussels) pose the most significant risk. Any serving may be contaminated with norovirus; there is no way to detect a contaminated oyster, clam, or mussel from a safe one.  Shellfish become contaminated when their waters become contaminated—e.g. when raw sewage is dumped overboard by recreational or commercial boaters). Shellfish are filter feeders and will concentrate virus particles present in their environment. With shellfish, only complete cooking offers reliable protection; steaming does not kill the virus or prevent its transmission.