Norovirus - A Main Cause of Viral Gastroenteritis

Norovirus is a member of the genus of viruses. It comes under the family of Caliciviridae whose members are class IV of the Baltimore scheme. They are composed of RNA, which are single stranded and non-segmented. Caliciviruses are found mostly in animals of the bovine category, such as cattle, pigs, chickens, reptiles, dolphins and amphibians as well as human beings. The calciviruses have a simple structure. The protein shell or capsule appears to be hexagonal or spherical. It has a diameter of 35 to 39 nm. They cannot be reproduced in a culture medium and only recent advancement in virology has enabled us to recognize this strain of virus. The norovirus consist of a single RNA strand with a genome of approximately 7.5 kbp, encoding a major structural protein (VP1) of about 58 to 60 kd along with a minor capsule protein (VP2). The viral particle has a smooth amorphous surface structure when visualized under an electron microscope. They range between 27 to 38 nm in size.

A recent survey has revealed that almost 50% of all the gastroenteritis found nowadays is caused by norovirus. The term norovirus now encompasses the whole group of viruses under the Norwalk virus group or Norwalk like virus group.

Norovirus can genetically be classified under 5 different groups of GI to GV. It can be further divided into different genetic groups depending upon the strains affecting different species. For example Genotype II is most prevalent in human beings, whereas genotype III affects the bovine population predominantly as in cows. Even Genotype II which affects the human beings can be further divided in to 19 varieties within the same genotype. Genotype V mostly affects species of mice. Norwalk viruses belonging to the genotype II and genotype IV are responsible for the outbreak of the majority of gastroenteritis in sheep all over the world. Last year there was an enormous increase in the number of Norovirus infection world wide.