Noroviruses

Noroviruses are a group of non-enveloped, single-stranded RNA viruses in the family Caliciviridae that cause acute gastroenteritis in humans and various animals. They are highly contagious and are a leading cause of gastroenteritis worldwide, producing both sporadic cases and numerous outbreaks, especially in crowded settings such as ships, schools, and care facilities.

Taxonomy and genome: The genus Norovirus includes several genogroups (GI–GVI), with GI, GII, and GIV known to infect humans. The virion is icosahedral, and the genome is about 7.5 kilobases of positive-sense RNA organized into several open reading frames, encoding a major capsid protein and nonstructural proteins. Antigenic diversity, particularly in GII.4 strains, underlies many epidemic waves.

Transmission and epidemiology: Noroviruses spread primarily by the fecal-oral route, including direct person-to-person contact, contaminated food or water, and contaminated surfaces. The infectious dose is low and the virus is highly stable in the environment, resisting many common disinfectants unless proper concentrations are used. The typical incubation period is 12–48 hours, and symptoms usually last 1–3 days.

Clinical features and diagnosis: Illness is characterized by sudden nausea, vomiting, watery diarrhea, abdominal cramps, and sometimes fever. Dehydration is a risk, especially in young children and older adults. Diagnosis can be clinical but is often confirmed by reverse transcription PCR on stool samples; antigen tests are also available but vary in sensitivity.

Treatment and prevention: Management is supportive, focusing on rehydration and electrolyte balance. There is no specific antiviral. Prevention relies on hand hygiene, thorough surface disinfection with chlorine-containing agents, safe food handling, and isolating ill individuals from food preparation. Immunity after infection is partial and short-lived; reinfections occur with different strains. Vaccines are under development.

History: Noroviruses were first identified in an outbreak in Norwalk, Ohio, in 1968 and were later named for the genus. They are now recognized as a leading global cause of gastroenteritis.