Norovirus, an RNA virus, is the leading cause of vomiting and diarrhea from acute gastroenteritis across all age groups in the United States. Molecular tests are the most reliable and rapid way to detect norovirus and to differentiate it from other causes of gastroenteritis. Early use of molecular tests may limit the spread of disease, especially in healthcare facilities and schools, and may play an important role in limiting the ineffectual use of antibiotics.
Norovirus is the most common source of foodborne disease outbreaks, accounting for 38% (1,130) of outbreaks between 2009 and 2015.1 It is also a common cause of sporadic illness. According to the most recent figures from the Centers for Disease Control and Prevention (CDC), norovirus causes an average of 570-800 deaths, 56,000-71,000 hospitalizations, 400,000 emergency department visits, almost 2 million outpatient visits, and at least 19 million illnesses each year in the United States.2 The highest rates of medical visits due to infection are in children less than five years old, while the risk for death from norovirus infection is greatest in those 65 years of age or older. Norovirus disease is most common from October to March but can occur year-round. Case rates may increase by 50% when a new strain of the virus emerges, typically every few years.3
Clinical features and routes of transmission
Noroviruses are highly contagious, with a minimal infective oral dose as low as 18 viral particles.4 The incubation period for norovirus is 12-48 hours. Affected individuals develop non-bloody diarrhea, vomiting, nausea, and/or abdominal cramps, along with low-grade fever and body aches. The symptoms develop rapidly, typically last 1-3 days, and resolve without treatment in healthy individuals. More prolonged courses of illness lasting 4–6 days can occur, particularly among young children, the elderly, and hospitalized patients. Norovirus infection in immunocompromised adults can become chronic and persist for weeks to years. The CDC estimates that about 10% of people with norovirus gastroenteritis seek medical attention.
Infection can be spread through contact with food, water, or contaminated surfaces, or through direct person-to-person contact. Approximately one-fourth of total norovirus infections are caused by foodborne transmission.5 The majority of norovirus outbreaks involve person-to-person transmission.
The most common settings for norovirus outbreaks in the United States are schools and healthcare facilities, including nursing homes and hospitals.6 Virus can be introduced into healthcare facilities by staff, visitors, and patients, or by contaminated food products. Outbreaks in healthcare facilities may last months and can have severe outcomes for hospitalized patients. Rapid identification of norovirus as the cause of illness in a school or healthcare setting may allow early implementation of control measures that can slow its spread. Measures include isolation or cohorting of symptomatic or exposed patient(s) or exclusion of affected staff, along with restricting visits and new admissions until the outbreak is under control.
Prevention and treatment
In the absence of a vaccine, prevention is the only protection against norovirus infection. Standard precautions, including hand hygiene, disinfection, and use of disposable gloves, when exposure is unavoidable, provide significant protection from spreading infection. In addition, food safety measures such as scrupulous washing can further limit food-borne transmission.
There is no specific treatment for norovirus illness. The biggest challenge is to keep the affected person hydrated to replace fluid lost from vomiting and diarrhea. Dehydration may require hospitalization for effective oral rehydration.
Diversity and Molecular Biology of Noroviruses
Noroviruses are classified into six genogroups (GI-GVI) based on amino acid identity in the major structural protein (VP1). The genogroups are further divided into distinct genotypes on the basis of sequence similarity in the VP1 genome. The strains that infect humans belong to genogroups GI, GII and GIV. GII.4 strains are more likely to be associated with person-to-person transmission, especially in long-term care facilities and hospital settings, whereas GI and other GII strains are more frequently associated with foodborne or waterborne transmission.
According to the CDC, genotyping the causative virus can help in epidemiologic investigations by linking cases, and thus potentially identifying the source of exposure, or by identifying emerging strains.6
Like other RNA viruses, noroviruses mutate rapidly. This constant changing of norovirus genomic sequence is one reason that those exposed to the virus do not develop immunity, and why vaccines are currently lacking. The rapidity of change in the viral genome is driven in part by the human immune system, which selects for those viral capsid epitopes that are not controlled by immune defenses. This leads to turnover in the dominant circulating viruses on about a three-year cycle, increasing the likelihood of an outbreak and a spike in cases.
