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Copyright © 2017 International Committee on Taxonomy of Viruses.
No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means electronic, mechanical, photocopying, recording or otherwise without the prior written permission of the ICTV.
The first internationally organized attempts to introduce order into the bewildering variety of viruses took place at the International Congress of Microbiology held in Moscow in 1966. A committee was created, later called the International Committee on Taxonomy of Viruses (ICTV), and was given the task of developing a single, universal taxonomic scheme for all the viruses infecting animals (vertebrates, invertebrates and protozoa), plants (higher plants and algae), fungi, bacteria and archaea. The ICTV was created as a committee of the Virology Division of the International Union of Microbiological Societies (IUMS) and is governed by Statutes approved by the Virology Division. The Statutes, available from this link and define the objectives of ICTV: (i) to develop an internationally agreed taxonomy for viruses (the term “viruses” for this purpose is taken to include viroids and some important groups of satellite viruses); (ii) to develop internationally agreed names for these taxa; (iii) to communicate taxonomic decisions to the international community of virologists; and (iv) to maintain an index of virus names. The Statutes also state that classification and nomenclature of viruses will be subject to rules set out in an International Code that is available from this link.
Virus taxonomy differs from other types of biological classification because the ICTV not only regulates a Code of Nomenclature but also considers and approves the creation of virus taxa (currently orders, families, subfamilies, genera and species). Priority of publication is not the determining factor. Species names are usually derived from the common (vernacular) name of the virus (usually in English) used to establish the species. The names of all recognized taxa are written in italics with an initial capital letter.
To date, the Executive Committee (EC) has established 88 international Study Groups (SGs) covering all major virus orders, families and genera. The Chair of each SG is appointed by the relevant Subcommittee Chair, who is a member of the EC. Each subcommittee is responsible for classes of viruses with different genome configurations infecting different hosts. The current subcommittees encompass Animal DNA Viruses and Retroviruses, Animal dsRNA and ssRNA- Viruses, Animal ssRNA+ Viruses, Bacterial and Archaeal Viruses, Fungal and Protist Viruses, and Plant Viruses. SG Chairs are responsible for (i) organizing discussions among SG members of emerging taxonomic issues in their field, (ii) overseeing the submission of proposals for new taxonomy, and (iii) preparing or revising relevant chapter(s) in ICTV Reports.
The ICTV welcomes taxonomic proposals from any interested individual,l although in practice most are prepared by the relevant SG. An all-purpose template and guidance notes are available for downloading from https://talk.ictvonline.org/files/taxonomy-proposal-templates/. Proposals will be forwarded to all relevant SGs and will also be made available on the website for public comment. Authors are then invited to respond to any comments made. Subcommittee Chairs then present taxonomic proposals to the EC for discussion and approval at the annual EC meeting. Straightforward proposals, such as those to create new species within existing genera for which species demarcation have been previously established, can normally be approved at a single meeting. More complex or controversial proposals are made available on the ICTV website for public comment for a further year before being re-considered by the EC.
Proposals approved by the EC do not become accepted taxonomy until they have been ratified by the full ICTV membership. As specified in the Statutes, this includes members of the various Subcommittees (mostly SG Chairs), National Members and Life Members. Lists of members are provided at https://talk.ictvonline.org/information/. Ratification proceeds by an email vote, after which the approved taxonomy is updated at https://talk.ictvonline.org/taxonomy/. This site should always be consulted for the most up-to-date ICTV taxonomic information. Summaries of the voting decisions are also prepared and published annually in an article in Archives of Virology within the Virology Division News section.
The ICTV has communicated its decisions to the international community through a series of published Reports. Between 1971 and 2012, nine ICTV Reports were published. Publication usually followed the formal approval of taxonomic proposals at plenary meetings of the ICTV held during International Congresses of Virology. These reports provide milestones by which the progress in virus taxonomy can be tracked.
Reporting ICTV Proceedings at the International Congress of Virology held in:
43 families and groups
Budapest, 1971 and Madrid, 1975
47 families and groups
The Hague, 1978
50 families and groups
54 families and groups
Francki et al. (1991)
Sendai, 1984; Edmonton, 1987; Berlin, 1990
73 families and groups classifying 2,420 viruses
Murphy et al. (1995)
1 order, 50 families, 9 subfamilies, 164 genera and more than 3,600 virus species
van Regenmortel et al. (2000)
3 orders, 63 families, 9 subfamilies, 240 genera, 1550 species*
Fauquet et al. (2005)
Sydney, 1999 and Paris, 2002
3 orders, 73 families, 11 subfamilies, 289 genera and 1898 species
King et al. (2011)
San Francisco, 2005; Istanbul, 2008; Sapporo, 2011
6 orders, 87 families, 19 subfamilies, 349 genera and 2285 species
*With the introduction of the current species definition and the adoption of formal species demarcation criteria in the Seventh Report (van Regenmortel et al., 2000), many virus strains that had hitherto been listed as separate species were reorganized into new, more broadly defined species. This explains the reduction in the number of "species" between the Sixth and Seventh Reports.
