1. Virology - Introduction
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2. Why are viruses important ?
• Viruses cause disease in animals of economic
and/or welfare importance
• Diagnose viral disease (clinical/lab tests)
• Advise clients control (risk to other animals)
• Animal viruses may pose risk to human health
(zoonosis)
• Can act as important models for human disease
•VIRUSES CAN BE USEFUL
VACCINE DEVELOPMENT
GENE THERAPY
TOOLS TO INVESTIGATE HOST CELLS

3. What are Viruses?
• A virus is a non-
cellular particle made
up of genetic material
and protein that can
invade living cells.
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4. Size of Viruses
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5. Virus Classification
• International Committee on
Taxonomy of Viruses
chemical characteristics, genome type,
replication strategy, diseases, vectors,
geographical distribution, host species
nucleotide sequence
order family subfamily genus species/strain/type
-virales -viridae -virinae -virus -virus
mononegavirales paramyxoviridae paramyxovirinae morbillivirus canine distemper
virus
herpesviridae alphaherpesvirinae varicellovirus equid herpesvirus 1
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6. Basic virus structure
DNA
Capsid Naked
or + Nucleocapsid =
protein capsid virus
RNA
Lipid membrane,
Nucleocapsid + Enveloped virus
glycoproteins
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7. Virion (virus particle) structure
1. genome
nucleocapsid
2. capsid
± 3. envelope
envelope glycoproteins
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8. DNA Viruses
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9. RNA Viruses
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10. 1- Viral Nucleic Acids
 DNA or RNA [cell genetic material is DNA]
 ss or ds
 ss -/+/mixed sense [mRNA= +sense]
 linear or circular
 segmented/non-segmented
 size 2-300 kb(p) [cell genome 3x106kbp]
 Genetic ‘heritage’
 Codes for virus proteins
 Controls virus protein production
- promoters, transcriptional enhancers, splice signals
 Contains elements necessary for replication and
genome packaging

11. Viral Proteins
Structural
Components of capsid (protective coat) and other
components of the virion
Non-structural
Required for viral replication and interaction with
host

12. 2- Nucleocapsid
 Capsid is protein coat that protects the nucleic acid:
physical, chemical, enzymatic attack
 Nucleocapsid comprises the capsid and enclosed
nucleic acid
 facilitates entry into cell and delivery of nucleic acid
 exposed to immune system
genome
nucleocapsid
capsid

13. Viruses come in a
variety of
shapes and sizes
dictated by
their protein
and nucleic acid
composition
- but there are
common elements in
their architecture
due to SYMMETRY
ICOSAHEDRAL
HELICAL
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14. Capsid symmetry
Icosahedral Helical
Naked capsid
Enveloped
Matrix
Lipid
Glycoprotein
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15. Icosahedral (or cubic)
20 faces each face an equilateral triangle
axes of 2-, 3- and 5-fold rotational symmetry Some icosahedral
Capsomer structure enclosing maximum volume animal viruses are
enveloped
Foot and mouth disease
virus (picornavirus)
herpesvirus
adenovirus

16. http://www.ncbi.nlm.nih.gov/ICTVdb/Images/Ackerman/Animalvi/Adenovir/799-
16.htm
03/04/12
Electron
Adenovirus
micrograph
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Icosahedral naked capsid viruses
Crystallographic model
16
Foot and mouth disease virus
http://virology.wisc.edu/virusworld/ICTV8/fmd-foot-and-mouth-
ictv8.jpg

17. Icosahedral enveloped viruses
http://web.uct.ac.za/depts/mmi/stannard/emimages.html
http://virology.wisc.edu/virusworld/images/herpescapsid.
GIF
Herpes simplex virus Herpes simplex virus
Electron micrograph Nucleocapsid cryoEM model
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18. Helical
Simple viruses with small
genomes use this architecture to
provide protection for the
genome without the need to
encode multiple capsid proteins.
Rabies virus
(rhabdovirus)

19. Helical
All animal viruses
with helical
symmetry are
ENVELOPED
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paramyxovirus

20. Helical enveloped viruses
http://web.uct.ac.za/depts/mmi/stannard/fluvirus.
http://web.uct.ac.za/depts/mmi/stannard/paramyx.
html
html
Influneza A virus Paramyxovirus
Electron Electron
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21. 3 – Virus Envelope
 Envelopes are LIPID BILAYERS acquired from cellular
membranes e.g. endoplasmic reticulum, nuclear
membrane, plasma membrane
 viral proteins are associated with/inserted into
membrane – surface proteins often glycosylated
Adsorption and entry of virus
into cells (and exit)
-access to target cells
-binding to receptors
-fusion of envelope with
cellular membranes to
release genome
Interaction with immune
system components
- binding of antibody
- Targets of immune system

22. Complex Virus Structures
Most animal viruses fall into three structural classes,
helical capsid (enveloped)
icosahedral capsid (nonenveloped)
or icosahedral capsid (enveloped)
However, more complex structures do exist e.g. pox viruses

