Classification of Viruses

Taxonomy and Classification of Viruses

Introduction
There are numerous types of virus present and new species are still being discovered today. They are present in almost every type or organism such as animals, plants, bacteria and fungi and thus require a system of classification so as to have a better understanding on the type of viruses present. According to the WHO, FAO and other international agencies, there are approximately 30,000 viruses, strains and subtypes but many still remain unassigned because they are not properly categorized.

An example of taxonomy of organisms

The first International Congress of Microbiology was held in Paris in 1930 but it was only during the International Congress of Microbiology held in Moscow in 1966, that the first attempt to come up with a system of classification of viruses was made.

The international Committee on Taxonomy of Viruses (ICTV) is a committee that handles the taxonomy of viruses. They have come up with a universal taxonomic scheme that follows the hierarchy taxa: Order - Family – Sub-family – Genus – Species. So far they have approved of 3 orders, 73 families, 9 subfamilies, 287 genera and 5450 viruses from 1950 species.Their first report was submitted in 1971 and the subsequent reports where released in 1976, 1979, 1982, 1991, 1995, 2000 and 2005.

There is also a classification criteria

• Nature of genome and sequence relatedness.
• Virus structure (symmetry, shape, etc).
• Natural host range.
• Cell and tissue tropism.
• Pathogenicity and cytopathology.
• Mode of transmission.
• Physiochemical properties of virions.
• Antigenic properties of viral proteins.

Despite having the ICTV, there are other people who come up with their own methods of classification for viruses. Two such examples would be Lwoff’s scheme of classification and David Baltimore’s system for classification. Lwoff’s scheme is based on several factors. They are shared properties of viruses instead of host properties, the physical properties of virus, nucleic acid of virus, symmetry of capsid, presence/absence of an envelope and the dimensions of virion and capsid. Baltimore’s system however, is solely based on the viral genome and its relationship to mRNA.

Baltimore’s system of classification separates the several kinds of DNA and RNA into seven classes as different viruses have different types DNA and RNA. Thus under this system of classification, the viruses are grouped according to their type of DNA and RNA. Below is a table of the classes.

dsDNA - double-stranded DNA.

ssDNA - single-stranded DNA

dsRNA - double-stranded RNA

(+) sense - positive(+) sense

(-) sense - negative(-) sense

(+) sense RNA is identical to mRNA while (-) sense RNA is complimentary to mRNA.

RNA reverse transcribing virus is transcribed from RNA to DNA.

DNA reverse transcribing virus is a DNA polymerase enzyme transcribed from ssRNA to DNA.

An example of classification

Reference
http://en.wikipedia.org/wiki/International_Committee_on_Taxonomy_of_Viruses
http://www.iums.org/Congresses/index.html)-17/12/08
http://www.ictvonline.org/virusTaxInfo.asp)-17/12/08
http://biology.about.com/od/evolution/a/aa092304a.htm)-17/12/08
http://en.wikipedia.org/wiki/Baltimore_classification