Introduction to Virology

Dr. Kevin Forward
R 315 Mackenzie Bldg.
5788 University Ave
473-4109
kevin.forward@cdha.nshealth.ca
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
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 > intracellular replication
History of viral diseases

1500 BC: Leg deformities indicative of poliomyelitis
Pock marks indicative of smallpox
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

RSV
Virus naming and classification
Usually 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
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
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
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
Viral structure
(cont’d)
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”
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
Examples to show differences in size and form
Figure
16.10
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
Some representative DNA viruses
Some representative RNA viruses
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
Attachment
Uncoating
Replication
Assembly
Egress
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 > RNA
RNA > RNA >RNA
RNA > DNA >RNA
Some proteins are processed further (glyosylated)
Viral assembly occurs
Virus exits cell by budding of lysis
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
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)