Aron J. Hall, Marc-Alain Widdowson

Infectious Agent

Norovirus infection is caused by nonenveloped, single-stranded RNA viruses of the genus Norovirus, also referred to as “Norwalk-like viruses,” Norwalk viruses, human caliciviruses, and small round-structured viruses.

Mode of Transmission

Transmission occurs primarily through the fecal-oral route, either directly from person to person via contaminated hands or indirectly via contaminated food or water. Norovirus is also indirectly spread through aerosols of vomitus and contaminated environments.

Occurrence

• Norovirus infections are found worldwide and can occur year round.
• In the United States, norovirus infections are estimated to cause 23 million illnesses a year and may cause up to 50% of all foodborne outbreaks.
• Seroprevalence studies in the Amazon, southern Africa, Mexico, Chile, and Canada have shown that norovirus infections are common throughout the world, and most children will have experienced at least one infection by the age of 5 years.
• Case-control studies of norovirus as a cause of travelers’ diarrhea (TD) have not been done. Norovirus has been looked for in the stools of visitors to Guatemala and Mexico, with positive findings ranging from 17% to 65% of stools. However, concomitant infection with known pathogens was common, and the rate of norovirus in asymptomatic controls tested in the same manner was not determined. The viral etiology of TD in studies in Asia was found to be 5%–8%, but the increasing prevalence of norovirus recognition around the world may change these statistics.

Risk for Travelers

• Travelers of all ages are potentially at risk for norovirus infection, and previous infection does not reliably result in subsequent immunity.
• 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.
• Large outbreaks of gastroenteritis are associated with settings where persons are living in close quarters, such as hotels, cruise ships, and camps, and can easily infect each other over several days.
• Inapparent viral contamination of inanimate objects may persist during outbreaks and act as a source of infection. On cruise ships, for instance, such environmental contamination has caused recurrent outbreaks of illness on successive cruises with newly boarded passengers.
• Transmission of norovirus on an airplane may be limited if vomiting is confined to restrooms and sick persons are kept separate from others.

Clinical Presentation

• Infected persons usually have acute-onset, violent vomiting and nonbloody diarrhea after an incubation period of 24–48 hours. Other symptoms include abdominal cramps, nausea, and occasionally a low-grade fever.
• Illness is generally self-limited, and full recovery can be expected in 1–4 days. In some cases, dehydration, especially in those who are very young or elderly, may require medical attention.

Diagnosis

• Generally diagnosed by clinical presentation. Laboratory testing is currently not FDA approved in the United States for clinical management, but 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 reverse transcriptase polymerase chain reaction (RT-PCR), which provides rapid and reliable detection of the virus in stool specimens.
• Several commercial enzyme immunoassays (EIAs) are available for detection of the virus in stool specimens. The specificity and sensitivity of these assays are relatively poor or unknown. These tests have been used occasionally by cruise lines during outbreaks on ships.

Treatment

• The mainstay of management is supportive care, such as rest and oral rehydration.
• No antiviral medication is available for treating norovirus infection.

Preventive Measures for Travelers

• No vaccines are available.
• Noroviruses are very common and highly contagious, but the risk for infection can be minimized by frequent and proper handwashing and avoidance of possibly contaminated food and water.
• In addition to handwashing, measures to prevent transmission of noroviruses between persons traveling together include careful clean up of fecal material or vomit and disinfection of contaminated surfaces and toilet areas with products approved for norovirus disinfection by the Environmental Protection Agency, or a high concentration of domestic bleach (at least a 1:50 solution of bleach and water). Soiled articles of clothing should be washed promptly and thoroughly and machine-dried at high heat.
• Confinement of ill persons to help prevent the spread of noroviruses has occasionally been implemented on cruise ships.

References

1. Widdowson MA, Vinjé J. Food-borne viruses—state of the art. In: Koopmans MP, Cliver DO, Bosch A, editors. Food-borne viruses: progress and challenges. Washington, DC: ASM Press; 2008. p. 29–64.
2. Widdowson MA, Cramer EH, Hadley L, et al. Outbreaks of acute gastroenteritis on cruise ships and on land: identification of a predominant circulating strain of norovirus—United States, 2002. J Infect Dis. 2004;190:27–36.
3. Bresee JS, Widdowson MA, Monroe SS, et al. Foodborne viral gastroenteritis: challenges and opportunities. Clin Infect Dis. 2002;35:748–53.
4. Ko G, Garcia C, Jiang ZD, et al. Noroviruses as a cause of traveler’s diarrhea among students from the United States visiting Mexico. J Clin Microbiol. 2005;43:6126–9.
5. Mead PS, Slutsker L, Dietz V, et al. Food-related illness and death in the United States. Emerg Infect Dis. 1999;5:607–25.
6. Chapin AR, Carpenter CM, Dudley WC, et al. Prevalence of norovirus among visitors from the United States to Mexico and Guatemala who experience traveler’s diarrhea. J Clin Microbiol. 2005;43:1112–7.
7. CDC. Norovirus activity—United States, 2006–2007. MMWR Morb Mortal Wkly Rep. 2007;56:842–6.
8. Widdowson MA, Glass R, Monroe S, et al. Probable transmission of norovirus on an airplane. JAMA. 2005;293:1859–60.
9. CDC. “Norwalk-like viruses.” Public health consequences and outbreak management. MMWR Recomm Rep. 2001;50:(RR-9):1–17.