Classification of viruses

In 1962, a taxonomy of viruses was introduced by André L Woff, Robert W. Horne and Paul Tournier in accordance with the binary classification of living organisms founded by Carl von Linné. In this taxonomy, analogous to the taxonomy of other living organisms, the following taxa are subdivided:

• Genome group
• Order (...virales)
• Family (...viridae)
• Subfamily (...virinae)
• Genus (...virus)
• Species (disease/..virus).

The defining characteristics for this classification were:

• Nature of the viral genome (DNA or RNA/positive-stranded or negative-stranded/single-stranded or double-stranded).
• Symmetry of the capsid
• Presence of lipid envelope (enveloped virus/naked virus)
• Size of virion and capsid (large viruses/small viruses).

The older Baltimore classification is considered obsolete. This classification was temporarily established based on knowledge of the molecular biology of viruses. It goes back to a proposal made by the Nobel Prize winner David Baltimore in 1971. For everyday medical practice, the differentiation between human pathogens and non-human pathogens is of great practical importance.

A virus is a particle (15-300 nm) composed of at least nucleic acids and proteins, which is able to penetrate a (living) host cell and trigger the production of progeny viruses by damaging this cell. Viruses that infect bacterial cells are called bacteriophages. The term virion refers exclusively to the extracellular, physico-chemically defined, complete particle. Its biological properties are not taken into account (Hof H et al. 2019).

On the basis of the collected morphological, molecular and biological properties of viruses, a virus classification was developed by the "International Committee on Taxonomy of Viruses", using the following order terms: family, subfamily, genus, species or species.

• Order: A virus order consists of groups of virus families that share common characteristics and are distinct from other orders and families.
• Virus families are groups of virus genera that share common characteristics and are distinct from other virus genera. For the most part, virus families share common viral morphology, genome structure, and/or virus replication strategy.
• Subfamilies are kept within the four families Pox-, Herpes- , Parvo- and Paramyxoviridae to allow a more complex division.
• Species: A viral species is defined as a polythetic class of viruses descended from the same lineage of replication and occupying a particular ecological niche.

Important criteria of this currently most internationally recognized virus taxonomy include: genome structure (DNA, RNA, single-stranded, double-stranded, polarity, linear, circular, segmented), capsid shape (symmetry), presence of an envelope, arrangement of genes within the genome, replication strategy, virus size. Criteria disregarded for taxonomy, but nevertheless important for grouping different viruses with common medical or epidemiological characteristics are:

• common organisms they infect
• common modes of transmission (e.g. transmission by arthropods: arboviruses)
• similar disease patterns or infection of the same organ (e.g. hepatitis viruses)

Newly emerging viruses are either added to existing families or to a new family, taking into account their biochemical, biological, structural and genetic properties. Therefore, there are only 3 orders so far. Many families are not yet assigned to any order. Currently, about 80 families and about 4000 species are known.

Overview of major human pathogenic virus families:

Positive single-stranded RNA/ ss(+)RNA (acts directly as mRNA).

Picornaviridae

• Enterovirus(coxsackie,rhino,echovirus,poliovirus).
• Parechovirus (Parechoviruses)
• Heptavirus (Hepatitis A virus)

Caliciviridae

• Norovirus (Norwalk virus)
• Sapovirus (Sapporovirus)

Hepeviridae

• Hepevirus (Hepatitis E virus = HEV, this is the only human pathogenic species of this virus family)

Coronaviridae

• SARS-CoV2 (causative agent of Covid-19)
• SARS-associated coronavirus (SARS-CoV)
• Human coronavirus 229E (HCoV)
• Human coronavirus OC43 (HCoV)

Togaviridae

• Rubivirus (rubellavirus = rubellavirus)
• Alphavirus(human and animal pathogenic species; the most widespread species is Sindbis virus)

Flaviviridae

• Hepacivirus
• Hepatitis C virus (HCV)
• West Nile virus
• Dengue virus
• Yellow fever virus
• Louping-ill virus
• St. Louis encephalitis virus
• Japan B encephalitis virus
• Powassan virus
• RSSE virus
• TBE virus

Retroviridae

• Deltaretrovirus (Human T-lymphotropic virus 1 - HTLV-1, Human T-lymphotropic virus 2 - HTLV-2)

• Lentivirus (HIV 1, HIV 2)

• Gammaretrovirus (Xenotropic murine leukemia virus-related virus)

Paramyxoviridae

• Respiroviruses (Parainfluenza virus 1, 3)
• Morbilliviruses(measles/measles virus)
• Rubula viruses(mumps/mumps virus)
• Parainfluenza virus (2, 4)
• Newcastle disease virus (NDV)

Negative single-stranded RNA/ss(-)RNA ( SS(-) (acts as a template for mRNA synthesis).

