Norovirus

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This page is about microbiologic aspects of the organism(s). For clinical aspects of the disease, see Norovirus infection.

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Overview

Norovirus is the cause of norovirus infection. Noroviruses (genus Norovirus) are a group of related, single-stranded RNA, nonenveloped viruses that cause acute gastroenteritis in humans. Noroviruses belong to the family Caliciviridae.

Causes

Common Causes

Less Common Causes

Norovirus

Norovirus was the first virus to present with gastroenteritis in human and was first identified from stool specimen^[1]. The illness caused by this virus was primarily given the term “winter vomiting disease”, due to its prominent gastrointestinal manifestations^[2]. An outbreak in 1968 at an elementary school in Norwalk lead to the identification of the virus. Norovirus is the leading cause of gastroenteritis among all age groups and is responsible for 64000 cases of diarrhea leading to hospitalization, 900,000 visits to the clinic among children of developed countries and 200,000 deaths among children below 5 in developing countries^[3]^[4].

Virology

Structure

Norovirus genome structure and protein coding regions: the genome is positive-sense single stranded RNA encoding three open reading frames (ORF). ORF1 encodes the nonstructural proteins. ORF2 and ORF3 encode the major capsid (VP1) and minor structural protein (VP2), respectively.^[5]
Structural proteins: Norovirus consists of 90 dimers of VP1 and one or two copies of the VP2.
- VP1: This major structural protein encoded by ORF2, consists of 530–555 amino acids with calculated molecular weights of 58–60 kDa. The protein has two conserved domains and a central variable domain with antigenic characteristics defining the specificity of the strain. VP1 assembles into virus-like particles^[6]. VP1 has two major domains; 1) the shell domain (S) and 2) the protruding domain (P). The S domain is on the N-terminal (225 amino acids), containing the elements for icosahedron formation^[7]. The P domain is comprised of the remaining amino acids and has two subdomains of P1 and P2. The P domain contributes to the stability of the capsid and formation of protrusions on the virion. P2 has a hypervariable region which is thought to play a role in receptor binding, immune reaction and interactions of ABO blood group antigens associated with susceptibility to the viral infection.^[8]^[5]
- VP2: This minor structural protein encoded by ORF3, ranges from 208–268 amino acids with calculated molecular weights of 22–29 kDa. VP2 shows high sequence diversity among strains. The exact function of this protein in the virus is not yet known. It is suggested that VP2 might contribute in RNA genome packaging. VP2 is not necessary for viral particles assembly but it is necessary for the formation of an infection virus. ^[5]
Nonstructural proteins^[5]
- p48 (p37)
- p22 (p20)
- VPg
- 3CLpro
- RdRp

Life Cycle

Norovirus has a cytoplasmic replication. It attaches to the host receptors and enters the cell through endocytosis. Since, it is a positive sense virus, replication and transcription follows the corresponding models for positive stranded RNA viruses. Translation occurs by leaky scanning, and RNA termination-reinitiation.^[9]

Genus	Host Details	Tissue Tropism	Entry Details	Release Details	Replication Site	Assembly Site	Transmission
Norovirus	Humans; mammals	Intestinal epithelium	Cell receptor endocytosis	Lysis	Cytoplasm	Cytoplasm	Oral-fecal

Gallery

Electron micrograph of the Norovirus. From Public Health Image Library (PHIL). ^[10]
Transmission electron micrograph (TEM) revealed some of the ultrastructural morphology displayed by Norovirus virions. From Public Health Image Library (PHIL). ^[10]
Transmission electron micrograph (TEM) revealed some of the ultrastructural morphology displayed by Norovirus virions. From Public Health Image Library (PHIL). ^[10]
Transmission electron micrograph (TEM) revealed some of the ultrastructural morphology displayed by Norovirus virions. From Public Health Image Library (PHIL). ^[10]
Transmission electron micrograph (TEM) revealed some of the ultrastructural morphology displayed by Norovirus virions. From Public Health Image Library (PHIL). ^[10]
Transmission electron micrograph (TEM) revealed some of the ultrastructural morphology displayed by Norovirus virions. From Public Health Image Library (PHIL). ^[10]
Transmission electron micrograph (TEM) revealed some of the ultrastructural morphology displayed by Norovirus virions. From Public Health Image Library (PHIL). ^[10]