UNIT 5: Introduction to Virology
Virology and viruses
Virology is the study of viruses and the diseases caused.
Viruses are the minute entities that carry genetic information in one type of
nucleic acid and require living cells for multiplication.
Viral structure
A virus consists of a nucleic acid genome (DNA or RNA) surrounded by a
symmetric shell of protein called capsid, which is made up of capsomeres.
Nucleic acid and genome together is called nuleocapsid. This nucleocapsid may
or may not be covered by lipid containing outer layer called envelope and
accordingly the virus is called enveloped or unenveloped virus. Figure below
shows a viral structure
Nucleic acid (DNA /
RNA
Capsid
Envelope
Structure of Virus
Viral replication
The process by which virus multiply in the host cell is called replication. After
the penetration into the host cell virus releases its nucleic acid into the cytoplasm,
which multiplies using the materials of the host cell producing large number of
viruses. Figure below shows viral replication (bacteriophage)
Various phases in the replication of virus are:
1. Initiation phase:
 Attachment: virus attaches on the animal cell
 Penetration: cell membrane is lysed and the virus penetrate into the cell
 Uncoating: the capsid is dissolved and the genetic material of the virus is
introduced into the cell.
2. Replication phase:
 DNA or RNA synthesis: as per the nucleic acid content of the particular virus
there is synthesis of viral nucleic acid within the infected animal cell.
 Protein synthesis: similarly the viral protein required for the formation of viral
capsid are synthesised in the infected cell.
3. Release phase:
 Assembly: the vir al particles, nucleic acid and the capsid assemble together
resulting into production of new virion
 Maturation: after the assemble the virion mature and exit from cell after the
lysis of cell to infect new cells.
Classification of virus
Ether sensitivity
Based on the sensitivity to ether virus can be said to be ether sensitive or ether
resistant. Ether sensitive viruses are those that get denatured when in contact with
ether and ether resistant are those viruses that do not get denatured.
Envelope
Based on the presence of envelope or not virus are said to be either enveloped if it has
envelope or unenveloped if there is no envelope.
Viruses are classified into families and genera based on the nucleic acid present in
genome, size, structure, biochemical characteristics and mode of replication. Two
major groups of viruses are DNA virus and RNA virus.
Based on the type of genomic material, viruses are classified as:
DNA viruses
1. Adenoviridae. Viruses in this family are unenveloped and ether resistant.
2. Herpesviridae. Viruses in this family are unveloped. Infectious bovine
rhinotracheitis belong to this group of virus.
3. Papovaviridae. Viruses in this family are unenveloped and ether resistant.
4. Parvoviridae. Viruses in this family are unenveloped and ether resistant.
5. Poxviridae. Viruses in this family are large and covered by a complex outer
cover.
RNA viruses
1. Arenaviridae. Viruses in this family are associated with human diseases.
2. Bunyaviridae. Viruses in this family are enveloped and ether resistant. Rift
valley fever virus is the member of this family.
3. Coronaviridae. Viruses in this family are enveloped and ether sensitive. Avian
bronchitis virus, transmissible gastroenteritis virus of swine and bovine
coronavirus are the members of this family.
4. Orthomyxoviridae. Influenza virus belongs to this group of virus.
5. Paramyxoviridae. Viruses of this family are enveloped and ether sensitive.
Rinderpest virus belongs to this family.
6. Picornaviridae. Viruses of this family are small and enveloped. FMD virus is
the member of this family.
7. Reoviridae. Viruses in this family are unenveloped and ether resistasnt.
Bluetongue virus belongs to this family.
8. Retroviridae. Viruses in this family are enveloped.
9. Rhabdoviridae. Viruses in this family are bullet shaped and enveloped. Rabies
virus and vesicular stomatitis virus are the members of this family.
10. Togaviridae. Viruses in this family are enveloped and ether sensitive. Bovine
viral diarrhoea virus belongs to this family.
Viruses and diseases
Poxviridae
• DNA virus, 200mµ x300mµ
• Usually enveloped but variable.
• Usually ether resistant.
• Diseases in animals: smallpox and vaccinia.
Cowpox, sheep pox, Swinepox, monkey pox, etc
Avian poxes in turkey, fowl, pigeons and canaries
Herpesviridae
• DNA virus, enveloped
• 110 mµ cubic symmetry
• Ether sensitive
• Diseases in animals: Pseudorabies, Infectious Rhinotracheitis, Malignant
catarrh,
Laryngotracheitis
Papovaviruses
• DNA virus
• 40mµ to 50 mµ, cubic symmetry
• Ether resistant,
• Non enveloped
• Diseases in animals: Common warts
Bovine papilloma
Canine oral papilloma
Rabbit oral papilloma
Rabbit papilloma
Polyoma
Genital papilloma of swine
Adenoviruses
• DNA virus
• 70 mµ to 80 mµ cubic symmetry
• Ether resistant
• Nonenveloped
• In animals:Canine hepatitis
Bovine adenovirus
Simian adenovirus
Avian adenovirus
Arboviruses
RNA virus 40-50 mµ
Ether sensitive, enveloped
Usually vector transmitted
In animals:
Grp A: Equine encephalitis
Grp B: Louping ill
Yellow fever
Rift valley fever
Dengue
Ungrouped:
• California encepahlitis
• African Horse Sickness’
• Blue tongue
Myxo viruses
• RNA, 80 to 200 mµ
• Helical symmetry, enveloped
• Ether sensitive
• Affinity for mucoproteins
Coronaviridae
Enveloped
Ether sensitive
Diseases in animals
 Avian bronchitis virus
 Transmissible gastroenteritis virus of swine
 Bovine coronavirus
Orthomyxoviridae
RNA virus
Ether sensitive
Helically symmetrical
Enveloped
Causes influenza
Paramyxoviridae
RNA virus