Introduction to Virology

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Introduction to Virology

• Dr. Kevin Forward
• R 315 Mackenzie Bldg.
• 5788 University Ave
• 473-4109
• kevin.forward_at_cdha.nshealth.ca

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A virus is

• A package of genetic information protected by a
  protein shell for delivery into a host cell to be
  expressed and replicated

"Virus" is from the Greek meaning for "poison
and was initially described by Edward Jenner in
1798
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Viruses as distinct groups of infectious agents

• Characteristics which differentiate viruses from
  other micro-organisms
• Nucleic acid (DNA or RNA)
• Lack of nuclear membrane and external cell wall
• They have very small genomes, produce limited
  numbers of proteins and do not possess many
  intracellular systems ie they are parasites gt
  intracellular replication

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• History of viral diseases

Pock marks indicative of smallpox
1500 BC Leg deformities indicative of
poliomyelitis
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Viral diseases have played a major role in human
history over whatever time scale we choose to
explore Over the past 1000 years Smallpox and
measles
Over the past 10 years West Nile, SARS, Avian
Flu
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Virus naming and classificationUsually based on
data available at the time of discovery

• Disease they are associated with
• HIV, Measles
• Cytopathology they cause
• Cytomegalovirus
• Site of isolation
• Adenovirus, Enterovirus, Rhinovirus
• Places discovered or people that discovered them
• Epstein-Barr virus, West Nile Fever
• Biochemical features
• Retrovirus

RSV
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Viral taxonomy

• Viral classification is based upon
• Shape
• Type and form of nucleic acid
• Enveloped or naked
• Mode of replication
• Organization of the genome and antigenic
  differences

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Genomic organization Viral nucleic acids

• RNA viruses
• RNA single stranded
• positive polarity
• negative polarity
• RNA double stranded
• one piece
• segmented
• DNA viruses
• single stranded
• double stranded

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Viral structure

• Capsomeres are structural subunits containing
  several proteins
• Capsomeres aggregate to produce the viral capsid.
• The viral capsid associates with the viral
  nucleic acid to produce a nucleocapsid
• Nucleocapsids are usually arranged in one of
  three ways
• cubic, eg. icosahedral (Herpes viruses)
• helical (Influenza viruses)
• complex

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Viral structure (contd)

• Lipid envelopes are derived from cellular
  membranes
• Studded with surface projections (spikes or
  peplomers).
• These are usually glysosylated by host systems
  prior to make them sticky

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• How are viruses classified ?
• Hierarchical virus classification Order- Family
  - subfamily - genus - species - strain/type
• All families have the suffix viridae, e.g
  Herpesviridae
• Genera have the suffix virus. For instance
  Coxsackie virus

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Examples to show differences in size and form
Figure 16.10
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Some representative DNA viruses

DNA
Icosahedral

Complex
Naked Enveloped
Papilloma Herpes viruses Hepatitis
B
Pox viruses
Parvovirus
Adenoviruses
Cytomegalovirus
Herpes simplex 1 and 2
Varicella Zoster
Epstein
Barr Virus
HHV 6-8
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Some representative RNA viruses
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How do you acquire these viral infections?

• Direct personal contact
• Herpes viruses, HIV, Influenza
• Airborne spread
• Chicken pox
• Parenteral
• HIV, Hepatitis B and C, cytomegalovirus (CMV)
• Fomites
• Enteroviruses and other sturdy drying resistant
  viruses
• Vectors
• West Nile
• Vertical transmission
• HIV, Herpes simplex, cytomegalovirus, rubella
  (German measles)
• Enteral (foodborne)
• Hepatitis A, gastroenteritis viruses

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Egress
Attachment
Replication
Uncoating
Assembly
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Virus adheres to specific receptors on
susceptible cells
Viruses are internalized by the cell, enter the
cytoplasm
Uncoating occurs, nucleocapsid is degraded by
cellular proteases releasing nucleic acids (DNA
or RNA)
DNA may move to the nucleus, replication
begins DNA gt RNA RNA gt RNA gtRNA RNA gt DNA gtRNA
Some proteins are processed further (glyosylated)
Viral assembly occurs
Virus exits cell by budding of lysis
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Consequences of virus-cell interaction

• Viral proliferation and cell lysis
• Cold sores, genital herpes,chicken pox
• Latent infection (non replicating virus)
• Most Herpes viruses, Human papilloma viruses
• Viral genome may be integrated or not
• Persistent infection (ongoing viral replication,
  few or no symptoms)
• Hepatitis C and B, HIV
• Tissue injury may be induced by the virus or the
  immune response to the viral infection
• Oncogenesis (cellular proliferation)
• Human papilloma virus, EBV
• No apparent disease
• eg Hepatitis G

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Diagnosis of viral infections

• Clinical features typical of infection.
• Laboratory diagnosis
• Histology and cytology
• Direct examination by electron microscopy
• Examination looking for viral antigens
• In tissue
• In serum
• Antibody testing (rising IgG, IgM, very high
  titer IgG, any IgG)
• Virus isolation
• DNA or RNA amplification (in tissue sections,
  from tissue, in serum and fluids)