Notes

Taxonomy/ Classification of Viruses

The origin and subsequent evolution of viruses are in mystery because of the lack of a fossil record. However, recent advances in the understanding of virus structure and reproduction have made possible more information on viruses.

Viruses are usually grouped according to properties as size, the type of nucleic acid they contain, the structure of the capsid and the number of protein subunits in it, host species, and immunological characteristics.

It also means that when a new species of known virus family or genus is investigated it can be done in the context of the information that is available for other members of that group. Without a classification scheme each newly discovered virus would be like a black box, everything would have to be discovered and rediscovered. The development of a classification scheme is therefore an important and inevitable consequence.

Virus classification is the process of naming viruses and placing them into a taxonomic system.

Currently there are two main schemes used for the classification of viruses: i.e. ICTV System and Baltimore classification.

ICTV System (International Committee on Taxonomy of viruses)

Viruses are divided into different taxonomic groups based on characteristics that are related to

1. Nature of host (animal, plant, bacteria)
2. Nucleic acid features (DNA or RNA)
3. Capsid symmetry (icosahedral, helical, complex)
4. Absence or presence of Envelope
5. On the basis of mode of transmission
6. Nature of host infected (animal, plant, bacteria)

• Animal virus-
• Herpes simplex virus type-I causes Human respiratory infections
• Herpes simplex virus type II- causes genital infections
• Pox virus- gastroenterititis
• HIV virus
  • Plant virus: Tobacco Mossaic Virus
  • Bacterial virus: Bacteriophage

2. On the basis of nucleic acid composition

DNA virus

• ssDNA – Parvo virus
• dsDNA – (Herpes simplex virus, small pox, hepatitis B virus)

RNA virus

• ds RNA virus- Rota virus
• ssRNA virus – infleunza virus, rabies virus, HIV virus)

• Positive sense: Dengue virus, Hepatitis E virus.
• Negative sense: Rabies virus, Influenza virus

3. On the basis of structure (capsid symmetry)

• Icosahedral (Adeno virus, Reo virus)
• Helical- (Measles, mumps, Rabies)
• Complex- ( Bacteriphage, Pox virus)

4. On the basis of presence /absence of envelope

Enveloped virus Non- enveloped (naked)

 HIV Rota virus

Measles Parvo virus

Mumps Reo virus

Rabies

5. On the basis of mode of transmission

• Air borne : Infuenza virus
• Faecal oral route: Polio virus, Hepatitis A and E virus
• Sexual route: HIV, HSV
• Blood borne : HIV virus, Ebola virus, Hepatitis B virus
• Vector borne: Dengue virus, Japanese encephalitis virus

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, and
• Method of replication. 

Baltimore Groups

Group I : dsDNA ( Adenovirus, Herpesvirus)

Group II : ssDNA (Parvovirus)

Group III: dsRNA (Reovirus)

Group IV: (+) ssRNA(Picornavirus, Togavirus

Group V: (-) ssRNA (Orthomyxovirus, Rhabdovirus)

Group VI: ssRNA-RT (Retrovirus)

Group VII: dsDNA-RT (Hepadnavirus)