Author(s): Sara Mirza, Aron Hall

Infectious Agent

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, norovirus has no biologic association with influenza or influenza viruses.

Transmission

Norovirus transmission occurs primarily through the fecal–oral route, either through direct person-to-person contact or indirectly via contaminated food or water. Norovirus also is spread through fomites and aerosols of vomitus.

Epidemiology

Norovirus outbreaks frequently occur in settings where people live in close quarters and can easily infect each other. Norovirus is a commonly reported cause of diarrhea among travelers in confined spaces (e.g., on cruise ships, and in camps, dormitories, hotels). Risk for infection is present anywhere food is prepared in an unsanitary manner and can be contaminated, or where drinking water is inadequately treated. Ready-to-eat cold foods (e.g., salads, sandwiches) are a particular risk. Raw shellfish, especially oysters, are a frequent source of infection because viral particles in contaminated water concentrate in the gut of these filter feeders. Contaminated ice has also been implicated in outbreaks.

Viral contamination of fomites can 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 domestic and international flights and likely results from contamination of lavatories or from symptomatic passengers in the cabin.

Norovirus infections are common throughout the world. Globally, most children will have ≥1 infection by the time they are 5 years old. Norovirus infections can occur year round, but in temperate climates, activity peaks during the winter. Noroviruses are common in low-, middle-, and high-income countries. Globally, norovirus causes ≈18% of acute gastroenteritis cases and could be responsible for ≈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; norovirus causes ≈19–21 million illnesses a year and ≈50% of all foodborne disease outbreaks.

Clinical Presentation

Infected people usually experience acute onset of vomiting and non-bloody 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 most patients fully recover in 1–3 days. In some cases, especially among the very young or elderly, dehydration can occur and require medical attention.

Diagnosis

Norovirus infection is generally diagnosed based on symptoms. Diagnostic testing is not widely performed to guide clinical management of individual patients, but laboratory testing is used to identify disease clusters during outbreak investigations.

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 at the Centers for Disease Control and Prevention (CDC) is real-time reverse-transcription quantitative PCR (RT-qPCR), which rapidly and reliably detects the virus in stool specimens. Several commercial enzyme immunoassays (EIAs) also are available to detect the virus in stool specimens, but the specificity and sensitivity of EIAs are relatively poor compared with RT-qPCR.

CDC recommends contacting local health departments for outbreak investigation and specimen testing. Whole stool specimens are preferred for testing; vomitus specimens might be acceptable. For more information on laboratory diagnostic testing and specimen collection, see CDC's webpages on Lab Testing and Test Directory.

Treatment

Supportive care is the mainstay of norovirus treatment, especially oral or intravenous rehydration. Antidiarrheals and antiemetics are not recommended for the routine management of acute gastroenteritis in children. For adults, antiemetic, antimotility, and antisecretory agents can be useful adjuncts to rehydration. Antibiotics are not useful in treating patients with norovirus disease.

Prevention

No norovirus vaccine is currently available, but vaccine development is advancing. Noroviruses are common and highly contagious, but travelers can minimize their risk for infection by frequently and properly washing hands and avoiding possibly contaminated food and water. Washing hands with soap and water for ≥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, people traveling together can use measures to prevent transmission of noroviruses, including carefully cleaning up fecal material or vomit and disinfecting contaminated surfaces and toilet areas. Travelers should use products approved by the US Environmental Protection Agency for norovirus disinfection; alternatively, they can use a dilute bleach solution (5–25 tablespoons bleach per gallon of water). Travelers should wash soiled articles of clothing for the maximum available cycle length and machine dry clothing on high heat.

To help prevent the spread of noroviruses, consider isolation for ill people on cruise ships and in institutional settings, including hospitals, long-term care facilities, and schools.

CDC website: Norovirus