TWiV 357: Mistletoe on the Tree of Life

On episode #357 of the science show This Week in Virology, Jens Kuhn joins the TWiVomics to discuss the strengths and weaknesses of viral taxonomy, including its history and evolution, how viruses are ordered, and why T. rex was classified without having a living isolate.

You can find TWiV #357 at

Is it Ebolavirus or Ebola virus?

Filovirus virionWhen I drafted my article for TakePart (Don’t Panic – Ebola Isn’t Heading For You), I used the term ‘ebolavirus’ throughout, but the editors changed every instance to ‘Ebola virus’. Understanding which term is correct is far more complicated than you might imagine.

A new virus was first isolated in 1976 from patients during an outbreak of hemorrhagic fever in southern Sudan and northern Zaire. The name Ebola virus was proposed to describe the agent of this outbreak:

…the name Ebola virus is proposed for this new agent. Ebola is a small river in Zaire which flows westward, north of Yambuku, the village of origin of the patient from whom the first isolate was obtained.

The name was further modified with the subsequent finding of distinct isolates of the virus (e.g. Zaire Ebola virus, Sudan Ebola virus, Reston Ebola virus). In 2002 the virus names were contracted (Zaire ebolavirus, Sudan ebolavirus).

The way that viruses are named is regulated by the International Committee on Taxonomy of Viruses (ICTV). The current virus nomenclature for the Ebolaviruses is as follows:

Family: Filoviridae
Genus: Ebolavirus
Species (5)Bundibugyo ebolavirus, Reston ebolavirus, Sudan ebolavirus, Tai Forest ebolavirus, Zaire ebolavirus

This is why I used ebolavirus in the original draft of my article.

However, this new nomenclature did not work well, as summarized in a 2010 article, Proposal for a revised taxonomy of the family Filoviridae:

Five to eight years have passed since the introduction of the names Cote d’Ivoire ebolavirus [sic], Reston ebolavirus, Sudan ebolavirus, and Zaire ebolavirus for the members of the four recognized ebolavirus species. Instead of using these names, the overwhelming majority of publications refer to “Ebola virus” instead of Zaire ebolavirus, a preference that is also followed by the public press.

The authors conclude that introducing the name ‘Zaire ebolavirus’ was an error, and recommend reverting to the traditional virus name, Ebola virus:

Retrospectively, the virus nomenclature in most published articles will then be correct. Likewise, press articles, which almost invariably refer to “Ebola virus,” and usually with that term aim at referring to the virus that is currently officially named “Zaire ebolavirus,” will be correct retrospectively and prospectively. As the traditional names are different from the species names, confusing species and virus names will be much more difficult, even in the absence of taxonomic education.

When this proposal is officially ratified by the ICTV the nomenclature will be as follows:

Family: Filoviridae
Genus: Ebolavirus
Species: Tai Forest ebolavirus
Virus: Tai Forest virus (formerly Cote d’Ivoire ebolavirus)
Species: Reston ebolavirus
Virus: Reston virus
Species: Sudan ebolavirus
Virus: Sudan virus
Species: Zaire ebolavirus
Virus: Ebola virus
Species: Bundibugyo ebolavirus
Virus: Bundibugyo virus

This discussion leads us to the important difference between a virus and a species.  A virus species is defined as a polythetic class of viruses that constitutes a replicating lineage and occupies a particular ecological niche. According to the ICTV rules of nomenclature, virus species names are italicized with the first letter of the name capitalized (Zaire ebolavirus). Virus names (poliovirus) are written in lower case (except if a part of the virus name is a proper noun, e.g. Coxsackievirus) in non-italicized script.

The editors at TakePart changed my ‘ebolavirus’ to Ebola virus because that is the term they are familiar with. Using this name is not correct because Sudan virus, not Ebola virus, is responsible for the current outbreak in Uganda.

Incidentally, the virus I work on, poliovirus, is a member of the family Picornaviridae, genus Enterovirus, species Human enterovirus C. Poliovirus is the name of the virus. But you will often find it incorrectly called ‘polio virus’ in the popular press. At one time this virus was called ‘poliomyelitis virus’ which was shortened to ‘poliovirus’, not ‘polio virus’.

TWiV 110: CSI virology

Alan DoveHosts: Vincent Racaniello, Dickson DespommierAlan Dove, and Rich Condit

On episode #110 of the podcast This Week in Virology, Vincent, Alan, Rich, and Dickson discuss bacteria that can utilize arsenic in place of phosphorus, the passing of Frank Fenner, polio outbreak in The Congo, solving criminal cases of HIV transmission, and classifying viruses by capsid structure.

Click the arrow above to play, or right-click to download TWiV #110 (68 MB .mp3, 93 minutes).

Subscribe to TWiV (free) in iTunes , at the Zune Marketplace, by the RSS feed, or by email, or listen on your mobile device with Stitcher Radio.

Links for this episode:

Weekly Science Picks

Dickson – CDC’s West Nile virus case count for 2010
Rich –
The red bees of Red Hook
Alan – Arsenic-based life at XKCD
Vincent –
PLoS iPad app

Send your virology questions and comments (email or mp3 file) to or leave voicemail at Skype: twivpodcast. You can also post articles that you would like us to discuss at and tag them with twiv.

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