By Dr Ananya Mandal, MD
Initially after viruses were discovered there was no system for classifying viruses. Consequently viruses were named haphazardly. Most of the vertebrate viruses have been named according to:
- the associated diseases (poliovirus, rabies)
- the type of disease caused (murine leukemia virus),
- the sites in the body affected or from which the virus was first isolated (rhinovirus, adenovirus)
- the places from where they were first isolated (Sendai virus, Coxsackievirus)
- the scientists who discovered them (Epstein-Barr virus), or
- due to common cultural perceptions e.g. influenza ‘influence’ of bad air or dengue ‘evil spirit’
When did the classification of viruses begin?
The actual classification of viruses began in the 1960’s when new viruses were being discovered and studied by electron microscopy. When structure was clarified the need for a new system of classification was felt.
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. Although the full scheme could not be adopted for viruses but animal viruses were soon classified by family, genus, and species.
Characteristics used to classify viruses
According to the classification, viruses are grouped according to their properties, not the cells they infect. The main criteria were the type of nucleic acid – DNA or RNA.
Four characteristics were to be used for the classification of all viruses:
- Type of the nucleic acid including size of the genome, strandedness (single or double), linear or circular, positive or negative (sense), segments (number and size), sequence and G+C content etc.
- Symmetry of the protein shell
- Presence or absence of a lipid membrane
- Dimensions or the size of the virion and capsid
Other properties include the physicochemical properties including molecular mass, pH, thermal stability, susceptibility to chemicals and physical extremes and to ether and detergents.
Naming convention primarily depends on the genome and nucleic acid material of the viruses with the development of nucleic acid sequencing technologies in the 1970s. Naming is performed by the International Committee on the Taxonomy of Viruses (ICTV). A complete catalog of known viruses is maintained by the ICTV at ICTVdb.
The order is as follows;
- Order – virales
- Family –viridae
- Subfamily –virinae
- Genus –virus
- Species –virus
In the 2011 ICTV classification there are six orders – Caudovirales, Herpoesvirales, Mononegavirales, Nidovirales, Picornavirales and Tymovirales. The seventh Ligamenvirales has been proposed.
The Baltimore classification
This classifies according to the viral mRNA synthesis. This came from Nobel prize winner David Baltimore.
ICTV and Baltimore classifications used together
At present both ICTV and Baltimore classification are used together. Group I for example possesses double stranded DNA and group II single stranded DNA, Group III with double stranded RNA and Group IV with positive single stranded RNA and Group V with negative sense single stranded RNA. Group VI further has single stranded RNA with reverse transcriptase that converts RNA to DNA like HIV virus and Group VII has double stranded DNA with reverse transcriptase and this includes Hepatitis B virus.
Reviewed by April Cashin-Garbutt, BA Hons (Cantab)