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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 and, in 2013, some preliminary efforts were underway.
Virus species definition
Species form the basis for any biological classification system. Before 1982, it was thought that viruses could not be made to fit Ernst Mayr's reproductive concept of species, and so were not amenable to such treatment. In 1982, the ICTV started to define a species as "a cluster of strains" with unique identifying qualities. In 1991, the more specific principle that a virus species is a polythetic class of viruses that constitutes a replicating lineage and occupies a particular ecological niche was adopted.
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." Viruses are real physical entities produced by biological evolution and genetics, whereas virus species and higher taxa are abstract concepts produced by rational thought and logic. The virus/species relationship thus represents the front line of the interface between biology and logic.
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, some differences exist, such as the universal use of italics for all taxonomic names, 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 taxonomic suffixes in parentheses:
- Realm (-viria)
Unlike the system of binomial nomenclature adopted in cellular species, there is currently no standardized form for virus species names. At present, the ICTV mandates that a species name must contain as few words as possible while remaining distinct, and must not only contain the word virus and the host name. Species names often take the form of [Disease] virus, particularly for higher plants and animals. In 2019, the ICTV published a proposal to adopt a more formalized system of binomial nomenclature for virus species names, to be voted on in 2020. However, some virologists later objected to the potential naming system change, arguing that the debate came while many in the field were preoccupied due to the COVID-19 Pandemic.
As of 2019, all levels of taxa except subrealm, subkingdom, and subclass are used. Four incertae sedis realms, one incertae sedis order, 24 incertae sedis families, and three incertae sedis genera are recognized:
Incertae sedis order: Ligamenvirales
Incertae sedis families:
Structure-based virus classification
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:
- I: dsDNA viruses (e.g. Adenoviruses, Herpesviruses, Poxviruses)
- II: ssDNA viruses (+ strand or "sense") DNA (e.g. Parvoviruses)
- III: dsRNA viruses (e.g. Reoviruses)
- IV: (+)ssRNA viruses (+ strand or sense) RNA (e.g. Coronaviruses, Picornaviruses, Togaviruses)
- V: (−)ssRNA viruses (− strand or antisense) RNA (e.g. Orthomyxoviruses, Rhabdoviruses)
- VI: ssRNA-RT viruses (+ strand or sense) RNA with DNA intermediate in life-cycle (e.g. Retroviruses)
- VII: dsDNA-RT viruses DNA with RNA intermediate in life-cycle (e.g. Hepadnaviruses)
Viruses with a DNA genome, except for the DNA reverse transcribing viruses, are members of three of the four recognized viral realms: Duplodnaviria, Monodnaviria, and Varidnaviria. But the incertae sedis order Ligamenvirales, and many other incertae sedis families and genera, are also used to classify DNA viruses. The domains Duplodnaviria and Varidnaviria consist of double-stranded DNA viruses; other double-stranded DNA viruses are incertae sedis. The domain Monodnaviria consists of single-stranded DNA viruses that generally encode a HUH endonuclease; other single-stranded DNA viruses are incertae sedis.
- Group I: viruses possess double-stranded DNA. Viruses that cause chickenpox and herpes are found here.
- Group II: viruses possess single-stranded DNA.
|Virus family||Examples (common names)||Virion |
|Nucleic acid type||Group|
|1. Adenoviridae||Canine hepatitis virus, Some types of the common cold||Naked||Icosahedral||ds||I|
|2. Papovaviridae||JC virus, HPV||Naked||Icosahedral||ds circular||I|
|3. Parvoviridae||Human 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, cowpox, myxoma virus, monkeypox, vaccinia virus||Complex coats||Complex||ds||I|
|6. Anelloviridae||Torque teno virus||Naked||Icosahedral||ss circular||II|
|7. Pleolipoviridae||HHPV1, HRPV1||Enveloped||ss/ds linear/circular||I/II|
- Group III: viruses possess double-stranded RNA genomes, e.g. rotavirus.
- Group IV: viruses possess positive-sense single-stranded RNA genomes. Many well known viruses are found in this group, including the picornaviruses (which is a family of viruses that includes well-known viruses like Hepatitis A virus, enteroviruses, rhinoviruses, poliovirus, and foot-and-mouth virus), SARS virus, hepatitis C virus, yellow fever virus, and rubella virus.
- Group V: viruses possess negative-sense single-stranded RNA genomes. Ebola and Marburg viruses are well known members of this group, along with influenza virus, measles, mumps and rabies.
Reverse transcribing viruses
All viruses that encode a reverse transcriptase (also known as RT or RNA-dependent DNA polymerase) are members of the class Revtraviricetes, within the phylum Arterviricota, kingdom Pararnavirae, and realm Riboviria. The class Blubervirales contains the single family Hepadnaviridae of DNA RT (reverse transcribing) viruses; all other RT viruses are members of the class Ortervirales.
- Group VI: viruses possess single-stranded RNA viruses that replicate through a DNA intermediate. The retroviruses are included in this group, of which HIV is a member.
- Group VII: viruses possess double-stranded DNA genomes and replicate using reverse transcriptase. The hepatitis B virus can be found in this group.
|Virus Family||Examples (common names)||Capsid |
|Nucleic acid type||Group|
|1. Retroviridae||HIV||Enveloped||dimer RNA||VI|
|2. Caulimoviridae||Caulimovirus, Cacao swollen-shoot virus (CSSV)||Naked||VII|
|3. Hepadnaviridae||Hepatitis B virus||Enveloped||Icosahedral||circular, partially ds||VII|
- Group I: Phaginae (attacks bacteria)
- Group II: Phytophaginae (attacks plants)
- Group III: Zoophaginae (attacks animals)
Viroids and virus-dependent agents
- Family Avsunviroidae
- Family Pospiviroidae
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.
Satellite-like nucleic acids resemble satellite nucleic acids, in that they replicate with the aid of helper viruses. However they differ in that they can encode functions that can contribute to the success of their helper viruses; while they are sometimes considered to be genomic elements of their helper viruses, they are not always found within their helper viruses.
- Satellite viruses
- Single-stranded RNA satellite viruses
- Double-stranded DNA satellite viruses
- Family Lavidaviridae (virophages)
- Single-stranded DNA satellite viruses
- Genus Dependoparvovirus
- Satellite nucleic acids
- Single-stranded satellite DNAs
- Double-stranded satellite RNAs
- Single-stranded satellite RNAs
- Satellite-like RNA
- Satellite-like DNA
Defective interfering particles
Defective interfering particles are defective viruses that have lost their ability to replicate except in the presence of a helper virus, which is normally the parental virus. They can also interfere with the helper virus.
- Defective interfering particles (RNA)
- Defective interfering particles (DNA)
Prions, named for their description as "proteinaceous infectious particles", do not have nucleic acids or virus-like particles. They resist inactivation procedures that normally affect nucleic acids.
- Mammalian prions:
- Agents of spongiform encephalopathies
- Fungal prions:
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