Virus classification


Virus classification is the process of naming viruses and placing them into a taxonomic system. Similar to the classification systems used 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 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 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 actual criteria used vary by the taxon, and can be inconsistent or unrelated to lineage at times. The matter is, for many, not yet settled.

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, 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:
Species names often take the form of virus, particularly for higher plants and animals.
As of 2019, all levels of taxa except subrealm, subkingdom, and subclass are used. Four realms, one incertae sedis order, 24 incertae sedis families, and three incertae sedis genera are recognized:
Realms: Duplodnaviria, Monodnaviria, Riboviria, and Varidnaviria
incertae sedis order: Ligamenvirales
incertae sedis families:
incertae sedis genera: Deltavirus, Dinodnavirus, Rhizidiovirus

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 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

Baltimore classification is a classification system that places viruses into one of seven groups depending on a combination of their nucleic acid, strandedness, 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:

DNA viruses

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.
Virus familyExamples Virion
naked/enveloped		Capsid
symmetry		Nucleic acid typeGroup
1. AdenoviridaeCanine hepatitis virus, Some types of the common coldNakedIcosahedraldsI
2. PapovaviridaeJC virus, HPVNakedIcosahedralds circularI
3. ParvoviridaeHuman parvovirus B19, canine parvovirusNakedIcosahedralssII
4. HerpesviridaeHerpes simplex virus, varicella-zoster virus, cytomegalovirus, Epstein–Barr virusEnvelopedIcosahedraldsI
5. PoxviridaeSmallpox virus, cowpox, myxoma virus, monkeypox, vaccinia virusComplex coatsComplexdsI
6. AnelloviridaeTorque teno virusNakedIcosahedralss circularII
HHPV1, HRPV1Envelopedss/ds linear/circularI/II

RNA viruses

All viruses that have an RNA genome, and that encode an RNA-dependent RNA polymerase, are members of the kingdom Orthornavirae, within the realm Riboviria.
Virus FamilyExamples Capsid
naked/enveloped		Capsid
Symmetry		Nucleic acid typeGroup
1. ReoviridaeReovirus, rotavirusNakedIcosahedraldsIII
2. PicornaviridaeEnterovirus, rhinovirus, hepatovirus, cardiovirus, aphthovirus, poliovirus, parechovirus, erbovirus, kobuvirus, teschovirus, coxsackieNakedIcosahedralssIV
3. CaliciviridaeNorwalk virusNakedIcosahedralssIV
4. TogaviridaeEastern equine encephalitisEnvelopedIcosahedralssIV
5. ArenaviridaeLymphocytic choriomeningitis virus, Lassa feverEnvelopedComplexssV
6. FlaviviridaeDengue virus, hepatitis C virus, yellow fever virus, Zika virusEnvelopedIcosahedralssIV
Influenzavirus A, influenzavirus B, influenzavirus C, isavirus, thogotovirusEnvelopedHelicalssV
8. ParamyxoviridaeMeasles virus, mumps virus, respiratory syncytial virus, Rinderpest virus, canine distemper virusEnvelopedHelicalssV
9. BunyaviridaeCalifornia encephalitis virus, Sin nombre virusEnvelopedHelicalssV
10. RhabdoviridaeRabies virus, Vesicular stomatitisEnvelopedHelicalssV
11. FiloviridaeEbola virus, Marburg virusEnvelopedHelicalssV
12. CoronaviridaeSARS-CoV-2, MERSEnvelopedHelicalssIV
13. AstroviridaeAstrovirusNakedIcosahedralssIV
14. BornaviridaeBorna disease virusEnvelopedHelicalssV
15. ArteriviridaeArterivirus, equine arteritis virusEnvelopedIcosahedralssIV
16. HepeviridaeHepatitis E virusNakedIcosahedralssIV

Reverse transcribing viruses

All viruses that encode a reverse transcriptase 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 viruses; all other RT viruses are members of the class Ortervirales.
Virus FamilyExamples Capsid
naked/enveloped		Capsid
Symmetry		Nucleic acid typeGroup
1. RetroviridaeHIVEnvelopeddimer RNAVI
2. CaulimoviridaeCaulimovirus, NakedVII
Hepatitis B virusEnvelopedIcosahedralcircular, partially dsVII

Holmes classification

Holmes used Carl Linnaeus's system of binomial nomenclature to classify viruses into 3 groups under one order, Virales. They are placed as follows:
The LHT System of Virus Classification is based on chemical and physical characters like nucleic acid, symmetry, presence of envelope, diameter of capsid, number of capsomers. This classification was approved by the Provisional Committee on Nomenclature of Virus of the International Association of Microbiological Societies. It is as follows:
The following infectious agents are smaller than viruses and have only some of their properties.

Viroids and virus-dependent agents

Viroids

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.
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.
s, 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.