Norovirus - MegaMicro

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Microbiology > Virology > RNA viruses > Single Stranded RNA viruses > Calicivirus
Structure:   Family: Caliciviridae. “positive”- sense, non-segmented, single-stranded RNA (+ssRNA) virus, non-enveloped, icosahedral nucleocapsid.
• Worldwide, the most common cause of epidemic gastroenteritis as well as endemic diarrhea in community settings.  In the USA, it is responsible for at least half of all cases of foodborne illness (23M cases/yr).
• Seroprevalence in young adults nears 90%, however, immunity is not long lasting and reinfection can occur.
• Noroviruses are subdivided into at least 7 genogroups, with multiple genotypes within each group.  Genogroups GI, GII and GIV include human pathogens.  Despite genetic differences, clinical disease is generally indistinguishable.  GII has been associated with worldwide epidemic infections and more severe outcomes.
• Outbreaks often occur in communal settings (such as schools, daycare, hospitals, long-term care facilities, cruise ships and restaurants).

• Norovirus is shed in the stool, with person-to-person transmission via the fecal-oral route.
• Infection is also spread via airborne droplets of vomitus containing virus, contamination of fomites/surfaces or consumption of contaminated food/water.
• Extremely stable in the environment; resists alcohol, chlorine levels in water, freezing and heating to 60 C.
• The exact mechanism of norovirus induced vomiting and diarrhea is uncertain; no enterotoxin has been detected.

Disease Manifestations:
• Manifests primarily as watery, non-bloody diarrhea with abdominal cramps.  Nausea/vomiting is common; less common are fever, headache and myalgias.
• The incubation period is 24-48 hrs, duration of illness 24-72 hrs and shedding may occur for over 2 weeks.

Laboratory Diagnosis:
• Generally diagnosed clinically, but RT-PCR on stool samples is available. Virus is detectable within 48 hrs and may remain detectable for up to 2 weeks.  
• Laboratory testing is useful to document outbreaks and for patients with severe or persistent disease.  

Differential Diagnosis: Other enteric viruses (astrovirus, adenovirus, rotavirus), however, vomiting is less frequently noted.

• No specific medication; infection is self-limited.
• Rehydration, especially in children and older adults.

Prevention and control:
• No vaccine available.
• Hand hygiene with soap and water and cleaning of environmental surfaces with an appropriate bleach solution.
• Individuals cleaning areas heavily contaminated with stool and vomitus should wear protective equipment (mask, gloves and gowns).
• Good food sanitation practices.

Related concepts
Foodborne disease

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