Norovirus

Introduction

Infectious agent

Norovirus infections are caused by non-enveloped, single-stranded RNA viruses of the genus Norovirus, which in the past have also been referred to as Norwalk-like viruses, Norwalk viruses, and small round-structured viruses. Norovirus is one of the leading causes of acute 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 (e.g., cruise ships, camps, dormitories, long-term care facilities, and hotels) where people live or congregate in close quarters and can easily infect each other. Norovirus is a commonly reported cause of acute diarrhea among travelers in confined spaces. Although noroviruses have been detected in different animal species, those viruses are genetically different from the noroviruses infecting humans. With humans as the only reservoir, risk for infection is present anywhere food is prepared in an unsanitary manner and can become contaminated (e.g., sick food handler stocking a salad bar) or where drinking water is inadequately treated. Contaminated 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 can 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 approximately 18% of acute gastroenteritis cases and is responsible for tens of thousands of deaths annually. In the United States, norovirus is the leading cause of medically attended gastroenteritis in young children and of outbreaks of gastroenteritis in people of all ages; norovirus causes approximately 19–21 million illnesses a year and approximately 50% of all foodborne disease outbreaks.

Clinical presentation

Infected people usually experience acute onset of vomiting and non-bloody diarrhea. The incubation period ranges from 12 to 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 young children, the elderly, and immunocompromised individuals, dehydration can occur which may require medical attention.

Diagnosis

Norovirus infection is generally diagnosed based on clinical symptoms. Diagnostic testing is not widely performed to guide clinical management of individual patients, but laboratory testing of stool specimens to confirm the etiology and typing of strains is used to identify disease clusters during outbreak investigations.

Polymerase chain reaction (PCR)-based multipathogen diagnostic panels are increasingly available for clinical 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 also are available, but the specificity and sensitivity of these assays 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 or antiviral agents are currently available, but, as of April 2025, at least two vaccines are in clinical trials. Candidate vaccines against GI/GII genotypes have indicated cross-protection against non-GII.4 viruses, indicating that some level of cross-protection is possible. Further studies to estimate vaccine efficacy and length of protection are underway.

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. If clean water is available, washing hands with soap and water for ≥20 seconds is considered the most effective way to reduce norovirus contamination. While alcohol-based hand sanitizers might be useful between handwashing sessions, they 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 U.S. Environmental Protection Agency for norovirus disinfection; alternatively, they can use a dilute bleach solution (5–25 tablespoons of 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, cohorting, or exclusion of symptomatic staff, patients, and visitors in institutional settings, including hospitals, long-term care facilities, schools, and on cruise ships.

Acknowledgments

The following author contributed to the previous version of this chapter: Aron Hall.