Norovirus infection is caused by nonenveloped, single-stranded RNA viruses of the genus Norovirus , which have also been referred to as “Norwalk-like viruses,” Norwalk viruses, and small round-structured viruses. Norovirus is a cause of viral gastroenteritis, sometimes referred to as “stomach flu”; however, there is no biologic association with influenza or influenza viruses.
Transmission occurs primarily through the fecal-oral route, either through direct person-to-person contact or indirectly via contaminated food or water. Norovirus is also spread through aerosols of vomitus and contaminated environmental surfaces and objects.
Norovirus infections are common throughout the world, and globally most children will have experienced ≥1 infection by the age of 5 years. Norovirus infections can occur year round, but in temperate climates, norovirus activity peaks during the winter. Noroviruses are common in both developing and developed countries. In the United States, norovirus is the leading cause of medically attended gastroenteritis in young children and of outbreaks of gastroenteritis; it is estimated to cause 19–21 million illnesses a year and approximately 50% of all foodborne disease outbreaks.
Norovirus is a cause of travelers’ diarrhea; prevalence ranges from 3% to 17% of travelers returning with diarrhea. However, coinfection and asymptomatic infection with norovirus are common, so focused studies are needed to determine exactly how frequently norovirus is the cause of disease. Risk for infection is present anywhere food is prepared in an unsanitary manner and may become contaminated, or where drinking water is inadequately treated. Of particular risk are “ready-to-eat” cold foods, such as sandwiches and salads. Raw shellfish, especially oysters, are also a frequent source of infection, because virus from contaminated water concentrates in the gut of these filter feeders. Contaminated ice has also been implicated in outbreaks.
Norovirus outbreaks frequently occur in settings where people live in close quarters and can easily infect each other, such as hotels, cruise ships, camps, dormitories, and hospitals. Viral contamination of inanimate objects or environmental surfaces (fomites) may persist during and after outbreaks and be a source of infection. On cruise ships, for instance, such environmental contamination has caused recurrent norovirus outbreaks on successive cruises with newly boarded passengers. Transmission of norovirus on airplanes has been reported during both domestic and international flights and likely results from contamination of lavatories or from symptomatic passengers in the cabin.
Infected people usually have an acute onset of vomiting with nonbloody diarrhea. The incubation period is 12–48 hours. Other symptoms include abdominal cramps, nausea, and sometimes a low-grade fever. Illness is generally self-limited, and full recovery can be expected in 1–3 days for most patients. In some cases, dehydration, especially in patients who are very young or elderly, may require medical attention.
Norovirus infection is generally diagnosed based on symptoms. Norovirus diagnostic testing is not widely available to guide clinical management of individual patients, but laboratory testing is used during outbreak investigations by public health agencies. Norovirus diagnostic testing is usually not available in developing countries.
The most common diagnostic test used at state public health laboratories and CDC is RT-PCR, which rapidly and reliably detects the virus in stool specimens. Several commercial EIAs are also available to detect the virus in stool specimens. The specificity and sensitivity of these assays are poor compared with RT-PCR.
Supportive care is the mainstay of treatment of norovirus disease, especially oral or intravenous rehydration. Antimotility agents should be avoided in children aged <3 years but may be a useful adjunct to rehydration in older children and adults. Antiemetic agents should generally be reserved for adults. Antibiotics are not useful in treating patients with norovirus disease.
No vaccine is currently available, although vaccine development efforts are advancing. Noroviruses are common and highly contagious, but the risk for infection can be minimized by frequent and proper handwashing and avoiding possibly contaminated food and water. Washing hands with soap and water for at least 20 seconds is considered the most effective way to reduce norovirus contamination; alcohol-based hand sanitizers might be useful between handwashings but should not be considered a substitute for soap and water.
In addition to handwashing, measures to prevent transmission of noroviruses between people traveling together include carefully cleaning up fecal material or vomit and disinfecting contaminated surfaces and toilet areas. Products should be approved by the Environmental Protection Agency for norovirus disinfection; alternatively, a solution of domestic bleach (5–25 tablespoons bleach per gallon of water) may be used. Soiled articles of clothing should be washed at the maximum available cycle length and machine-dried at high heat.
To help prevent the spread of noroviruses, ill people may be isolated on cruise ships and in institutional settings.
CDC website: www.cdc.gov/norovirus
Aron J. Hall, Ben Lopman