Norwalk Virus

Norwalk Virus

Norwalk virus is one of the most common causes of viral gas-troenteritis in adults.

Classification

Norwalk virus is a calcivirus in the family Calciviridae. The calciviruses are small, nonenveloped, single-stranded RNA viruses. The calciviruses although show many features similar to those of picornaviruses, they differ from those by having a large genome and by having distinct spikes on the surface. There are two important calciviruses, which cause infection in humans: Norwalk virus and Hepatitis E virus.

Properties of the Virus

Norwalk virus shows following features:

·           Norwalk viruses are round viruses with a ragged outline.

·           The genome is a nonsegmented, single-stranded, positive-polarity RNA genome. The RNA genome is present in a 27-nm naked capsid consisting of one 60,000-Da capsid protein. The genome does not contain any virion polymerase.

·           Ten prominent spikes and 32 cup-shaped depressions can be seen on the virion by microscopy.

◗ Viral replication

The Norwalk virus replicates in the cytoplasm with release of viral particles on cell destruction. The virus is presumed to replicate in a manner similar to that of picornaviruses.

◗ Antigenic and genomic properties

The number of serotypes is not known.

◗ Other properties

Norwalk viruses are stable and can survive with freezing or at temperatures of up to 140°F. They are also resistant to inactivation by chlorination of water and to drying in the environment.

Pathogenesis and Immunity

Norwalk viruses are highly contagious. As low as 100 viral particles can establish infection, which is typically limited to the mucosal cells of the intestinal tract. Infection is charac-terized by damage to microvilli in the small intestine, causing malabsorption. The virus-mediated changes in gastric mucosa and delayed gastric emptying cause vomiting. It is a noninvasive virus and does not cause invasion of the colon; therefore leukocytes or erythrocytes in the feces are typically absent. No histopathological lesions are seen in the gastric mucosa.

◗ Host immunity

Norwalk virus infection confers a brief and short immunity. Recurrent infection occurs throughout life, because of the absence of long-term immunity, lack of cross-strain immunity, and because of the diversity of Norwalk virus strains.

Clinical Syndromes

Norwalk viruses cause gastroenteritis in adults. The illness in symptomatic cases typically begins after an incubation period of 24–48 hours. The illness is characterized by sudden onset of nausea, vomiting, and watery diarrhea. It is accompanied by low-grade fever, abdominal cramps, and myalgia. Fecal leuko-cytes are absent. Norwalk virus gastroenteritis is short-lived and typically lasts for 24–48 hours.

Epidemiology

Norwalk virus gastroenteritis is found worldwide in adults. It is a strict human infection, and humans are the major source of infection. The virus is excreted in the vomitus and feces for several weeks after recovery; hence vomitus and feces are important sources of infection. Infection is transmitted from person to person by ingestion of food or water contaminated with the virus. The infection typically occurs in group settings, such as schools, hospitals, nursing homes, etc.

Laboratory Diagnosis

Immunoelectron microscopy is used to detect the virus in the stool. Radioimmunoassay (RIA) or ELISA are also used to detect the virus and viral antigen in the stool. Polymerase chain reaction (PCR) is also being evaluated to demonstrate viral genome in the stool for diagnosis of the cases. Both ELISA and RIA are the serodiagnostic tests frequently used to detect specific antibodies to Norwalk virus in the serum.

Treatment

No specific treatment is available for Norwalk virus.

Prevention and Control

No vaccine is available against the virus. Strict personal hygiene, such as hand washing is useful to prevent the transmission of the disease.