The document discusses different systems for classifying viruses. The main systems are the International Committee on Taxonomy of Viruses (ICTV) system and the Baltimore classification system. The ICTV system is based on phenotypic characteristics and defines virus taxonomy, naming conventions, and species. It aims to develop and maintain a universal virus taxonomy. The Baltimore classification system places viruses into one of seven groups based on their nucleic acid and replication strategy.

1. VIRAL TAXONOMY
By Bikram Kumar Das

2. INTRODUCTION
• Virus classification is the process of naming viruses and placing
them into a taxonomic system.
• Viruses are mainly classified by phenotypic characteristics, such
as morphology, nucleic acid type, mode of replication, host
organisms, and the type of disease they cause.
• Currently there are two main schemes used for the classification of
viruses:
1. The International Committee on Taxonomy of Viruses (ICTV)
system and
2. Baltimore classification system,
which places viruses into one of seven groups. Accompanying this
broad method of classification are specific naming conventions and
further classification guidelines set out by the ICTV.

3. International Committee on
Taxonomy of Viruses (ICTV)
• The ICTV (originally the International Committee on
the Nomenclature of Viruses, ICNV) was established in
1966.
• The only body charged by the International Union of
Microbiological Societies with the task of developing,
refining, and maintaining a universal virus taxonomy.
• this system of nomenclature differs from other
taxonomic codes on several points.
• A minor point is that names of orders and families are
italicized, unlike in the International Code of
Nomenclature for algae, fungi, and plants and
International Code of Zoological Nomenclature.

4. International Committee on
Taxonomy of Viruses (ICTV)
• The ICTV had adopted the principle that A VIRUS SPECIES IS A
POLYTHETIC* CLASS OF VIRUSES THAT CONSTITUTES A REPLICATING
LINEAGE AND OCCUPIES A PARTICULAR ECOLOGICAL NICHE.
• The formal definition of a polythetic class is “a class whose
members always have several properties in common although no
single common attribute is present in all of its members”
• The qualification of a replicating lineage implies that members of a
species experience evolution over time with consequent variation,
but that members share a common ancestor.
• The qualification of occupation of an ecologic niche acknowledges
that the biology of a virus, including such properties as host range,
pathogenesis, transmission, and habitat, are fundamental
components of the characterization of a virus.

5. International Committee on
Taxonomy of Viruses (ICTV)
• In the most recent report of the ICTV, study group
members have listed the criteria that identify
each species, then listed species according to the
criteria.
• In addition, some viruses are listed as tentative
species because their taxonomic status cannot
currently be unambiguously determined.
• Last, a type species, the species used to define
the taxon, has been identified for each genus.

6. International Committee on
Taxonomy of Viruses (ICTV)
The official objectives of the ICTV are:
• To develop an internationally agreed taxonomy for viruses
• To develop internationally agreed names for virus taxa,
including species and sub-viral agents
• To communicate taxonomic decisions to all users of virus
names, in particular the international community of
virologists, by publications and via the Internet
• To maintain an index of virus names
• To maintain an ICTV database on the Internet, that records
the data that characterize each named viral taxon, together
with the common names of each taxon in all major
languages

7. International Committee on
Taxonomy of Viruses (ICTV)
The ICTV's essential principles of virus
nomenclature are:
• Stability
• To avoid or reject the use of names which
might cause error or confusion
• To avoid the unnecessary creation of names

8. International Committee on
Taxonomy of Viruses (ICTV)
• In July 2013, the ICTV definition of species changed to state: "A
species is a monophyletic group of viruses whose properties can be
distinguished from those of other species by multiple criteria.“
Viral classification starts at the level of order and continues as follows,
with the taxon suffixes given in italics:
• Order (-virales)
• Family (-viridae)
• Subfamily (-virinae)
• Genus (-virus)
• Species
Species names generally take the form of [Disease] virus.
**Of, pertaining to, or affecting a
single phylum (or other taxon) of organisms.

9. International Committee on
Taxonomy of Viruses (ICTV)
• The establishment of an order is based on the inference
that the virus families it contains have most likely evolved
from a common ancestor.
• The majority of virus families remain unplaced.
• Currently (2012), 7 orders, 96 families, 22 subfamilies, 420
genera, and 2,618 species (2,619 acc. to ICTV master
species list) of viruses have been defined by the ICTV.
• The orders are the Caudovirales, Herpesvirales,
Ligamenvirales, Mononegavirales, Nidovirales,
Picornavirales, and Tymovirales.
• These orders span viruses with varying host ranges.
The Ligamenvirales, infecting archaea, are the most recent
addition to the classification system.

11. International Committee on Taxonomy
of Viruses (ICTV)
As an example, the full formal written
description of human respiratory syncytial virus
is
• Order: Mononegavirales
• Family: Paramyxoviridae
• Subfamily: Pneumovirinae
• Genus: Pneumovirus
• Species: Human respiratory syncytial virus.

13. International Committee on Taxonomy
of Viruses (ICTV)
• Caudovirales are tailed dsDNA (group I) bacteriophages.
• Herpesvirales contain large eukaryotic dsDNA viruses.
• Ligamenvirales contains linear, dsDNA (group I) archaean viruses.
• Mononegavirales include nonsegmented (-) strand ssRNA (Group
V) plant and animal viruses.
• Nidovirales are composed of (+) strand ssRNA (Group IV) viruses
with vertebrate hosts.
• Picornavirales contains small (+) strand ssRNA viruses that infect a
variety of plant, insect and animal hosts.
• Tymovirales contain monopartite (+) ssRNA viruses that infect
plants.
• Other variations occur between the orders: Nidovirales, for
example, are isolated for their differentiation in expressing
structural and nonstructural proteins separately.

14. Structure-based virus classification
• It has been suggested that similarity in virion assembly
and structure observed for certain viral groups
infecting hosts from different domains of life (e.g.,
bacterial tectiviruses and eukaryotic adenoviruses or
prokaryotic Caudovirales and eukaryotic herpesviruses)
reflects an evolutionary relationship between these
viruses.
• Therefore, structural relationship between viruses has
been suggested to be used as a basis for defining
higher-level taxa - structure-based viral lineages - that
could complement the existing ICTV classification
scheme

15. BALTIMORE CLASSIFICATION
Baltimore classification (first defined in 1971) is a
classification system that places viruses into one of seven
groups depending on
• a combination of their nucleic acid (DNA or RNA),
• strandedness (single-stranded or double-stranded),
• Sense (+ or -), and
• method of replication.
Named after David Baltimore, a Nobel Prize-winning
biologist, these groups are designated by Roman
numerals

16. BALTIMORE CLASSIFICATION

17. BALTIMORE CLASSIFICATION
• I: dsDNA viruses (e.g. Adenoviruses, Herpesviruses,
Poxviruses)
• II: ssDNA viruses (+ strand or "sense") DNA (e.g.
Parvoviruses)
• III: dsRNA viruses (e.g. Reoviruses)
• IV: (+)ssRNA viruses (+ strand or sense) RNA (e.g.
Picornaviruses, Togaviruses)
• V: (−)ssRNA viruses (− strand or antisense) RNA (e.g.
Orthomyxoviruses, Rhabdoviruses)
• VI: ssRNA-RT viruses (+ strand or sense) RNA with DNA
intermediate in life-cycle (e.g. Retroviruses)
• VII: dsDNA-RT viruses (e.g. Hepadnaviruses)

18. BALTIMORE CLASSIFICATION

19. 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)

20. 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)