Classification of Viruses
Basis of Classification
The following properties have been used as a basis for the
classification of viruses:
(1) Virion morphology, including size, shape, type of symmetry, presence
or absence of peplomers, and presence or absence of membranes.
(2) Virus genome properties, including type of nucleic acid (DNA or
RNA), size of genome in kilobases (kb) , strandedness (single or double),
whether linear or circular, sense (positive, negative), segments (number,
size), nucleotide sequence and G + C content.
(3) Physicochemical properties of the virion, including molecular mass,
pH stability, thermal stability, and susceptibility to physical and chemical
agents, especially ether and detergents.
(4) Virus protein properties, including number, size, and functional
activities of structural and nonstructural proteins, amino acid sequence,
and modifications (glycosylation, phosphorylation).
(5) Genome organization and replication, including gene order, number
and position of open reading frames, strategy of replication (patterns of
transcription, translation), and cellular sites (accumulation of proteins,
virion assembly, virion release).
(6) Antigenic properties.
(7) Biologic properties, including natural host range, mode of
transmission, vector relationships, pathogenicity, tissue tropisms, and
pathology.
Classification Systems
1- ICTV classification
The International Committee on Taxonomy of Viruses (ICTV)
developed the current classification system and put a greater certain virus
properties to maintain family uniformity. The general taxonomic structure
is as follows:
Order (-virales)
Family (-viridae)
Subfamily (-virinae)
Genus (-virus)
Species (-virus)
In the current (2011) ICTV taxonomy, six orders have been
established, the Caudovirales, Herpesvirales, Mononegavirales,
Nidovirales, Picornavirales and Tymovirales. A seventh order
Ligamenvirales has also been proposed. The committee does not
formally distinguish between subspecies, strains, and isolates.
2- Baltimore classification
Main article: Baltimore classification
The Baltimore Classification of viruses is based on the method of
viral mRNA synthesis.
The Nobel Prize-winning biologist David Baltimore devised the
Baltimore classification system. The ICTV classification system is
used in conjunction with the Baltimore classification system in
modern virus classification.
The Baltimore classification of viruses is based on the mechanism of
mRNA production. Viruses must generate mRNAs from their
genomes to produce proteins and replicate themselves, but different
mechanisms are used to achieve this in each virus family. Viral
genomes may be single-stranded (ss) or double-stranded (ds), RNA or
DNA, and may or may not use reverse transcriptase (RT). In addition,
ssRNA viruses may be either sense (+) or antisense (−). This
classification places viruses into seven groups as above.
As an example of viral classification, the chicken pox virus, varicella
zoster (VZV), belongs to the order Herpesvirales, family
Herpesviridae,
subfamily
Alphaherpesvirinae,
and
genus
Varicellovirus. VZV is in Group I of the Baltimore Classification
because it is a dsDNA virus that does not use reverse transcriptase.
DNA viruses


Group I: viruses possess double-stranded DNA.
Group II: viruses possess single-stranded DNA.
Virus Family
1.Adenoviridae
Examples (common
Virion
Capsid
names)
naked/enveloped Symmetry
Adenovirus, hepatitis
Naked
virus
2.Papillomaviridae Papillomavirus
3.Parvoviridae
4.Herpesviridae
5.Poxviridae
Naked
Parvovirus B19,
Naked
Canine parvovirus
Herpes simplex virus,
varicella-zoster virus,
Enveloped
cytomegalovirus,
Epstein-Barr virus
Smallpox virus, cow
pox virus, sheep pox
virus, orf virus,
Complex coats
monkey pox virus,
vaccinia virus
6.Hepadnaviridae Hepatitis B virus
Enveloped
Nucleic
acid Group
type
Icosahedral ds
Icosahedral
I
ds
I
circular
Icosahedral ss
II
Icosahedral ds
I
Complex
I
ds
circular,
Icosahedral partially VII
ds
Polyoma virus; JC
virus (progressive
7.Polyomaviridae
Naked
multifocal
leukoencephalopathy)
Icosahedral
ds
I
circular
8.Anelloviridae
Icosahedral
ss
II
circular
Torque teno virus
Naked
RNA viruses


Group III: viruses possess double-stranded RNA genomes
Group IV: viruses possess positive-sense single-stranded RNA genomes.
Group V: viruses possess negative-sense single-stranded RNA genomes
Virus Family
Examples (common names)
1.Reoviridae
Reovirus, Rotavirus
Enterovirus, Rhinovirus,
Hepatovirus, Cardiovirus,
Aphthovirus, Poliovirus,
2.Picornaviridae
Parechovirus, Erbovirus,
Kobuvirus, Teschovirus,
Coxsackie
3.Caliciviridae
Norwalk virus, Hepatitis E virus
4.Togaviridae
Rubella virus
Lymphocytic choriomeningitis
5.Arenaviridae
virus
Dengue virus, Hepatitis C virus,
6.Flaviviridae
Yellow fever virus
Influenzavirus A, Influenzavirus
7.Orthomyxoviridae B, Influenzavirus C, Isavirus,
Thogotovirus
Measles virus, Mumps virus,
Respiratory syncytial virus,
8.Paramyxoviridae
Rinderpest virus, Canine
distemper virus
California encephalitis virus,
9.Bunyaviridae
Hantavirus
10.Rhabdoviridae Rabies virus
11.Filoviridae
Ebola virus, Marburg virus
12.Coronaviridae Corona virus
13.Astroviridae
Astrovirus
14.Bornaviridae
Borna disease virus
Arterivirus, Equine Arteritis
15.Arteriviridae
Virus
Naked
Nucleic
acid Group
type
Icosahedral ds
III
Naked
Icosahedral ss
IV
Naked
Enveloped
Icosahedral ss
Icosahedral ss
IV
IV
Enveloped
Complex
V
Enveloped
Icosahedral ss
IV
Enveloped
Helical
ss(-)
V
Enveloped
Helical
ss(-)
V
Enveloped
Helical
ss(-)
V
Enveloped
Enveloped
Enveloped
Naked
Enveloped
Helical
Helical
Helical
Icosahedral
Helical
ss(-)
ss(-)
ss
ss
ss(-)
V
V
IV
IV
V
Enveloped
Icosahedral ss
Capsid
Capsid
naked/enveloped Symmetry
ss(-)
IV
Reverse transcribing viruses


Group VI: viruses possess single-stranded RNA genomes and replicate using
reverse transcriptase. The retroviruses are included in this group, of which HIV is
a member.
Group VII: viruses possess double-stranded DNA genomes and replicate
using reverse transcriptase. The hepatitis B virus can be found in this group.
3- Holmes classification
Holmes (1948) used Carolus Linnaeus's system of binomial nomenclature to
classify viruses into 3 groups under one order, Virales. They are placed as follows:



Group I: Phaginae (attacks bacteria)
Group II: Phytophaginae (attacks plants)
Group III: Zoophaginae (attacks animals)
4- LHT System of Virus Classification
The LHT System of Virus Classification is based on chemical and
physical characters like nucleic acid (DNA or RNA), Symmetry (Helical
or Icosahedral or Complex), presence of envelope, diameter of capsid,
number of capsomers. This classification was approved by the
Provisional Committee on Nomenclature of Virus (PNVC) of the
International Association of Microbiological Societies (1962).