Calicivirus - Classification, Norovirus, Sapovirus, Morphology, Genome, Habitat, Transmission

Classification of Calicivirus

The classification of Calicivirus is done by phenotypic characteristics.

Realm: Riboviria

Kingdom: Orthornavirae

Phylum: Pisuviricota

Class: Pisoniviricetes

Order: Picornavirales

Family: Caliciviridae

Among the Calciviridae family, the two genera Norovirus and Sapovirus are medically important.

Habitat, Transmission of Calicivirus

Calicivirus is the common cause of diarrhoeal disease occurring worldwide and is transmitted by the fecal-oral route and fomites.

Norovirus is a major cause of food and water-associated diarrhoeal outbreaks and vomiting throughout the world while Sapovirus are associated with sporadic cases of diarrhea and vomiting.

Very young and elderly are at the most risk and require rehydration prolonged excretion is seen in immunocompromised patients. Cold foods and shellfish harvested from contaminated seawater are important sources of infection as the Calicivirus habitats there.

Morphology of Calicivirus

Sapovirus are non-enveloped viruses with icosahedral symmetry. The calices formed by 32 cups are characteristic surface morphology. Factors such as freezing and thawing, proteolytic enzymes, or incorrect staining can affect the appearance of the virion.

Norovirus has a similar structure but the tips of the capsomers are bent and partially obscure the hollous resulting in an amorphous surface structure with a ragged outline.

Morphologically, the complete virion measures 35-39nm in diameter with a solid inner shell radius of 11.5-15nm surrounding RNA.

Genome of Calicivirus

Calicivirus are single-stranded RNA viruses with a linear genome that is 7.5kb in size.

The Sapovirus contains 2 ORFs:

• ORF 1 = encodes non-structural proteins and major structural protein (capsid) [VP1]

• ORF 2 = In the Manchester strain, it encodes a putative protein with an unknown function

The Norovirus contains 3 0RFs:

• ORF 1 = encodes a polyprotein that is subsequently cleaved into a set of non-structural proteins during replication

• ORF 2 = encodes major structural (capsid) protein

• ORF 3 = encodes a minor structural protein (VP2)

Calciviruses possess highly conserved regions within ORF1 encoding helicase (2C), a protease (3C), and RNA-dependent RNA polymerase (3D). These sites can be used to direct primers for RT-PCR. The VPg protein is present at the 5’ terminal while the 3’-terminal has a poly(A) tail

Difference between Sapovirus and Norovirus

The Sapovirus and Norovirus can be differentiated into following types.