23. 5 BASIC TYPES OF VIRAL STRUCTURE
icosahedral nucleocapsid nucleocapsid
lipid bilayer
ICOSAHEDRAL ENVELOPED ICOSAHEDRAL
helical nucleocapsid
COMPLEX
nucleocapsid
lipid bilayer
glycoprotein spikes
= peplomers
HELICAL ENVELOPED HELICAL
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Adapted from Schaechter et al., Mechanisms of Microbial Disease

24. Stability of Viruses
•Non enveloped viruses more ‘hardy’ than enveloped viruses
(e.g. foot and mouth disease hardier than influenza virus)
•Different viruses have differential ability to survive
• sensitive to temperature, pH, dessication, lipid solvents,
detergents
Most inactivated at >55-60oC
Detergents used to disrupt viral envelopes
Rotavirus survives pH of stomach
•Clinical sample collection / Diagnostics

25. Properties of enveloped viruses
• Envelope is sensitive • Consequences
to – Must stay wet during
– Drying transmission
– Heat – Transmission in large
droplets and secretions
– Detergents – Cannot survive in the
– Acid gastrointestinal tract
– Do not need to kill
cells in order to spread
– May require both a
humoral and a cellular
immune response
Adapted from Murray, P.R. Rosenthal K.S., Pfaller, M.A. (2005) Medical Microbiology, 5th edition,
Elsevier Mosby, Philadelphia,
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26. Properties of naked capsid viruses
• Capsid is resistant to • Consequences
– Drying – Can survive in the
– Heat gastrointestinal tract
– Detergents – Retain infectivity on drying
– Acids – Survive well on
– Proteases environmental surfaces
– Spread easily via fomites
– Must kill host cells for
release of mature virus
particles
– Humoral antibody response
may be sufficient to
neutralize infection
Adapted from Murray, P.R. Rosenthal K.S., Pfaller, M.A. (2005) Medical PADAGA 5th edition, Elsevier Mosby, Philadelphia, PA , 26 6-4
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27. BASIC STEPS IN VIRAL LIFE CYCLE
• ADSORPTION
• PENETRATION (injection) of viral DNA or
RNA
• UNCOATING AND ECLIPSE
• SYNTHESIS OF VIRAL NUCLEIC ACID AND
PROTEIN (REPLICATION)
• ASSEMBLY (maturation) of the new viruses
• RELEASE of the new viruses into the
environment (cell lyses)
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28. ADSORPTION
• TEMPERATURE INDEPENDENT
• REQUIRES VIRAL ATTACHMENT
PROTEIN
• CELLULAR RECEPTORS
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29. PENETRATION enveloped virus
•FUSION WITH PLASMA MEMBRANE
•ENTRY VIA ENDOSOMES
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31. PENETRATION
- ENVELOPED VIRUSES
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from Schaechter et al, Mechanisms of Microbial Disease, 3rd ed, 1998 PADAGA 31

32. PENETRATION
NON-ENVELOPED VIRUSES
entry directly
across plasma
membrane:
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34. UNCOATING
• NEED TO MAKE GENOME
AVAILABLE
• ONCE UNCOATING OCCURS, ENTER
ECLIPSE PHASE
• ECLIPSE PHASE LASTS UNTIL FIRST
NEW VIRUS PARTICLE FORMED
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35. SYNTHESIS OF VIRAL
NUCLEIC ACID AND
PROTEIN
• MANY STRATEGIES
• NUCLEIC ACID MAY BE MADE IN
NUCLEUS OR CYTOPLASM
• PROTEIN SYNTHESIS IS ALWAYS IN
THE CYTOPLASM
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36. ASSEMBLY AND
MATURATION
• NUCLEUS RELEASE
• CYTOPLASM • LYSIS
• AT MEMBRANE • BUDDING
THROUGH PLASMA
MEMBRANE
• NOT EVERY
RELEASED VIRION
IS INFECTIOUS
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37. epithelial cells - adenovirus
uninfected
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early infection
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late infection
37
slides from CDC

38. Bacteriophages
• Bacteriophages ( phages ) are obligate
intracellular parasites that multiply inside
bacteria by making use of some or all of the
host biosynthetic machinery . They are
viruses that infect bacteria.
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39. Composition of Bacteriophage
• Nucleic acid: either DNA or RNA but not
both
– ds DNA, ss RNA, ss DNA
– unusual or modified bases
– encode 3-5 gene products to over 100 gene
products
• Protein: function in infection and protect the
nucleic acid
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40. Structure of Bacteriophage
Capsid
Head
DNA
Contractile
Sheath
Tail
Tail Fibers
Base Plate
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42. Prions
•Prions are “infectious
proteins”
• They are normal body
proteins that get
converted into an alternate
configuration by contact
with other prion proteins
• They have no DNA or
RNA
•The main protein involved
in human and mammalian
prion diseases is called
“PrP”
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43. Prion Diseases
•Prions form insoluble
deposits in the brain
•Causes neurons to
rapidly degeneration.
•Mad cow disease (bovine
spongiform encephalitis:
BSE) is an example
•People in New Guinea
used to suffer from
kuru, which they got
from eating the brains
of their enemies
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