7/29/2019 Appendix C - Virus Taxonomy

1/2

Viruses are classified on the basis of:

Morphology

Size, capsid structure (icosahedral symmetry,

helical symmetry, complex symmetry) and

appearance in the electron microscope, pre-

sence of envelope, other structural features

(presence of cellular structures or unusual

viral structures, e.g. ribosomes in Arenav ir i-dae, tegument in Herpesviridae).

Classification Suffi x Example Notes

Order -virales Mononegavira/es

Family -viridae ParamyxoviridaeSubfamily -virinae Pneumovirinae

Genus -virus PneumovirusSpecies None Respiratory

syncytial virus

Only one orderdefined to date.

Always defined.Not alwaysdefined.

Always defined.Always defined.

Genome

Type of nucleic acid (RNA or DNA), number of

strands (single or double) and relationship to

messenger RNA if single stranded (termed

'positive sense' if of the same base sequence

as mRNA, 'negative sense' if of complemen-

tary sequence) or segments present, sequence

homology and similarities of genetic organiza-

tion to other viruses.

Physical and biochemical propertiesBuoyant density (in cesium chloride) and sedi-

mentation constant, pH stability, number and

size of proteins present, presence of lipid

(usually as an envelope).

Serology

Presence of antigens cross-reactive with otherviruses.

Pathology

Nature of

involved.

disease caused, mechanisms

A novel virus wil l be assigned to a family

(ending -viridae) and genus (ending -virus),

and possibly to other taxonomical subdivi-

sions, on the basis of these properties. The

heirarchy of classification is:

The size of individual virus families varies enor-

mously, for example:

Filoviridae: Two members, Marburg and Ebola

viruses

Picornaviridae: Over two hundred members,

including poliovirus, rhinoviruses (common

cold), hepatitis A virus.

As an example of the processes involved in

virus classification:

Human herpes virus 6 was assigned to thefamily Herpesviridae on the basis of its

morphology (enveloped, with a 100-110 nm

icosahedral nucleocapsid and visible tegu-

ment between the nucleocapsid and the envel-

ope) and the presence of a large double-

stranded DNA genome, and noted to be

related to Epstein-Barr virus (human herpes

virus 5, subfamily Gammaherpesvirinae) onthe basis of its behaviour in cell culture, has

since been reclassified as closer to cytomega-

Iovirus (human herpes virus 4, subfamily Beta-

herpesvirinae) on the basis of its genomic

structure and sequence.

While the nature of the disease caused by a

virus has little effect on its placement in tax-

onomy, it may influence the name used for the

virus itself. For example: The human hepatitis

Virology Methods ManualISBN 0-12-465330-8

Copyright 9 1996 Academic Press LtdAll rights of reproduction in any form reserved

7/29/2019 Appendix C - Virus Taxonomy

2/2

V i r o l og y m e t h o d s m a n u a lviruses A to E are only very distantly related,and belong to four different virus families (A,Picornaviridae; B, Hepadnaviridae; C, Flavivir-idae; E, Caliciviridae), while Hepatit is D 'virus'is a sub-viral infectious agent related to theviroids of plants. Despite this, they have verysimilar names, based on their role in hepatitis.A virus is usually named, often from the dis-ease caused, before it is assigned a place inthe taxonomical structure, and it may take agreat deal of time before a final classification isagreed by the International Committee on theTaxonomy of Viruses (ICTV), the responsible

body in these matters, and even this maychange when new evidence becomesavailable. For the current classification ofmost known viruses, readers are directed tothe latest report of the International Commit-tee on Taxonomy of Viruses, whos e details areas fol lows:Virus Taxonomy. Sixth report of the Interna-tional Committee on Taxonomy of Viruses ed.FA Murphy, CM Fauquet, DHL Bishop, SAGhabriel, AW Jarvis, GP Martelli, MA Mayo,MD Summers. Archives of Virology, Supp. 10,Vienna: Springer Verlag, 1995.

3 6 6 A p p e n d i x C