Molecular testing provides a definitive diagnosis of norovirus infection and eliminates the use of unnecessary antibiotic treatment. Molecular diagnosis also provides critical information for healthcare personnel in planning how to control an outbreak of gastroenteritis in hospitals, nursing homes, and other facilities. Rapid determination of the causative agent saves time and expense that would otherwise be spent on less definitive tests.
FDA-approved diagnostic procedures for norovirus are based on the detection of virus in stool samples by enzyme immunoassays (EIA) or reverse transcription polymerase chain reaction (PCR) or other molecular methods. Norovirus is also a target in the larger gastrointestinal panels which simultaneously test for the presence of multiple disease-causing microbes in a stool sample.
Stool specimens contain the highest concentration of norovirus; therefore, stool is a preferred specimen for laboratory diagnosis of norovirus, and the only validated specimen for FDA-approved methods for clinical diagnosis. Stool specimens should be obtained during the acute phase of illness (ie, within 48-72 hours after onset) while the stools are still liquid or semisolid and viral load is at its peak.
Enzyme immunoassays (EIA) detect norovirus antigen. While they are rapid and relatively inexpensive, they have low sensitivity (36% to 80%) and specificity (47% to 100%) in comparison to molecular methods, leading to a high rate of false negatives and a somewhat lower rate of false positives. The development of a more efficient EIA is hindered by the high number of norovirus strains and constant evolution of new strains with distinct antigens. Because of the modest performance of EIA, this method is not recommended for clinical diagnosis of norovirus infection in sporadic cases of gastroenteritis. In outbreaks, it may have utility as a rapid screen for a large number of samples, but negative samples should still be tested with a high-sensitivity technique such as polymerase chain reaction (PCR), according to the CDC.7 Thus, it may be cost-effective to consider PCR as the initial test, despite its higher cost.
Real-Time PCR (RT-PCR) assays
Sensitivity and specificity of PCR-based assays range from 90% to 100%, depending on the assay. Because of this, RT-PCR assays have become the gold standard for norovirus detection. The molecular assays detect a highly conserved region in the norovirus genome to identify not only the virus, but the specific genogroup. High sensitivity permits detection of norovirus in samples from people without disease but who are shedding the virus.
Quest Diagnostics norovirus testing
Quest Diagnostics offers an RT-PCR test for genogroup I and II (test code 35980), performed on an unpreserved frozen stool sample. The report is returned within 1-2 days.
1. Dewey-Mattia D, Manikonda K, Hall AJ, Wise ME, Crowe SJ. Surveillance for Foodborne Disease Outbreaks — United States, 2009–2015. MMWR Surveill Summ 2018;67(No. SS-10):1–11.
2. “Burden of Norovirus Illness and Outbreaks Figure.” Centers for Disease Control and Infection,October 5, 2018. www.cdc.gov/norovirus/trends-outbreaks/illness-outbreaks-figure.html
4. Teunis PF, Moe CL, Liu P, Miller SE, Lindesmith L, Baric RS, Le Pendu J, Calderon RL. Norwalk virus: how infectious is it? J Med Virol. 2008 Aug;80(8):1468-76. doi: 10.1002/jmv.21237.
3. Hall AJ, Lopman BA, Payne DC, et al. Norovirus Disease in the United States. Emerging Infectious Diseases. 2013;19(8):1198-1205.
5. Hall AJ , Vinjé J, Lopman B, et al. Updated Norovirus Outbreak Management and Disease Prevention Guidelines MMWR Recommend Rep 2011;60(RR03);1-15.
6. Kaplan JE, Feldman R, Campbell DS, Lookabaugh C, Gary GW. The frequency of a Norwalk-like pattern of illness in outbreaks of acute gastroenteritis. Am J Public Health. 1982 Dec;72(12):1329-32.
7. “Laboratory Diagnosis.” Centers for Disease Control and Infection,October 5, 2018. www.cdc.gov/norovirus/lab/diagnosis.html