1. Classification of viruses
Virology 1.3
Classification of viruses

2. Some actual virus names
Cricket paralysis virus
Molluscum contagiosum
Bean golden mosaic virus
Barley yellow dwarf virus
Virus names are so cute Why not stop there? Why bother to classify at all?

3. 1) Classification appeals to sense of logic or beauty
There is an intrinsic simplicity of nature, and the ultimate contribution of science resides in the discovery of unifying and simplifying generalizations, rather than in the description of isolated situations—in the visualization of simple overall patterns rather than in the analysis of patchworks.
“Salvador Luria”, General Virology, Wiley:New York 1953

4. 2. Understanding Virus Evolution
Viruses infect members of all three kingdoms and their evolution is intimately tied to the evolution of their hosts.
Charles Darwin
Tree of Life

5. 3. Utility and Practicality- a. Historical method….
Places where viruses were found or people who discovered them: e.g.:
Epstein-Barr virus,
Rous sarcoma virus,
Rift Valley fever virus

6. Utility and practicality b. social impact
Viruses cause diseases
Suffering, economic damage, etc.
Classification may aid in understanding

7. The pragmatic approach tends to win out
A time-honored method is to base names on disease properties…… Diseases viruses are associated with, e.g.: Poxvirus, Hepatitis virus, HIV, Measles virus. Cytopathology occuring during infection, e.g.: Respiratory Syncytial virus, Cytomegalovirus. Site of infection, e.g.: Adenovirus, Rhinovirus, Enterovirus

8. “Hepatitis” Viruses illustrate some of the problems with this pragmatic approach
Hepatitis A Virus- a picornavirus
Hepatitis B Virus- a hepatovirus
Hepatitis C Virus- a flavivirus
(Partial listing only! There are more viruses that cause hepatitis.)
Similar diseases but different viruses

9. Similar viruses-different diseases
Related Herpesviruses Cause Different Diseases
HSV Herpes Simplex Virus Cold sores (type 1), Genital lesions (type 2)
VZV Varicella Zoster Virus Chicken pox
CMV Cytomegalovirus Organ failure
EBV Epstein-Barr Virus Mononucleosis, Burkitt’s lymphoma,
Nasopharyngeal carcinoma
and HHV-6, HHV-7, HHV-8…..
Similar viruses-different diseases
Therefore if these viruses were classified based on their symptoms their relationships would be missed.

10. “Simple” classification systems did not work well
Different viruses can cause (nearly) the same symptoms. e.g., the hepatitis viruses
However, distinct members of the same group can cause different symptoms. e.g., the herpes viruses
“Simple” classification systems did not work well
Virologists had to devise more orderly systems

11. “informal” systems proliferated “national” naming systems
Host-based naming systems
Strong personalities were involved
A profusion of data
The problems with virus nomenclature and classification led to chaos by the 1960s

12. International Committee on the Taxonomy of Viruses (ICTV)
Federation of European Microbiology Societies (FEMS)
Virology Division
International Committee
on the Nomenclature of Viruses (1966)
Renamed in 1971
International Committee on the Taxonomy of Viruses (ICTV)

13. Study groups practice the opinionated use of data.
The ICTV is charged to devise an internationally-acceptable system for naming and classification of viruses and to communicate that system.
The ICTV seeks input from a wide range of virologists and meets every three years to revise the taxa.
Study groups practice the opinionated use of data.
Seven orders, 104 families, 23 subfamilies, >350 genera, >2,500 species

14. Characteristics of the ICTV system
“Non-systematic”
Polythetic
Hierarchical w/species as lowest level
Universal
Limited (very conservative)
Uses “order of presentation” not a universal tree
Latinized, but no binomials
Preserves common or trivial names at species level

15. The virus species is the fundamental unit.
ICTV Definition
The virus species is the fundamental unit.
A virus species (or quasi-species) is defined as a polythetic class of viruses that constitutes a replicating lineage and occupies a particular ecological niche.

16. All Viruses 1-5 are in the same virus species
Polythetic class
Individual
Properties 1 A B C D 2 E 3 4 5
All Viruses 1-5 are in the same virus species

17. ICTV Hierarchical nomenclature
Taxonomic Level
ICTV Suffix
Order
-virales
Family
-viridae
Subfamily
-virinae
Genus
-virus
Species
none (trivial name preserved)

18. Example 1 Taxonomic Level ICTV Name Order (none-conservative!) Family
Poxviridae
Subfamily
Chordopoxvirinae
Genus
Orthopoxvirus
Species
Vaccinia virus

19. Potato yellow dwarf virus
Example 2
Taxonomic Level
ICTV Name
Order
Mononegavirales
Family
Rhabdoviridae
Subfamily
Genus
Nucleorhabdovirus
Species
Potato yellow dwarf virus

20. XXXXXXXXXXXXXXXXXXXXX
Order of Presentation
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21. Virus Classification is Based Primarily on Properties of the Virus Particle (see virosphere)
Genetic material Is Most Important!!! (See Baltimore Scheme)
form of nucleic acid (RNA, DNA, ss or ds)
genetic organization
sequence homology
Morphology:
by electron microscopy
Serology:
antigenic cross-reactivity
Biological Properties have a low priority

22. Baltimore classification of viral genomes -1971
Based on gene expression
Relationship to mRNA is central feature-defined by properties of genome nucleic acid
All viruses divided into classes based on genome structure
Molecular property

23. Classes in the Baltimore Scheme
Virion nucleic acid
Example 1
dsDNA
Adenoviridae 2a
+ssDNA
Microviridae 2b
-ssDNA
Parvoviridae 3
dsRNA
Reoviridae 4
+ssRNA
Picornaviridae 5
-ssRNA
Filoviridae 6
+ssRNA (RT)
Retroviridae 7
dsDNA (RT)
Caulimoviridae

24. Diagram of Baltimore scheme

25. Comparisons of Vertebrate and Plant Viruses
Vertebrate Infecting Viruses
Plant Infecting Viruses

26. Hi Dr. Luciano, I'm getting some terrific questions from your virology class about TWiV episodes. Thanks so much for having them do that as an assignment - I think it's a great way for the students to get someone else's perspective.
We'll likely read many of these s over the next several shows, starting this Friday - first names only of course. Would you prefer that I respond by as well? ______________________________ Vincent Racaniello, Ph.D. | Higgins Professor Department of Microbiology & Immunology Columbia University, New York virology.ws | twiv.tv | microbeworld.org/twip "Tanto sa ciascuno quanto opera"