Norovirus infection overview
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]
Overview
Noroviruses are a group of viruses that cause gastroenteritis, which is commonly known as the "stomach flu" or "food poisoning". Norovirus is a highly contagious virus. It is often transmitted via contaminated food or water, or by touching contaminated surfaces. The virus is one of the most common causes of acute gastroenteritis. Norovirus gastroenteritis results in abdominal pain (usually diffuse, stabbing pain), nausea, diarrhea (usually watery), and vomiting. There is no age or gender predisposition to the development of norovirus infection. Also, norovirus infection can occur upon subsequent exposures. Norovirus illness can be serious, especially for young children and older adults. Each year, it causes approximately 21 million illnesses and contributes to approximately 70,000 hospitalizations and 800 deaths. Norovirus is also the most common cause of foodborne-disease outbreaks in the United States. Preventive measures include proper hand washing and general cleanliness.
Historical Perspective
Originally called the "Norwalk agent" after Norwalk, Ohio, Norovirus was first discovered after an outbreak in an elementary school in 1968. The first documentation of the virus on electron microscopy was in 1972.
Classification
Norovirus constitutes a genus of genetically diverse single-stranded RNA, non-enveloped viruses belonging to the Caliciviridae family. The Norwalk virus is the only species belonging to that genus; however, several strains have been identified including: Hawaii virus, Mexico virus, and Desert Shield virus. These strains can genetically be classified into five different genogroups (GI, GII, GIII, GIV, and GV).
Pathophysiology
Noroviruses are highly contagious, given that as few as 10 particles are enough to cause the disease, transmission is via the fecal-oral route and peaks during the period when symptoms are most severe, and up to 3 days after recovery. As many strains of noroviruses exist, immunity is not gained following an infection. A non-functional fucosyltransferase FUT2 mutation has been associated with protection against the most common norovirus strains as FUT2 is required for viral transport in the small intestine.
Causes
Norovirus is the cause of norovirus infection. Noroviruses (genus Norovirus) are a group of related, single-stranded RNA, nonenveloped viruses that cause acute gastroenteritis in humans. Noroviruses belong to the family Caliciviridae.
Risk Factors
Noroviruses are transmitted directly via person to person or indirectly via contaminated water and foods. A CDC study of eleven outbreaks in New York State lists the suspected mode of transmission as person-to-person in seven outbreaks, foodborne in two, waterborne in one, and one unknown. The source of waterborne outbreaks may include water from municipal supplies, wells, recreational lakes, swimming pools and ice machines. [1]
Shellfish and salad ingredients are the foods most often implicated in Norwalk outbreaks. Ingestion of raw or insufficiently steamed clams and oysters poses a high risk for infection with the Norwalk virus. Foods other than shellfish are contaminated by ill food handlers.[2]
Natural History, Complications and Prognosis
The incubation period for norovirus-associated gastroenteritis in humans is usually between 24 and 48 hours (median in outbreaks 33 to 36 hours), but cases can occur within 12 hours of exposure. Recovery is usually complete and there is no evidence of any serious long-term sequelae.
Diagnosis
History and Symptoms
Norovirus infection usually presents as acute-onset vomiting, watery non-bloody diarrhea with abdominal cramps, and nausea. Low-grade fever also occasionally occurs, and vomiting is more common in children. Dehydration is the most common complication, especially among the young and elderly, and may require medical attention. Symptoms usually last 24 to 60 hours.
Physical Examination
Norovirus infection can present as mild febrile illnesss or a severe febrile illness with temperatures of 101-102 F.
Laboratory Findings
In the last 10 years, diagnosis of norovirus as cause of outbreaks of acute gastroenteritis has improved with the increasing use of the reverse transcriptase polymerase chain reaction (RT-PCR). Currently, state public health laboratories of 47 states have the capability to test for noroviruses by (realtime) RT-PCR. RT-PCR detects the norovirus RNA and can be used to test stool and emesis samples, as well as environmental swabs in special studies. Identification of the virus can be best made from stool specimens taken within 48 to 72 hours after onset of symptoms, although good results can be obtained by using RT-PCR on samples taken as long as 5 days after symptom onset. Virus can sometimes be found in stool samples taken as late as 2 weeks after recovery.
Sequencing of norovirus strains found in clinical and environmental samples has greatly helped in conducting epidemiologic investigations by linking cases to each other and to a common source and by differentiating outbreaks that were mistakenly connected. Sequences can be entered into CaliciNet, a recently developed sequence database on the basis of the PulseNet model. In the next years CaliciNet will be further implemented to be able to help to determine links (e.g., norovirus contaminated foods) between outbreaks across the U.S.
Older methods for diagnosis include direct and immune electron microscopy of fecal specimens, and detection of a fourfold increase of specific antibodies in acute- and convalescent-phase blood samples. Several commercially available enzyme-linked immunosorbent assays for detection of virus in stools have been developed but await evaluation further evaluation regarding sensitivity and specificity.
Treatment
Medical Therapy
There is no specific medicine to treat people with norovirus illness. Norovirus infection cannot be treated with antibiotics because it is a viral (not a bacterial) infection. Patients with norovirus infection should drink plenty of liquids to replace fluid lost from throwing up and diarrhea. This will help prevent dehydration. Sports drinks and other drinks without caffeine or alcohol can help with mild dehydration. But, these drinks may not replace important nutrients and minerals. Oral rehydration fluids that you can get over the counter are most helpful for mild dehydration. Dehydration can lead to serious problems. Severe dehydration may require hospitalization for treatment with intravenous fluids).
Primary Prevention
There is no vaccine to prevent norovirus infection. Prevention of foodborne norovirus disease is based on the provision of safe food and water. Noroviruses are relatively resistant to environmental challenge: they are able to survive freezing, temperatures as high as 60°C, and have even been associated with illness after being steamed in shellfish. Moreover, noroviruses can survive in up to 10 ppm chlorine, well in excess of levels routinely present in public water systems. Despite these features, it is likely that relatively simple measures, such as correct handling of cold foods, strict hand washing after using the bathroom and before handling food items, and paid sick leave, may substantially reduce foodborne transmission of noroviruses. Prevention of norovirus disease spread via droplets from vomitus (person to person transmission) should focus on methods to limit transmission including isolation precautions (e.g., cohort sick patients in a healthcare facility) and environmental disinfection.
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References
- ↑ Hedberg CW, Osterholm MT (1993). "Outbreaks of food-borne and waterborne viral gastroenteritis". Clin. Microbiol. Rev. 6 (3): 199–210. PMID 8395330.
- ↑ Parashar UD, Monroe SS (2001). ""Norwalk-like viruses" as a cause of foodborne disease outbreaks". Rev. Med. Virol. 11 (4): 243–52. doi:10.1002/rmv.321. PMID 11479930.