The International Committee on Taxonomy of Viruses (ICTV) is a committee which authorizes and organizes the taxonomic classification of viruses.^[1] They have developed a universal taxonomic scheme for viruses and aim to describe all the viruses of living organisms. Members of the committee are considered to be world experts on viruses^[2]. The committee formed from and is governed by the Virology Division of the International Union of Microbiological Societies. Detailed work such as delimiting the boundaries of species within a family is typically done by study groups, which consist of experts in the families.^[1]

The committee also operates an authoritative database (ICTVdB) containing taxonomic information for 1,950 virus species, as of 2005.^[1] It is open to the public and is searchable by several different means.

Objectives

The official objectives of the ICTV are:

1. To develop an internationally agreed taxonomy for viruses
2. To develop internationally agreed names for virus taxa, including species and subviral agents
3. To communicate taxonomic decisions to all users of virus names, in particular the international community of virologists, by publications and via the Internet
4. To maintain an index of virus names
5. To maintain an ICTV database on the Internet, that records the data that characterize each named viral taxon, together with the common names of each taxon in all major languages

Principles of nomenclature

The ICTV's essential principles of virus nomenclature are:

• Stability
• To avoid or reject the use of names which might cause error or confusion
• To avoid the unnecessary creation of names

The ICTV's universal virus classification system uses a slightly modified version of the standard biological classification system. It only recognises the taxa order, family, subfamily, genus, and species. When it is uncertain how to classify a species into a genus but its classification in a family is clear, it will be classified as an unassigned species of that family. Many taxa remain unranked. There are also, as of 2005, GenBank sequences assigned to 3,142 "species" which are not accounted for in the ICTV report (due to the way GenBank works, however, the actual number of proper species is probably significantly smaller).^[1] The number of unidentified virus sequences is only expected to increase as the rate of virus sequencing increases dramatically.^[1]

The ICTV has been strikingly successful in achieving stability, since their inception in 1962. Every genus and family recognized in the 1980s continued to be in use as of 2005, for example.^[1]

Naming and changing taxa

Proposals for new names, name changes, and the establishment and taxonomic placement of taxa are handled by the Executive Committee of the ICTV in the form of proposals. All relevant ICTV subcommittees and study groups are consulted prior to a decision being taken.

The name of a taxon has no status until it has been approved by ICTV, and names will only be accepted if they are linked to approved hierarchical taxa. If no suitable name is proposed for a taxon, the taxon may be approved and the name be left undecided until the adoption of an acceptable international name, when one is proposed to and accepted by ICTV. Names must not convey a meaning for the taxon which would seem to either exclude viruses which are rightfully members of that taxa, exclude members which might one day belong to that taxa, or include viruses which are members of different taxa.

Rules for taxa

Species

A species name shall consist of as few words as practicable but must not consist only of a host name and the word virus. A species name must provide an appropriately unambiguous identification of the species. Numbers, letters, or combinations thereof may be used as species epithets where such numbers and letters are already widely used. However, newly designated serial numbers, letters or combinations thereof are not acceptable alone as species epithets. If a number or letter series is in existence it may be continued.

Genera

A virus genus is a group of related species that share some significant properties and often only differ in host range and virulence. A genus name must be a single word ending in virus. Approval of a new genus must be accompanied by the approval of a type species.

Subfamilies

A subfamily is a group of genera sharing certain common characters. The taxon shall be used only when it is needed to solve a complex hierarchical problem. A subfamily name must be a single word ending in virinae.

Families

A family is a group of genera, whether or not these are organized into subfamilies, sharing certain common characters. A family name must be a single word ending in viridae.

Orders

An order is a group of families sharing certain common characters. An order name must be a single word ending in virales.

Rules for sub-viral agents

Rules concerned with the classification of viruses shall also apply to the classification of viroids. The formal endings for taxa of viroids are the word viroid for species, the suffix -viroid for genera, the suffix -viroinae for sub-families, should this taxon be needed, and -viroidae for families.

Retrotransposons are considered to be viruses in classification and nomenclature. Satellites and prions are not classified as viruses but are assigned an arbitrary classification as seems useful to workers in the particular fields.

Rules for orthography

1. In formal taxonomic usage the accepted names of virus orders, families, subfamilies, and genera are printed in italics and the first letters of the names are capitalized.
2. Species names are printed in italics and have the first letter of the first word capitalized. Other words are not capitalized unless they are proper nouns, or parts of proper nouns.
3. In formal usage, the name of the taxon shall precede the term for the taxonomic unit.

Decimal code

The ICTV classification uses an eight position decimal code to represent order, family, subfamily, genus, species, subspecies, serotype or subtype, and strain or isolate. Each position in the code uses 1, 2, or 3 numerical digits. The decimal code identifies a virus to the level of strain or isolate. Two examples are shown in the table below.

Database

The development of ICTVdB has been supported by the ICTV since 1991 and was initially intended to aid taxonomic research. The database classifies viruses based primarily on their chemical characteristics, genomic type, nucleic acid replication, diseases, vectors, and geographical distribution, among other characteristics.

The database was developed at the Australian National University with support of the US National Science Foundation, and sponsored by the American Type Culture Collection. It uses the Description Language for Taxonomy (DELTA) system, a world standard for taxonomic data exchange, developed at Australia's Commonwealth Scientific and Industrial Research Organisation (CSIRO). DELTA is able to store a wide diversity of data and translate it into a language suitable for traditional reports and web publication. For example, ICTVdB does not itself contain genomic sequence information but can convert DELTA data into NEXUS format. It can also handle large data inputs and is suited to compiling long lists of virus properties, text comments, and images.

ICTVdB has grown in concept and capability to become a major reference resource and research tool, in 1999 it was receiving over 30,000 combined online hits per day from its main site at the Australian National University, and two mirror sites based in the UK and USA^[3].