Pneumoviridae

• Genus: Orthopneumovirus (human pathogenic species: human respiratory syncytial virus, HRSV; occurs in the two most common subtypes A and B and the rarer types S2 and RSS-2).
• Genus: Metapneumovirus (human pathogenic species: human metapneumovirus (HMPV, types A1 to 2, B1 to B2)

Rhabdoviridae

• Lyssaviruses (rabies virus)
• Rabies virus (RABV)

Filoviridae

• Lake Victoria Marburg virus
• Ebola-like viruses (serogroup)
• Ebola zaïre virus
• Ebola Sudan virus
• Ebola Côte d'Ivoire virus
• Ebola Reston virus

Arenaviridae

• Arena virus
• Lassa virus(Lassa fever)
• Lymphocytic choriomeningitis virus (LCMV)
• Tacaribe virus
• Junin virus(Argentine hemorrhagic fever)
• Machupo virus

Bunyaviridae

• Orthobunyavirus (>150 species or subtypes; they are transmitted by mosquitoes). Significant for Europe are:
• Tahyna virus
• Inkoo virus
• California encephalitis virus
• La Crosse virus
• Jamnestown canyon virus
• Snowshoe-hare virus
• Phlebovirus (3 serogroup types, more than 45 species) carriers are butterfly mosquitoes- Phlebotomus species)
• Phlebotomus fever virus: In Europe, Phlebotomus fever virus plays a role (Tuscany type). It causes Pappataci fever (sandfly fever) with photophobia, neck stiffness and arthralgias.
• Rift Valley fever virus: 1977 epidemic in Egypt with 20,000 cases, complicating encephalitis.
• Nairovirus (6serogroups, 34 species) vector is ticks. Clinically significant is the
• Crimean-Congo hemorrhagic fever syndrome, which can be associated with high lethality.
• Hantavirus (infection does not occur through arthropods but through smear infections and dust injections with excrements of rodents, e.g. mice). Of importance for Europe are:
• Hantaan virus
• Puumula virus
• Dobrava-Belgrade virus

Orthomyxoviridae

• Influenza viruses A
• Influenza virus A variant (H1N1)
• Influenza virus A variant (H2N2)
• Influenza virus A variant (H3N2)
• (avian) influenza virus A variant (H5N1), highly pathogenic avian influenza virus (HPAIV)
• (avieres) influenza virus A variant (H7N2), low pathogenic avian influenza virus (LPAIV)
• (avieres) influenza virus A variant (H7N3), low pathogenic avian influenza virus (LPAIV)
• (avieres) influenza virus A variant (H7N7), highly pathogenic avian influenza virus (HPAIV)
• (avieres) influenza virus A variant (H9N2), low pathogenic avian influenza virus (LPAIV)
• Influenza virus B
• Influenza virus B/Victoria lineage
• Influenza virus B/Yamagata lineage
• Influenza virus C

Double-stranded RNA genome/dsRNA

Reoviridae

• Subfamily Sedoreovirinae with genus Rotavirus and species Rotavirus (A to C).
• Spinareovirans subfamily with the genus Coltvirus and the species Colorado tick fiber virus (endemic in mountainous regions above 1700 meters in USA and Canada).

Double-stranded DNA - genome/ dsDNA (normal genome form of all life)

Herpesviridae ( human pathogenic species)

• Alphaherpesvirinae
• Herpessimplex virus 1 (HSV-1) or (HHV-1)
• Herpes simplex virus 2 (HSV-2) or (HHV-2)
• Varicella virus
• Varicella-zoster virus (VZV) or (HHV-3)
• Betaherpesvirinae
• Cytomegalovirus (CMV)
• Human cytomegalovirus (also human herpes virus 5 - HHV-5)
• Reseolovirus (HHV-6,HHV-7)
• Gammaherpesvirinae
• Lymphocryptovirus (Epstein-Barr virus/ EBV, also called human herpes virus 4/HHV-4)
• Rhadinovirus (human herpes virus 8/HHV-8)

Papillomaviridae

• Papillomavirus (Humanpapillomavirus/HPV )

Polyomaviridae

• Human polyomavirus (BK and JC virus/BKV, JCV, MCPyV, PSPyV; HPyV6/7)

Adenoviridae

• Mastadenovirus (human adenoviruses)

Poxviridae

• Chordopoxviridae
• Orthopoxvirus (variola, vaccinia virus)
• Parapoxvirus (Milking Node, Orfvirus)
• Molluscipoxvirus(Molluscum contagiosum virus)
• Tanapoxvirus(Tanapox)
• Human monkeypox virus(monkeypox)

Ds/ssDNA

Hepadnaviridae

• Orthohepadnaviruses
• Hepatitis B virus (HBV)

Single-stranded DNA/ ssDNA. Virions contain DNA of positive or negative polarity.

Parvoviridae

• Erythrovirus (Parvovirus B19)

New classifications are developed by the International Committee on Taxonomy of Viruses (ICTV). This committee is charged with the development, refinement and maintenance of universal virus taxonomy. The task includes classifying virus species and higher-level taxa according to the genetic and biological properties of their members; naming virus taxa; maintaining a database detailing the currently approved taxonomy; and making the database, supporting proposals, and other virus-related information available from a public, open-access website. The ICTV website (http://ictv.global) provides access to the current taxonomy database in online and downloadable formats and maintains a complete history of virustaxa back to the first publication in 1971, with comprehensive descriptions of all virustaxa covering virus structure, genome structure, biology, and phylogenetics. The ninth ICTV report, published in 2012, is available as an open access online publication on the ICTV website. The current, 10th report (http://ictv.global/report/) is published online and replaces the previous hardcopy edition with a fully open, continuously updated account.

1. Simmonds P et al (2018) Virus classification - where do you draw the line? Arch Virol 163:2037-2046.
2. Lefkowitz EJ et al (2018) Virus taxonomy: the database of the International Committee on Taxonomy of Viruses (ICTV). Nucleic Acids Res 46: D708-D717.

Selection of cutaneous viral diseases