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 are not yet definitively classified as living or non-living. 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. Currently there are two main schemes used for the classification of viruses: the ICTV system and Baltimore classification system, which places viruses into one of seven groups. Accompanying this broad method of classification are specific naming conventions and further classification guidelines set out by the International Committee on Taxonomy of Viruses.
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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.
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 day. The ICTV is the only body charged by the International Union of Microbiological Societies (IUMS) 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. Viral classification starts at the level of order and follows as thus, with the taxon suffixes given in italics:
So far, six orders have been established by the ICTV: the Caudovirales, Herpesvirales, Mononegavirales, Nidovirales, Picornavirales, and Tymovirales. These orders span viruses with varying host ranges. Caudovirales are tailed dsDNA (group I) bacteriophages, Herpesvirales contains large eukaryotic dsDNA viruses, Mononegavirales includes non-segmented (-) strand ssRNA (Group V) plant and animal viruses, Nidovirales is composed of (+) strand ssRNA (Group IV) viruses with vertebrate hosts, Picornavirales contains small (+) strand ssRNA viruses that infect a variety of plant, insect, and animal hosts, and Tymovirales contains monopartite ssRNA viruses that infect plants. Other variations occur between the orders, for example, Nidovirales are isolated for their differentiation in expressing structural and non-structural proteins separately. 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, as in the ICBN.[1] Most notably, species names generally take the form of [Disease] virus. The establishment of an order is based on the inference that the virus families contained within a single order have most likely evolved from a common ancestor. The majority of virus families remain unplaced. Currently (2011) 6 orders, 87 families, 19 subfamilies, 349 genera, and 2,284 species of virus have been defined.[2][3]
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 and discriminate viruses depending on their mode of replication, and genome type. 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:[4]
| Virus Family | Examples (common names) | Virion naked/enveloped |
Capsid Symmetry |
Nucleic acid type | Group |
|---|---|---|---|---|---|
| 1.Adenoviridae | Adenovirus, Infectious canine hepatitis virus | Naked | Icosahedral | ds | I |
| 2.Papillomaviridae | Papillomavirus | 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.Polyomaviridae | Polyoma virus; JC virus (progressive multifocal leukoencephalopathy) | Naked | Icosahedral | ds circular | I |
| 8.Anelloviridae | Torque teno virus | Naked | Icosahedral | ss circular | II |
Holmes (1948) used Carolus 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 (DNA or RNA), Symmetry (Helical or Icosahedral or Complex), presence of envelope, diameter of capsid, number of capsomers.[5] 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 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.[9]
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