The First Report was published in 1971 by the then International Committee on Nomenclature of Viruses (ICNV), which had been established in 1966 (Wildy, 1971). This report, covering the period 1966 to 1970, established five Subcommittees: Bacteriophage (now Prokaryote Virus), Invertebrate Virus, Plant Virus, Vertebrate Virus and Cryptograms. The Subcommittees were responsible for approving taxonomic proposals relevant to their groups of viruses and presenting these proposals for approval by the Executive Committee (EC), the ICNV, with the provision that “a sizable number of virologists working in the relevant field were to be consulted”. This report included the designations family, genus (group) and type species, thus establishing and setting the foundations for viral taxonomy.
By the Second Report (Fenner, 1976), the name change to ICTV had been approved (in 1973), and a formal structure of ICTV officers had been established, consisting of the President, Vice-President, two Secretaries, Chairs of the Subcommittees, Elected Members and Life Members. There was now a Fungal Virus Subcommittee (created from the disbanded Cryptograms Subcommittee) and a Coordination Subcommittee (disbanded 1995) charged with ensuring that the Study Groups included virologists with interests in particular virus groups in each class of host affected.
In the Third Report (Matthews, 1979), the viruses were listed on the basis of the kind and strandedness of the nucleic acid making up the viral genome, and the presence or absence of an envelope. It was noted that although the ICTV had approved families and genera, there was no such approved “taxon equivalent to species, lying between genus and strain or variant”. The problem of defining species for viruses and naming these species was presented, and an extensive discussion of the various points of view as presented by representatives of different groups of viruses was outlined for the first time. It was suggested that it might take 10–20 years to provide these taxa, and an appeal was made to Study Groups to put forward species proposals for consideration by the ICTV. This was also the first ICTV Report to include virus diagrams, grouped according to the major hosts (animal, bacteria or plant). There was also a list of some unclassified viruses and virus-like agents, including the agents of scrapie, Kuru and Creutzfeldt–Jakob diseases, viroids, and satellite viruses and satellite RNAs in plants.
The same general arrangement was followed in the Fourth (Matthews, 1982) and Fifth (Francki et al., 1991) Reports. By the time of the Sixth Report (Murphy et al., 1995), the ICTV had finally accepted the controversial category of virus species based on a proposal made in 1990 (van Regenmortel, 1990). A virus species was defined as “a polythetic class of viruses that constitutes a replicating lineage and occupies a particular ecological niche”. There was also a clear description of the usage of formal taxonomic nomenclature and an attempt to explain its appropriate usage and how it differs from informal vernacular usage. More than 15 years later, this issue continues to elicit controversy.
In the Seventh Report (van Regenmortel et al., 2000), there was an extensive discussion of the species concept in virus taxonomy, and an appeal to the virology community to establish demarcation criteria whichcould be used to discriminate between different virus species within a genus. Some, but not all, genus descriptions in the Seventh Report included criteria by which species could be differentiated.
The Eighth Report (Fauquet et al., 2005) continued this process and provided an epic compilation of virus taxonomy illustrated with 436 electron micrographs, diagrams of virus particles, diagrams of genome organization and phylogenetic trees in a book of 1259 pages. As this approached the limit for publication in a single volume, it was becoming obvious that a different vehicle for transmission of virus taxonomy would soon be required.
Nevertheless, the Ninth Report was also published as a book of 1327 pages listing 2284 virus and viroid species distributed amongst 349 genera, 19 subfamilies, 87 families and 6 orders. There was also a chapter of unassigned viruses that provided information on several viruses that had not yet been classified but which were probably representatives of new genera or families. The final chapters described the satellites (and other virus-dependent nucleic acids) and prions (which include the agents of spongiform encephalopathies of humans), which are not formally classified by ICTV but were simply listed for historical reasons.
The current ICTV Online (10th) Report on Virus Taxonomy is published as a freely available, online resource. Over a three-year cycle, ICTV Study Groups will update the information in the chapters of the 9th Report as well as produce chapters for newly created viral taxa. These chapters will include direct links to the latest taxonomic database as well as to external sequence and taxonomic databases. In addition, Journal of General Virology will publish ICTV Virus Taxonomy Profiles – a new series of concise review-type articles that will summarize individual chapters from the Online (10th) Report. Written by ICTV Study Groups, these reviews will provide overviews of the classification, structure and properties of individual virus orders, families and, in some cases, genera. These summaries will be linked to the full Online Report chapters on the ICTV website and will be indexed in PubMed. Together, the summaries and online chapters are will provide researchers with up-to-date and authoritative taxonomic and descriptive information on virus taxa.
Support for preparation of the Online Report and Report Summaries has been provided by: