Virus classification is the process of naming viruses and placing them into a taxonomic system. Similar to the classification systems used for cellular organisms, virus classification is the subject of ongoing debate and proposals. This is mainly due to the pseudo-living nature of viruses, which is to say they are non-living particles with some chemical characteristics similar to those of life, or non-cellular life. As such, they do not fit neatly into the established biological classification system in place for cellular organisms.
Viruses are mainly classified by phenotypic characteristics, such as morphology, nucleic acid type, mode of replication, host organisms, and the type of disease they cause. The formal taxonomic classification of viruses is the responsibility of the International Committee on Taxonomy of Viruses (ICTV) system, although the Baltimore classification system can be used to place viruses into one of seven groups based on their manner of mRNA synthesis. Specific naming conventions and further classification guidelines are set out by the ICTV.
A catalogue of all the world's known viruses has been proposed; some related preliminary efforts have been accomplished.
Species form the basis for any biological classification system. The ICTV had adopted the principle that a virus species is a polythetic class of viruses that constitutes a replicating lineage and occupies a particular ecological niche. In July 2013, the ICTV definition of species changed to state: "A species is a monophyletic group of viruses whose properties can be distinguished from those of other species by multiple criteria."
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 realm and continues as follows, with the taxon suffixes given in italics:
Species names often take the form of [Disease] virus, particularly for higher plants and animals. As of November 2018, only phylum, subphylum, class, order, suborder, family, subfamily, genus, and species are used.
The establishment of an order is based on the inference that the virus families it contains have most likely evolved from a common ancestor. The majority of virus families remain unplaced.
As of 2018, just one single phylum, two subphyla (Haploviricotina and Polyploviricotina), six classes, 14 orders, five suborders, 143 families, 64 subfamilies, 846 genera, and 4,958 species of viruses have been defined by the ICTV. The orders are as follows:
These orders span viruses with varying host ranges. The Ortervirales (Groups VI and VII), containing also retroviruses (infecting animals including humans e.g. HIV), retrotransposons (infecting invertebrate animals, plants and eukaryotic microorganisms) and caulimoviruses (infecting plants), are recent additions to the classification system orders.
Other variations occur between the orders: Nidovirales, for example, are isolated for their differentiation in expressing structural and nonstructural proteins separately.
It has been suggested that similarity in virion assembly and structure observed for certain viral groups infecting hosts from different domains of life (e.g., bacterial tectiviruses and eukaryotic adenoviruses or prokaryotic Caudovirales and eukaryotic herpesviruses) reflects an evolutionary relationship between these viruses. Therefore, structural relationship between viruses has been suggested to be used as a basis for defining higher-level taxa – structure-based viral lineages – that could complement the existing ICTV classification scheme.
Baltimore classification (first defined in 1971) is a classification system that places viruses into one of seven groups depending on a combination of their nucleic acid (DNA or RNA), strandedness (single-stranded or double-stranded), sense, and method of replication. Named after David Baltimore, a Nobel Prize-winning biologist, these groups are designated by Roman numerals. Other classifications are determined by the disease caused by the virus or its morphology, neither of which are satisfactory due to different viruses either causing the same disease or looking very similar. In addition, viral structures are often difficult to determine under the microscope. Classifying viruses according to their genome means that those in a given category will all behave in a similar fashion, offering some indication of how to proceed with further research. Viruses can be placed in one of the seven following groups:
|Virus family||Examples (common names)||Virion
|Nucleic acid type||Group|
|1. Adenoviridae||Adenovirus, infectious canine hepatitis virus||Naked||Icosahedral||ds||I|
|2. Papovaviridae||Papillomavirus, polyomaviridae, simian vacuolating virus||Naked||Icosahedral||ds circular||I|
|3. Parvoviridae||Parvovirus B19, canine parvovirus||Naked||Icosahedral||ss||II|
|4. Herpesviridae||Herpes simplex virus, varicella-zoster virus, cytomegalovirus, Epstein–Barr virus||Enveloped||Icosahedral||ds||I|
|5. Poxviridae||Smallpox virus, cow pox virus, sheep pox virus, orf virus, monkey pox virus, vaccinia virus||Complex coats||Complex||ds||I|
|6. Hepadnaviridae||Hepatitis B virus||Enveloped||Icosahedral||circular, partially ds||VII|
|7. Anelloviridae||Torque teno virus||Naked||Icosahedral||ss circular||II|
|Virus Family||Examples (common names)||Capsid
|Nucleic acid type||Group|
|1. Reoviridae||Reovirus, rotavirus||Naked||Icosahedral||ds||III|
|2. Picornaviridae||Enterovirus, rhinovirus, hepatovirus, cardiovirus, aphthovirus, poliovirus, parechovirus, erbovirus, kobuvirus, teschovirus, coxsackie||Naked||Icosahedral||ss||IV|
|3. Caliciviridae||Norwalk virus||Naked||Icosahedral||ss||IV|
|4. Togaviridae||Rubella virus, alphavirus||Enveloped||Icosahedral||ss||IV|
|5. Arenaviridae||Lymphocytic choriomeningitis virus||Enveloped||Complex||ss(-)||V|
|6. Flaviviridae||Dengue virus, hepatitis C virus, yellow fever virus, Zika virus||Enveloped||Icosahedral||ss||IV|
|7. Orthomyxoviridae||Influenzavirus A, influenzavirus B, influenzavirus C, isavirus, thogotovirus||Enveloped||Helical||ss(-)||V|
|8. Paramyxoviridae||Measles virus, mumps virus, respiratory syncytial virus, Rinderpest virus, canine distemper virus||Enveloped||Helical||ss(-)||V|
|9. Bunyaviridae||California encephalitis virus, hantavirus||Enveloped||Helical||ss(-)||V|
|10. Rhabdoviridae||Rabies virus||Enveloped||Helical||ss(-)||V|
|11. Filoviridae||Ebola virus, Marburg virus||Enveloped||Helical||ss(-)||V|
|12. Coronaviridae||Corona virus||Enveloped||Helical||ss||IV|
|14. Bornaviridae||Borna disease virus||Enveloped||Helical||ss(-)||V|
|15. Arteriviridae||Arterivirus, equine arteritis virus||Enveloped||Icosahedral||ss||IV|
|16. Hepeviridae||Hepatitis E virus||Naked||Icosahedral||ss||IV|
The LHT System of Virus Classification is based on chemical and physical characters like nucleic acid (DNA or RNA), symmetry (helical or icosahedral or complex), presence of envelope, diameter of capsid, number of capsomers. This classification was approved by the Provisional Committee on Nomenclature of Virus (PNVC) of the International Association of Microbiological Societies (1962). It is as follows:
The following agents are smaller than viruses but have only some of their properties.
Satellites depend on co-infection of a host cell with a helper virus for productive multiplication. Their nucleic acids have substantially distinct nucleotide sequences from either their helper virus or host. When a satellite subviral agent encodes the coat protein in which it is encapsulated, it is then called a satellite virus.
Prions, named for their description as "proteinaceous and infectious particles", lack any detectable (as of 2002) nucleic acids or virus-like particles. They resist inactivation procedures that normally affect nucleic acids.
Alvernaviridae is a family of viruses. Dinoflagellates serve as natural hosts. There is currently only one species in this family: the type species Heterocapsa circularisquama RNA virus 01. Diseases associated with this family include: control of the host population possibly through lysis of the host cell.Bacillarnavirus
Bacillarnavirus is a genus of viruses in the order Picornavirales. Marine diatoms serve as natural hosts. There are currently three species in this genus, including the type species Rhizosolenia setigera RNA virus 01.Baltimore classification
The Baltimore classification, developed by David Baltimore, is a virus classification system that groups viruses into families, depending on their type of genome (DNA, RNA, single-stranded (ss), double-stranded (ds), etc..) and their method of replication.Barnaviridae
Barnaviridae is a family of viruses. Cultivated mushroom serve as natural hosts. There are currently only one genus (Barnavirus) and one species in this family: the type species Mushroom bacilliform virus. Diseases associated with this family include: La France disease.Carmotetraviridae
Carmotetraviridae is a family of viruses; there is currently only one genus in this family, Alphacarmotetravirus, and one species in this genus, the type species Providence virus. Lepidopteran insects serve as natural hosts.DsDNA-RT virus
dsDNA-RT viruses are the seventh group in the Baltimore virus classification.
