Norovirus (a.k.a. Norwalk Virus)
Viral structure: Calcivirus family, positive-sense single-stranded RNA (+ssRNA) virus with naked capsule
Clinical Disease: Manifests primarily as watery, non-bloody diarrhea with abdominal cramps. Nausea/Vomiting is common and may distinguish it from similar viruses (e.g. astrovirus). Low-grade fever, myalgias, headaches are less common. Symptom onset is 12-48 hours after exposure. Duration of illness is 24-72 hours and is usually self-limiting.
Diagnosis: Diagnosis is by RT-PCR of a stool sample, which will be positive 48-72 hours after symptom onset and remain positive for up to 2 weeks. For epidemic situations, a clinical diagnosis may be made.
Treatment: There is no specific treatment. Therapy is supportive with rehydration, particularly for children, older adults and immunocompromised patients.
Epidemiology: Norovirus is the most common cause of foodborne illness in the USA (half of all cases, 23M cases/yr). It is usually from a point-source of contamination of food or water, although contamination of surfaces is also linked to transmission. It is transmitted by fecal-oral route and is found in stool and vomitus. 30% of infected people are asymptomatic but may spread the infection. Outbreaks have occurred in communal settings (schools, hospitals, cruise ships, restaurants.)
Prevention and control: There is no vaccine. Hygiene measures in food handlers will reduce transmission. Because of its naked capsule, it is highly resistant to detergents, desiccation, and acid. For outbreak settings, careful attention to hand hygiene and environmental surface cleaning with bleach are necessary.