How viruses are classified

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