Virus Classification - ICTV Classification

ICTV Classification

The International Committee on Taxonomy of Viruses began to devise and implement rules for the naming and classification of viruses early in the 1970s, an effort that continues to the present. The ICTV is the only body charged by the International Union of Microbiological Societies with the task of developing, refining, and maintaining a universal virus taxonomy.

The system shares many features with the classification system of cellular organisms, such as taxon structure. However, this system of nomenclature differs from other taxonomic codes on several points. A minor point is that names of orders and families are italicized, unlike in the International Code of Nomenclature for algae, fungi, and plants and International Code of Zoological Nomenclature.

Viral classification starts at the level of order and follows as thus, with the taxon suffixes given in italics:

Order (-virales)

Family (-viridae)

Subfamily (-virinae)

Genus (-virus)

Species

Species names generally take the form of virus.

The establishment of an order is based on the inference that the virus families contained within a single order have most likely evolved from a common ancestor. The majority of virus families remain unplaced. Currently (2011), six orders, 87 families, 19 subfamilies, 349 genera, and 2,284 species of viruses have been defined.

Six orders have been established to date by the ICTV: the Caudovirales, Herpesvirales, Mononegavirales, Nidovirales, Picornavirales, and Tymovirales. These orders span viruses with varying host ranges. A seventh order (Ligamenvirales) infecting archaea has been proposed.

Caudovirales are tailed dsDNA (group I) bacteriophages.

Herpesvirales contain large eukaryotic dsDNA viruses.

Mononegavirales include nonsegmented (-) strand ssRNA (Group V) plant and animal viruses.

Nidovirales are composed of (+) strand ssRNA (Group IV) viruses with vertebrate hosts.

Picornavirales contains small (+) strand ssRNA viruses that infect a variety of plant, insect and animal hosts.

Tymovirales contain monopartite (+) ssRNA viruses that infect plants.

Other variations occur between the orders: Nidovirales, for example, are isolated for their differentiation in expressing structural and nonstructural proteins separately.