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. Globally, norovirus is estimated to cause 18% of acute gastroenteritis cases and may be responsible for approximately 200,000 deaths annually. 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 outbreaks frequently occur in settings where people live in close quarters and can easily infect each other. Although most norovirus outbreaks occur in health care, school, and food-service settings, outbreaks also occur on cruise ships, and in hotels, camps, and dormitories. Norovirus is a well-known cause of travelers’ diarrhea; prevalence in some settings is known to be greater than in others. Because coinfection and asymptomatic infection with norovirus are common, additional controlled 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.
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, 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 performed to guide clinical management of individual patients, but laboratory testing is used during outbreak investigations by public health agencies.
PCR-based multipathogen diagnostic panels are increasingly available for clinical and research purposes; these panels have good sensitivity and specificity to detect norovirus. 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 enzyme immunoassays (EIAs) are also available to detect the virus in stool specimens. The specificity and sensitivity of EIAs are relatively poor compared with RT-PCR. For more information on laboratory diagnostic testing and specimen collection, see www.cdc.gov/norovirus/lab-testing/index.html.
Supportive care is the mainstay of treatment of norovirus disease, especially oral or intravenous rehydration. For routine management of acute gastroenteritis in children, antidiarrheals and antiemetics are not recommended because high-quality evidence for efficacy and their potential toxicity is lacking. For adults, antimotility, antisecretory, and antiemetic agents may be useful adjuncts to rehydration. 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 dilute bleach solution (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, isolation may be considered for ill people on cruise ships and in institutional settings, including hospitals, long-term care facilities, and schools.
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