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
They are four types of classification system can be summarized as
follow :
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
The Baltimore classification of viruses is based on the method of
viral mRNA synthesis.
Figure 1: Baltimore Classification of viruses
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.
Table 1: Examples of DNA viruses
Virus Family
1.Adenoviridae
2.Papillomaviridae
3.Parvoviridae
4.Herpesviridae
5.Poxviridae
6.Hepadnaviridae
Examples (common
Virion
Capsid
names)
naked/enveloped Symmetry
Nucleic
acid Group
type
Adenovirus, hepatitis
virus
Naked
Icosahedral
ds
I
Papillomavirus
Naked
Icosahedral
ds
circular
I
ss
II
ds
I
ds
I
Parvovirus B19,
Naked
Icosahedral
Canine parvovirus
Herpes simplex virus,
varicella-zoster virus,
Enveloped
Icosahedral
cytomegalovirus,
Epstein-Barr virus
Smallpox virus, cow
pox virus, sheep pox
virus, orf virus,
Complex coats Complex
monkey pox virus,
vaccinia virus
Hepatitis B virus
Enveloped
circular,
Icosahedral partially
ds
VII
Polyoma virus; JC
virus (progressive
7.Polyomaviridae
multifocal
leukoencephalopathy)
Naked
Icosahedral
ds
circular
I
8.Anelloviridae
Naked
Icosahedral
ss
circular
II
Torque teno virus
The genetic material is important for determining the structure and
behavior of a cell. In a virus, a protein coat called a "capsid" surrounds
the nucleic acid. This coat serves to protect the nucleic acid and aid in its
transmission between host cells. The capsid is made of many small
protein particles called "capsomeres," and can be formed in three general
shapes – helical, icosahedral (a 20-sided figure with equilateral triangles
on each side), and complex. Some of the more advanced viruses have a
third structure that surrounds the capsid. This is called the "envelope" and
is composed of a bilipid layer, like the membrane on a cell, and
glycoproteins, which are protein and carbohydrate compounds. The
envelope serves to disguise the virus to look like a 'real' cell, protecting it
from appearing as a foreign substance to the immune system of the host.
The structure of a virus is closely related to its mode of reproduction
Viral Shapes
Name
Basic Shape
Example
(electron micrograph)
Helical
Tobacco mosaic virus
Icosahedral
Herpes simplex
Complex
T-4 Bacteriophage
Figure 2: Viral shapes
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
Table 2: Examples of RNA viruses
Virus Family
Examples (common names)
1.Reoviridae
Reovirus, Rotavirus
Enterovirus, Rhinovirus,
Hepatovirus, Cardiovirus,
Aphthovirus, Poliovirus,
Parechovirus, Erbovirus,
Kobuvirus, Teschovirus,
Coxsackie
Norwalk virus, Hepatitis E virus
Rubella virus
Lymphocytic choriomeningitis
virus
Dengue virus, Hepatitis C virus,
Yellow fever virus
Influenzavirus A, Influenzavirus
B, Influenzavirus C, Isavirus,
Thogotovirus
Measles virus, Mumps virus,
Respiratory syncytial virus,
Rinderpest virus, Canine
distemper virus
California encephalitis virus,
Hantavirus
Rabies virus
Ebola virus, Marburg virus
Corona virus
Astrovirus
Borna disease virus
Arterivirus, Equine Arteritis
Virus
2.Picornaviridae
3.Caliciviridae
4.Togaviridae
5.Arenaviridae
6.Flaviviridae
7.Orthomyxoviridae
8.Paramyxoviridae
9.Bunyaviridae
10.Rhabdoviridae
11.Filoviridae
12.Coronaviridae
13.Astroviridae
14.Bornaviridae
15.Arteriviridae
Naked
Nucleic
acid Group
type
Icosahedral
ds
III
Naked
Icosahedral
ss
IV
Naked
Enveloped
Icosahedral
Icosahedral
ss
ss
IV
IV
Enveloped
Complex
ss(-)
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
IV
Capsid
Capsid
naked/enveloped Symmetry
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).