How viruses are classified

7 August 2009

virosphere-2005For the first 60 years of virus discovery, there was no system for classifying viruses. Consequently viruses were named haphazardly, a practice that continues today.

Vertebrate viruses may be named according to the associated diseases (poliovirus, rabies), the type of disease caused (murine leukemia virus), or the sites in the body affected or from which the virus was first isolated (rhinovirus, adenovirus). Some viruses are named for where they were first isolated (Sendai virus, Coxsackievirus), for the scientists who discovered them (Epstein-Barr virus), or for the way people imagined they were contracted (dengue = ‘evil spirit'; influenza = ‘influence’ of bad air).

By the early 1960s, new viruses were being discovered and studied by electron microscopy. As particles of different sizes, shapes, and composition were identified, it became clear that a systematic nomenclature was needed. Lwoff, Horne, and Tournier suggested a comprehensive scheme for classifying all viruses in 1962. Their proposal used the classical Linnaean hierarchical system of phylum, class, order, family, genus and species. The complete scheme was not adopted, but animal viruses were soon classified by family, genus, and species.

An important part of the scheme proposed by Lwoff and colleagues is that viruses are grouped according to their properties, not the cells they infect. The nucleic acid genome was also recognized as a primary criterion for classification. Four characteristics were to be used for the classification of all viruses:

  1. Nature of the nucleic acid in the virion
  2. Symmetry of the protein shell
  3. Presence or absence of a lipid membrane
  4. Dimensions of the virion and capsid

Other characteristics which were subsequently added include the type of disease caused, and which animals and tissues are infected. With the development of nucleic acid sequencing technologies in the 1970s, genomics has played an increasingly important role in taxonomy. Today new viruses are assigned to families based on the nucleic acid sequence of their genome.

The International Committee on the Taxonomy of Viruses (ICTV) is charged with the task of developing, refining, and maintaining a universal virus taxonomy. A complete catalog of known viruses is maintained by the ICTV at ICTVdb. Although the ICTV nomenclature is used to classify animals viruses, plant virologists do not place their viruses into families and genera, but use group names derived from the prototype virus.

Because the viral genome carries the blueprint for producing new viruses, virologists consider it the most important characteristic for classification. Next we’ll discuss the Baltimore classification, an alternative scheme based on the viral genome.

Lwoff, A., Horne, R., & Tournier, P. (1962). A system of viruses. Cold Spring Harb Symp Quant Biol., 27, 51-55

Buchen-Osmond, C. (2003). The universal virus database ICTVdB Computing in Science & Engineering, 5 (3), 16-25 DOI: 10.1109/MCISE.2003.1196303

  • mdubuque

    I'm assuming viruses have been around for billions of years. Perhaps I am mistaken, but it seems quite plausible to me.

    In the plant world we say that horsetails and fungi are older because they preceded monocots and dicots. Similarly, the centipede and the turtle are older than homo sapiens.

    Are some viruses therefore clearly more ancient than others? Are some viral genomes clearly more primitive than other more “advanced” viral genomes?

    And sorry to go to far afield, but where the heck do prions fit into all this taxonomy? I know they aren't viruses, but is it possible they shared a joint ancestor with viruses? I just don't know how they fit into the big picture of life/non life.

  • rajkumarvats

    The International Committee on the Taxonomy of Viruses database should be continuously updates as in future also we are going to see many new variants also .

  • rajkumarvats

    The International Committee on the Taxonomy of Viruses database should be continuously updates as in future also we are going to see many new variants also.

    http://www.epernicus.com/people/rajkumarvats

  • http://theartfulamoeba.com/ Jennifer Frazer

    Call me crazy, but I actually prefer some of the old names for viruses to the acronyms that have come into use today. Something about Rift Valley Fever or Ebola virus or Spanish Influenza sounds much better than SARS or HIV or even H1N1 virus, the PC term for swine flu. I wish there was some way to standardize viral names by site of first detection or discoverer rather than giving them an industrial, generic sounding acronym as a name.

    Given that viruses probably evolved independently many times, I also think it makes sense not to go above the family level taxonomy-wise. It's too bad the plant and animal virus people can't get together on nomenclature and taxonomy, though.

  • Pingback: Simplifying virus classification: The Baltimore system

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  • http://www.virology.ws profvrr

    HCV specific inhibitors against the NS3/4A polymerase and the NS5B
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    Several new protease inhibitors, nucleoside and non-nucleoside
    polymerase inhibitors and compounds with anti-HCV activity such as
    cyclophilin inhibitors, silibinin, and nitazoxanide are currently in
    clinical evaluation.

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  • Anonymous

    Computer viruses can be classified by many methods.

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  • http://rybicki.wordpress.com/ Ed Rybicki

    Ummmmmm….”plant virologists do not place their viruses into families and genera, but use group names derived from the prototype virus”…fail!

    There are some 18 families of plant viruses, with a plethora of genera, and species in good standing.  These include pararetroviruses (Caulimoviridae), ssDNA viruses (Gemini- and Nanoviridae), ss(-)RNA viruses (Bunyaviridae), picornavirus-like viruses with ss(+)RNA (Sesquiviridae), alpha-like viruses (Tobamoviridae, Bromoviridae)….

    Plant virologists are in fact the most compliant with ICTV standards: viruses are never named after anyone; phylogenies are frequently used as the basis for generic and familial classification and nomenclature, unlike the case with “human papillomavirus”, for example, some of which are more closely related to primate viruses than they are to each other.  Or HIV, for that matter: HIV-1 = SIVch, and HIV-2 = SIVsm.

    Your up-to-date reference would be Virus Taxonomy VIIIth Report of the ICTV (http://www.amazon.co.uk/Virus-Taxonomy-International-Committee-Viruses/dp/0122499514)

  • http://rybicki.wordpress.com/ Ed Rybicki

    Viruses have certainly been around for billions of years: if you consider bacterio- and archaeophages [=bacterial and archaeal viruses] you could postulate that they date back to at least the divergence of these, and probably further.

    There is a case to be made for some virus families being older than others: some viruses seem to be reasonably recent assemblages of components; the whole Order Mononegavirales [Filo-, Rhabdo-, Paramyxo- and Bornaviridae] is probably of more recent vintage than (for example) Poxviridae, because the viruses preserve more gene order and composition than many other virus families.

    Prions are an infectious state of protein folding – and so have no obvious relationship to viruses at all.  They are on the far, far end of the spectrum of life, from cells through to molecules.

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