The most common cause of gastroenteritis in the U.S. among patients of all age groups.

Norwalk agent the leading cause of epidemics of gastroenteritis and an important cause of sporadic gastroenteritis in children and adults.

Single positive-strand RNA virus, approximately 7.7 kb in size, enclosed in a non-enveloped protein coat with cuplike depressions.

Genus norovirus and family Caliciviridae.

At least 25 genotypes exist, along with numerous subgroups, but genogroup II, genotype four is responsible for majority of cases, and outbreaks.

Have not been grown in cell culture.

The virus is known to persist on environmental surfaces for up to two weeks, and resists many commercial disinfectants.

As few as 18 virus particles are enough to cause an infection.

Susceptibility to Norwalk virus is dependent on the expression of a functional focosyl-transferase 2 (FUT2) gene, and individuals who had a nonfunctional FUT2 gene are genetically resistant to such virus infections.

Mutations are responsible for genetic drift, and the emergence of variance responsible for epidemics.

Humans are the only known hosts.

Estimated 23 million cases per year in the U.S.

Estimated 570-800 deaths per year.

Estimated to be responsible for 56000-71000 hospitalizations and 400,000 ED visits and 1.8 million outpatient visits per year.

US resident has a lifetime risk of having a norovirus gastroenteritis in his or her lifetime.

Average incubation period is 24-48 hours.

Infection manifested by acute onset of vomiting and/or diarrhea that lasts 12-60 hours.

Approximately two thirds of people infected are symptomatic.

Generally mild and of short duration, but at times can be severe and even fatal, especially in the young and elderly.

Primary route of transmission of noroviruses is fecal-oral, with consumption of contaminated water or food, person-to-person contact, and from contaminated objects or environments.

Can be transmitted by airborne vomit droplets.

Primary cases may come from fecal contamination and secondary cases may result from person-to-person contact.

Infectious vomitus may play a role in infection transmission.

Associated with transmission with a low infectious dose of approximately 18 to 1000 viral particles to allow spread through droplets, fomites, person-to-person contact and environmental contamination (Teunis PF).

Low dose exposure can lead to infection, virus can be shed in a prolonged and asymptomatic fashion.

Secondary attack rates of 30% or more among family members and close contacts, indicates the significant infectious nature of the virus.

Viral shedding precedes the clinical onset of illness in up to 30% of exposed patients and may continue long after the illness abates, and can increase the potential risk of secondary spread among family members and food handlers.

The virus can survive in temperatures from freezing to 60°C and 10 persist on environmental surfaces, in recreation bodies of water, drinking water, and in foods such as raw oysters and fruits and vegetables exposed to sewage and uncooked.

Fruit and vegetables, particularly leafy vegetables, have been implicated in nearly half the gastrointestinal infections caused by norovirus in the United States. 

Patients having an infection lack a lasting immunity and ca nhave repeated infections throughout life.

Substantial strain diversity exists, and epidemic strains may be more virulent and persistent than other nonepidemic strains.

Prevention and control of outbreaks emphasizes the exclusion of ill caregivers, and food handlers from work settings and the utilization of hand washing techniques.

Infected individuals must utilized hand washing techniques and avoid food preparation.

Foods contaminated by persons with gastroenteritis should be discarded.

Polymerase-chain-reaction is the standard method for detecting the virus in feces, water and food specimens.

PCR analysis is indicated that more than 90% of gastroenteritis outbreaks that were previously unknown or presently attributed to an oral virus (Fankhauser RL).

Outbreaks involved individuals in nursing homes, day care centers, hospitals, cruise ships, and restaurants.

High-risk groups for outbreaks include young children, elderly, soldiers, travelers and immunocompromised individuals.

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