Rotavirus: Properties of the Virus

Rotavirus

Rotavirus is the most common agent of gastroenteritis in child-ren aged 6 months to 2 years.

Properties of the Virus

◗ Morphology

Rotavirus shows following features:

·           The rotavirus has a distinctive morphological appearance by negative stain EM. The virus measures 70 nm in diam-eter and possesses three-layer icosahedral capsid without an envelope.

·           The sharply defined circular outline of the outer capsid gives the appearance of the rim of a wheel placed on short pokes radiating from a wide arm. This appearance gives the virus the name “rota” (Latin word which means wheel).

·           The genome consists of a segmented double-stranded RNA genome. The outer shell is composed of a major glycopro-tein with a molecular weight of 34,000 Da. This protein is known as viral protein (VP7). It also consists of a minor trypsin-sensitive protein with a molecular weight of 84,000 Da. This protein is now designated as VP4, which was earlier called VP3. The minor shell or core protein of the virus core consists of four proteins: VP1, VP2, VP3, and VP6. The virus also consists of six nonstructural proteins (NS53, NS34, NS35, NS28, NS26, and NS12).

◗ Viral replication

Rotaviruses replicate in cytoplasm of the host cell. Infection of the cell begins by attachment to the cell surface at the site of beta-adrenergic receptor. After entry into the cell, the virion uncoats and in the cytoplasm the RNA-dependent RNA poly-merase synthesizes mRNA from each of the 10–11 segments. These mRNAs encode a variety of structural and nonstructural proteins. RNA polymerase is one of those proteins that syn-thesizes minus strands, which become a part of progeny virus.

Capsid proteins, subsequently, form an incomplete capsid around the minus strands. Later on, the plus strands of progeny genome segment are synthesized. Finally, the virions are released by lysis of the cell.

◗ Antigenic and genomic properties

Human and animal rotaviruses have been classified into differ-ent serotypes, groups, and subgroups. Human rotaviruses have been classified to a total of 11 G serotypes primarily based on the VP7 (glycoprotein G) and VP4 (protease-sensitive protein P) proteins. Serotypes G1, G2, G3, or G4 have been identified with a majority of infections and are designated as serotype 1, 2, 3, or 4, respectively. There are six different HRP-P types. P type 1 is usually associated with G type 1, 3, or 4, whereas P type 1b is usually associated with a type 2.

Rotaviruses are classified into seven groups (from A to G) based on the antigenicity of VP6 and the electrophoretic mobil-ity of the genomic segments. Human disease is caused mostly by group A and occasionally by group B and C rotaviruses.

◗ Other properties

Rotaviruses are relatively stable at low temperature. They are also stable at extremes of pH (3.5–10) and also to repeated freezing and thawing. Treatment with proteolytic enzymes, such as trypsin, enhances the infectivity of the virus.

Virus Isolation

◗ Culture

Human rotaviruses are also difficult to grow. They do not grow readily in cell cultures. Rotavirus growth is facilitated if the viruses are pretreated with proteolytic enzyme, trypsin, and if low levels of trypsins are included in tissue culture medium. This causes cleavage of outer capsid protein of the virus and thereby facilitates uncoating.