What's In A Name?

Viruses Are Cool #2
Click here to read the introduction to this series.

One of the most interesting features of viruses is their diversity. Viruses vary greatly in physical characteristics, genetic material, replication strategy, route of transmission, species and cell tropism, and pathogenesis. And there are lots of them, so it's not really so surprising that the naming and classification of viruses can be a bit confusing.

For example, variola, orf, myxoma, ectromelia, and molluscum contagiosum viruses are all poxviruses, but you wouldn't be able to tell that from their names. And chickenpox virus? It's actually a herpesvirus and not a poxvirus at all. Another example is pseudorabies virus, which is also a herpesvirus and not to be confused with rabies virus, which is a rhabdovirus.

The problem is the inconsistent, and sometimes idiosyncratic, ways in which viruses are named. Some are named after their discoverers (i.e., Epstein-Barr virus) and others are named after the places where they were discovered (i.e., Nipah and Coxsackie viruses). Viruses are also named after the types of diseases they cause. This may sound like a perfectly rational thing to do, but it can still lead to confusion. For example, hepatitis A, hepatitis B, and hepatitis C viruses are, respectively, a picornavirus, a hepadnavirus, and a flavivirus. And viruses often have multiple names--chickenpox virus is the colloquial name for varicella zoster virus, which is also known as human herpesvirus 3.

So how do virologists manage to keep all of this straight? Um...well, we don't. But there are two virus classification systems currently being used that are helpful. The Baltimore system segregates viruses into seven categories based on their genetic material (i.e., double-stranded DNA, single-stranded RNA, etc.) and replication intermediates.

Additionally, rules established by the International Committee on Taxonomy of Viruses (ICTV) classify viruses by order, family, subfamily, genus, and species. Viruses are assigned to genera and families based on shared combinations of traits related to virion morphology and biological properties of the viruses. An accompanying decimal system uniquely identifies individual viruses and denotes their linkage to their genus and family. Here is an example:

• 00. Order Unassigned
• 00.058. Family Poxviridae
• 00.058.1. Subfamily Chordopoxvirinae
• 00.058.1.03. Genus Avipoxvirus
• 00.058.1.03.001 Species Fowlpox virus

So there is some order in viral taxonomy after all, but not too much--that would spoil the fun. For those who are interested in further exploring the viral smorgasbord, the ICTV Universal Virus Database is a great place to start. You can browse through the alphabetical listing of viruses or search for a specific one to read about. And they also have a fantastic picture gallery of just about every virus family. Happy virus hunting!

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