They are not considered DNA viruses (class I of Baltimore classification),
but rather reverse transcribing viruses because they replicate through an RNA intermediate. It includes the families Hepadnaviridae and Caulimoviridae.
The term "pararetrovirus" is also used for this group. The term was introduced in 1985.Emaravirus
Emaravirus is a genus of plant viruses. The genus has nine species. The type species European mountain ash ringspot-associated emaravirus is associated with a leaf mottling and ringspot disease of European mountain ash Sorbus aucuparia. It can be transmitted by grafting and possibly mites.Gammaflexiviridae
Gammaflexiviridae is a family of viruses in the order Tymovirales. Fungi serve as natural hosts. There are currently only one genus, Mycoflexivirus, and one species in this family: the type species Botrytis virus F.Hantaviridae
Hantaviridae is a family of viruses in the order Bunyavirales. It is named for the Hantan River area in South Korea where an early outbreak of one of its species was observed.Higrevirus
Higrevirus is a genus of viruses. Plants serve as natural hosts. There is currently only one species in this genus: the type species Hibiscus green spot virus 2.Megabirnaviridae
Megabirnaviridae is a family of viruses with one genus Megabirnavirus. Fungi serve as natural hosts. There is only one species in this family: the type species Rosellinia necatrix megabirnavirus 1. Diseases associated with this family include: reduced host virulence.Nairoviridae
Nairoviridae is a family of viruses in the order Bunyavirales. Its name derives from Nairobi sheep disease, caused by the member Nairobi sheep disease orthonairovirus.Nomenclature codes
Nomenclature codes or codes of nomenclature are the various rulebooks that govern biological taxonomic nomenclature, each in their own broad field of organisms. To an end-user who only deals with names of species, with some awareness that species are assignable to families, it may not be noticeable that there is more than one code, but beyond this basic level these are rather different in the way they work.
The successful introduction of two-part names for species by Linnaeus was the start for an ever-expanding system of nomenclature. With all naturalists worldwide adopting this approach to thinking up names there arose several schools of thought about the details. It became ever more apparent that a detailed body of rules was necessary to govern scientific names. From the mid-nineteenth century onwards there were several initiatives to arrive at worldwide-accepted sets of rules. Presently nomenclature codes govern the naming of:
Algae, Fungi and Plants – International Code of Nomenclature for algae, fungi, and plants (ICN), which in July 2011 replaced the International Code of Botanical Nomenclature (ICBN) and the earlier International Rules of Botanical Nomenclature.
Animals – International Code of Zoological Nomenclature (ICZN)
Bacteria – International Code of Nomenclature of Prokaryotes (ICNP), which in 2008 replaced the International Code of Nomenclature of Bacteria (ICNB)
Cultivated plants – International Code of Nomenclature for Cultivated Plants (ICNCP)
Plant associations – International Code of Phytosociological Nomenclature (ICPN)
Viruses – The International Code of Virus Classification and Nomenclature (ICVCN); see also virus classificationPandoravirus
Pandoravirus is a genus of giant virus, first discovered in 2013. It is the second largest in physical size of any known viral genus. Pandoraviruses have double stranded DNA genomes, with the largest genome size (2.5 million base pairs) of any known viral genus.Peribunyaviridae
Peribunyaviridae is a family of viruses in the order Bunyavirales. Its name partially derives from Bunyamwera, Uganda, where the founding species was first isolated.Phasmaviridae
Phasmaviridae is a family of viruses with negative stranded RNA genomes. They are a member of the order Bunyavirales.Pithovirus
Pithovirus, first described in a 2014 paper, is a genus of giant virus known from one species, Pithovirus sibericum, which infects amoebas. It is a double-stranded DNA virus, and is a member of the nucleocytoplasmic large DNA viruses clade. The 2014 discovery was made when a viable specimen was found in a 30,000-year-old ice core harvested from permafrost in Siberia, Russia.Spiraviridae
Spiraviridae is a family of single stranded DNA viruses that infect archeae.Turriviridae
Turriviridae is a family of viruses; it contains only one genus, Alphaturrivirus. The archaea Sulfolobus solfataricus serve as natural hosts. There are currently only two species in the genus Alphaturrivirus.
|Viral life cycle|
Baltimore (virus classification)