What Is Norovirus?

When someone talks about having “the stomach flu,” they are probably describing acute-onset gastroenteritis caused by one of the noroviruses, which are members of the “calcivirus” family (Caliciviridae).[1] Noroviruses are entirely unrelated to influenza viruses.[2]

The Centers for Disease Control and Prevention (CDC) estimates that noroviruses cause nearly 21 million cases of acute gastroenteritis annually, making noroviruses the leading cause of gastroenteritis in adults in the United States. Norovirus is highly contagious and transmitted by infected individuals at an enormous rate. It is simple genetically and evolves quickly, and exposure does not lead to lasting immunity. It is estimated that each individual experiences norovirus five times during the course of their life.[3]

Nature has created an ingenious bug in norovirus. The round blue ball structure of norovirus is actually a protein surrounding the virus’s genetic material. [4]  The virus attaches to the outside of cells lining the intestine, and then transfers its genetic material into those cells. Once the genetic material has been transferred, norovirus reproduces, finally killing the human cells and releasing new copies of itself that attach to more cells of the intestine’s lining. 

The virus, due to its structure, is also very stable in the environment and is resistant to many sanitizers/disinfectants. It is a major concern for several sectors including health care, education, and tourism, and in food, shellfish, and produce. But most foodborne norovirus outbreaks occur in restaurants or institutional food service settings.

How Common is Norovirus?

Currently, norovirus is the most common cause of acute gastroenteritis in the United States.[5] While bacterial causes of foodborne illness, such as pathogenic Escherichia coli and Salmonella, are often cited as commonly reported sources of food poisoning, noroviruses cause 58% of foodborne illnesses acquired in the U.S. Thus, norovirus is the leading cause of outbreaks from contaminated food in this country. According to CDC estimates, this translates into about 2,500 reported norovirus outbreaks in the United States each year. Norovirus outbreaks occur throughout the year but are most common from November to April. About 1 in every 15 individuals in the U.S. will get norovirus illness annually. By five years of age, one out of every 14 children will visit an emergency room, and one out of 278 children will be hospitalized due to norovirus.

Norovirus outbreaks have been reported in many settings, including healthcare facilities, restaurants and catered events, schools, and childcare centers.[6] Cruise ships account for a small percentage (1%) of reported norovirus outbreaks overall.

Sources of Norovirus and Challenges Associated with the Virus

In addition to humans, norovirus can infect a broad range of hosts including livestock, pets, and wild animals (e.g., marine mammals and bats). Little is known about norovirus infections in most non-human hosts, but the close genetic relatedness between some animal and human noroviruses, coupled with a lack of understanding of where newly appearing human norovirus genotypes and variants are emerging from, has led to the hypothesis that norovirus may not be host restricted and might be able to jump the species barrier. To date, no animal noroviruses have been detected in human stool, but some serological evidence hints to possible transmission from animals to humans.

In humans, norovirus is transmitted primarily through the fecal-oral route, with as few as 10 to 100 virion particles needed to initiate infection. Transmission occurs either person-to-person or through contamination of food or water. It can also transmit through several other means such as droplets of vomit and fomites (surfaces contaminated with either feces or vomit) contaminated with norovirus. The infection spreads mainly among the people in crowded and enclosed places such as schools, shelters, hotels, resorts, nursing homes, cruise ships, and airline flights, even those of short duration. The probability of norovirus infections is increased among the people with compromised immune systems, as well as people following unhygienic practices. 

Historically, food is the most common vehicle of transmission for noroviruses; of 232 outbreaks of norovirus between July 1997 and June 2000, 57% were foodborne, 16% were spread from person-to-person, and 3% were waterborne. When food is the vehicle of transmission, contamination occurs most often through a food handler improperly handling a food directly before it is eaten.

How is norovirus transmitted?

Norovirus outbreaks can result from the evolution of one strain due to the pressure of population immunity. Typically, norovirus outbreaks are dominated by one strain, but they can also involve more than one strain. For example, some outbreaks associated with shellfish have been found to contain up to seven different norovirus strains. Swedish outbreak studies also reveal a high degree of genetic variability, indicating a need for wide detection methods when studying these outbreaks.[7]

Once infected, individuals begin viral shedding in stool and vomitus as soon as eight hours after exposure, with viral shedding peaking four days after exposure and, in some instances, continuing for up to 56 days. Given the high titers produced during viral shedding and the low number of norovirus particles that are needed to establish infection, it is possible that one individual could potentially infect thousands.[8]

Although norovirus-infected individuals shed the highest number of viral particles while they are ill, asymptomatic shedding is also possible for a short period of time before the onset of symptoms. Concentrations of shed norovirus particles are high both for symptomatic and asymptomatic infected individuals, and peak titers of shedding usually occur during the first five days after infection.

Food handlers and health-care workers are considered important contributing factors in the spread of norovirus both in foodborne outbreaks and in person-to-person transmitted outbreaks occurring in healthcare institutions. Several case reports have confirmed the role of infected food handlers in causing outbreaks, and the percentage of outbreaks in which food handlers were involved has been reported to be as high as 34% to 70%. In healthcare settings, nosocomial (hospital-acquired) norovirus transmission is mainly caused by symptomatic shedders, but asymptomatic transmission is not uncommon.[9]

Role of Restaurants

Food service establishments are routinely linked to foodborne illness outbreaks, and out of the over 9,000 foodborne illness outbreaks reported to the CDC between 1998 to 2004 (a vast underestimation of disease burden), over half (52%) were associated with restaurants or delicatessens.[10] Not surprisingly, noroviruses were identified in almost half of the foodborne outbreaks when a specific cause was determined, and restaurants were by far the most common setting/source for these outbreaks (64%). These findings have led the CDC and others to call upon the food service industry to instigate preventive measures and policies to curtail norovirus transmission in their facilities.[11]

Yet noroviruses are especially difficult to manage once introduced into a retail foodservice or grocery facility. Sometimes described as the “perfect human pathogen,” these viruses are able to last for up to two weeks on surfaces, are resistant to most commercially available disinfectants, and are easily spread by touch. Only a few virus particles are needed to make someone sick. In the case of a retail food setting, the presence of a single norovirus-infected staff member or customer can, within hours, lead to numerous cases of disease in the local population. Logistically, this results in staff absences, temporary closures of facilities for disinfection, and the need to dispose of potentially contaminated food, although there are also broader impacts such as legal expenses and damage to brand.[12]

The history of norovirus outbreaks has led the CDC and others to call upon the restaurant industry to instigate preventive measures and policies to curtail norovirus transmission in their facilities including handwashing, cleaning, and sanitizing, and implementing employee health policies.

Symptoms & Risks of Norovirus Infection

The most common symptoms are sudden onset of vomiting and watery diarrhea, although stomach cramps and pain also often occur. Some people experience fever and body aches. Symptoms usually start 12 to 48 hours after being exposed and typically last about 1 to 3 days.[13]

Although symptoms usually only last 1 to 2 days in healthy individuals, norovirus infection can become quite serious in children, the elderly, and immune-compromised individuals. In some cases, severe dehydration, malnutrition, and even death can result from norovirus infection, especially among children, and among older and immune-compromised adults in hospitals and nursing homes. 

Diagnosing a Norovirus Infection

Diagnosis of norovirus illness is based on the combination of symptoms, particularly the prominence of vomiting, fever, and short duration of illness. If a known norovirus outbreak is in progress, public health officials may obtain specimens from ill individuals for testing in a lab.[14] Lab tests consist of identifying norovirus under an electron microscope. A reverse transcriptase polymerase chain reaction test (RT-PCR assay) can also detect norovirus in food, water, stool samples, and on surfaces. These tests isolate and replicate the suspected virus’s genetic material for analysis. An ELISA can also be performed, which detects antigens; they are easier to perform than RT-PCR, but less sensitive and can also result in many false negatives.

Treating a Norovirus Infection

Collecting a stool sample and using molecular methods to find viral RNA is the preferred method to test for norovirus in the public health world. In many cases, however, diagnosis is made based on symptoms. Most people who become ill recover within 24 to 48 hours after symptoms with just rest and fluids to prevent dehydration. Antibiotics are not effective, as the illness is caused by a viral pathogen.[15]

There is no specific treatment available for norovirus. In most healthy people, the illness is self-limiting; however, outbreaks among infants, children, elderly, and immune-compromised populations may result in severe complications among those affected. Death may result without prompt measures. A vital part of therapy for norovirus is the replacement of fluids and minerals such as sodium, potassium, and calcium—otherwise known as electrolytes—lost due to persistent vomiting and diarrhea. This rehydration can be done either by drinking large amounts of liquids, or intravenously.

Preventing Norovirus Infection

The role of food handlers has been documented substantially, highlighting that keeping ill food handlers out of the kitchen coupled with proper handwashing is the best strategy to prevent norovirus. Outbreaks linked to infected restaurant staff in the past have led to multiple meals served from the same kitchen causing illness. In a 2006 outbreak in Michigan, the investigation revealed that several foodservice workers had been ill and continued to work, ultimately leading to environmental contamination and persistence when a line cook had vomited in the restaurant. At least 364 restaurant patrons became ill with gastroenteritis consistent with norovirus. The investigation also identified deficiencies with employee handwashing practices, cleaning, and sanitizing of food and nonfood contact surfaces.

The good news about norovirus is that it does not multiply in foods as many bacteria do. In addition, thorough cooking destroys the virus. To avoid norovirus, make sure the food you eat is cooked completely. While traveling in areas that have polluted water sources, raw vegetables should be washed thoroughly before being served, and travelers should drink only boiled drinks or carbonated bottled beverages without ice.[16]

Shellfish (i.e., oysters, clams, mussels) pose the greatest risk, and any particular serving may be contaminated with norovirus; there is no way to detect a contaminated oyster, clam, or mussel from a safe one. Shellfish become contaminated when their waters become contaminated—e.g., when raw sewage is dumped overboard by recreational or commercial boaters. Shellfish are filter feeders and will concentrate virus particles present in their environment. With shellfish, only complete cooking offers reliable protection; steaming does not kill the virus or prevent its transmission. Some researchers suggest that norovirus monitoring in shellfish areas could be a good preventive strategy. Waterborne norovirus outbreaks are ubiquitous, but difficult to recognize. Improved analysis of environmental samples would have the potential to significantly improve the detection of norovirus in shellfish waters.[17]

[1]           Tripathi, M., & Kumar, S. (2019). Developments of an Emerging Infectious Agent: Norovirus. Annual Research & Review in Biology31(4), 1-6. https://doi.org/10.9734/arrb/2019/v31i430054

[2]           Desai AN. (2019). What Is Norovirus? JAMA. 322(20):2032.

[3]           “Updated Norovirus Outbreak Management and Disease Prevention Guideline.” Centers for Disease Control and Prevention, 4 Mar 2011. Available at: https://www.cdc.gov/mmwr/preview/mmwrhtml/rr6003a1.htm.

[4]           Kallem, S., & Bigback, K. (2005, September 1). Caliciviridae. Retrieved September 07, 2020, from https://web.stanford.edu/group/virus/calici/2005kallem/caliciviridae.html

[5]           Chikara Ogimi, Yae Jean Kim, Emily T Martin, Hee Jae Huh, Cheng-Hsun Chiu, Janet A Englund, What’s New With the Old Coronaviruses?, Journal of the Pediatric Infectious Diseases Society, Volume 9, Issue 2, June 2020, Pages 210–217, https://doi.org/10.1093/jpids/piaa037

[6]           “Common Settings of Norovirus Outbreaks.” Centers for Disease Control and Prevention, 8 Oct 2020. Available at: https://www.cdc.gov/norovirus/trends-outbreaks/outbreaks.html.

[7]           Lopman B, et al. (2012). Environmental transmission of norovirus gastroenteritis. Curr Opin Virol. 2(1):96-102.

[8]           Marsh Z, et al. (2018). Epidemiology of Foodborne Norovirus Outbreaks – United States, 2009-2015. Food Safety. 6(2):58-66.

[9]           Lopman B, Simmons K, Gambhir M, Vinjé J, Parashar U. (2014). Epidemiologic implications of asymptomatic reinfection: a mathematical modeling study of norovirus. Am J Epidemiol. 179(4):507-12.

[10]         Hall AJ, Wikswo ME, Pringle K, Gould LH, Parashar UD. (2014). Vital Signs: Foodborne Norovirus Outbreaks – United States, 2009-2012. MMWR. 63(22):491-5.

[11]         Sabrià A, et al. (2016). Norovirus shedding among food and healthcare workers exposed to the virus in outbreak settings. J Clin Virol. 82:119-25.

[12]         Kosa KM, Cates SC, Hall AJ, Brophy JE, Fraser A. (2014) Gaps in Food Safety Professionals’ Knowledge about Noroviruses. J Food Prot. 77(8):1336-41.

[13]         “Facts About Noroviruses.” Centers for Disease Control and Prevention, 15 Oct 2018. Available at: https://www.cdc.gov/nceh/vsp/pub/Norovirus/Norovirus.htm.

[14]         Hall AJ, et al. (2011). Updated Norovirus Outbreak Management and Disease Prevention Guidelines. MMWR. 60(RR03):1-15.

[15]         “Norovirus infection.” Mayo Clinic, 5 Feb 2020. Available at: https://www.mayoclinic.org/diseases-conditions/norovirus/symptoms-causes/syc-20355296.

[16]         Lopman B, et al. (2012). Environmental transmission of norovirus gastroenteritis. Curr Opin Virol. 2(1):96-102

[17]         “Facts About Noroviruses.” Centers for Disease Control and Prevention, 15 Oct 2018. Available at: https://www.cdc.gov/nceh/vsp/pub/Norovirus/Norovirus.htm.