CLASSIFICATION
OF VIRUSES
Following the
discovery of viruses, earliest studies on them were based on their
filterability, and
observations on the diseases which they caused. Early classification
systems were premised
on pathogenic effects of the viruses and their transmission
patterns. However,
with the invention of electron microscope and sophisticated molecular
techniques that
permitted ultra-structural studies, details of the structures and
compositions of
viruses began to emerge. Thereafter, it became possible to group viruses
on the basis of
shared features of the virions.
Consequently, the
following general parameters are have been used for classification of
viruses:
1.
Pathogenicity
2.
Ecological characteristics
3.
Physico-chemical characteristics
Pathogenicity:
In this classification,
viruses affecting same tissues producing similar syndrome and
pathological
manifestations are grouped together.
-
Viruses affecting the respiratory tract: Influenza, rhinoviruses,
parainfluenza, adenovirus
-
Vesicular viruses: Foot-and-Mouth-Disease, vesicular stomatitis
-
Central nervous system viruses: rabies, equine encephalitis, entroviruses
(polio), mumps
-
Mucous membrane viruses: Myxomaviruses
-
Enteric viruses: rotaviruses,
-
Limitations of this classification system: some viruses affect more than one
system of
the
body and they will belong to several groups. Pantropic viruses affecting
multiple
systems
such as canine distemper, Newcastle disease, rinderpest, pestes des petits
ruminants
will belong to respiratory, enteric and CNS groups.
Ecological characteristics:
Ecological features
of viruses such as the involvement of vectors or vertebrate reservoirs
in their transmission
cycles and maintenance in nature can be used for classification.
Viruses are
classified into arboviruses and non-arboviruses or roboviruses and
nonroboviruses.
Arboviruses: these
are viruses that are transmitted biologically between blood sucking
arthropods (such as
ticks, culicoides, mosquitoes) and vertebrate hosts. They cause
disease in the
vertebrate host but not in the arthropods. Examples include African swine
36
fever virus (soft
ticks), Yellow fever virus (mosquitoes), Equine encephalitis virus
(mosquitoes), African
horse sickness virus (culicoides).
Roboviruses: These
are viruses with rodent reservoirs. Infected rodents are
asymptomatic. They
shed the virus in their urine and contaminate the human
surroundings, food,
drinks and formites. Example include Lassa fever virus with rat as
reservoir.
Physico-chemical
characteristics:
These are the most
reliable, verifiable and satisfactory parameters for classifying viruses.
Viruses are
classified based on the following criteria:
Type
of nucleic acid (RNA or DNA)
Number
of strands of the nucleic acid (single or double stranded)
Physical
construction of the nucleic acid (linear, circular, circular with break,
segmented,
non-segmented)
Polarity
of the viral genome: positive polarity (viral genome can be used directly as
mRNA)
and negative polarity(viral genome must be transcribed into mRNA)
Symmetry
of the nucleocapsid
Presence
or absence of envelope
Size
of the virus
Antigenic
and chemical compositions
Susceptibility
to physical and chemical changes
Based on these
criteria, viruses are grouped into families, subfamilies and genera. Further
subdivision is based
on the degree of antigenic similarity and serological tests
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