viruses and what they do
TRANSCRIPT
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Viruses and what they do -
An overview
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Viruses (Encyclopedia Britannica)
..infectious agents of small size and simple composition that can multiply only in living cells of animals, plants and bacteria. Viruses are obligate parasites that are metabolically inert when they are outside their hosts. They all rely, to varying extents, on the metabolic processes of their hosts to reproduce themselves. The viral diseases we see are due to the effects of this interaction between the virus and its host cell (and/or the host’s response to this interaction).
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Viruses• Infectious agents of small size and simple composition that can multiply only in living cells of animals, plants and bacteria.
• Viruses are obligate parasites that are metabolically inert when they are outside their hosts.
• The viral diseases we see are due to the effects of this interaction between the virus and its host cell (and/or the host’s response to this interaction).
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Viruses are Not Cells
“Smaller” “Bigger”
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Virion
capsomeres
Capsid
envelopedvirus
envelope
a Virion
nucleocapsid
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Viru
s A
rchi
tect
ure
Minimally, a virus is a proteinaceous carrier of nucleic acid.
Many viruses are more complicated than that, such as having a lipid envelope surrounding the protein capsid.
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Viru
s A
rchi
tect
ure
Note spikes projecting from protein capsid surrounding nucleic acid.
Note spikes projecting from lipid envelope-surrounding capsid (which in turn surrounds the nucleic acid).
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Some viral shapes
adenovirus
parvovirus
papillomavirus
100 nm1 nm = 1 millionth of a mm100 nm = 1 ten thousandth of a mm
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Some viral shapes
herpesvirus
parainfluenzavirus
influenzavirus
poxvirus
1 nm = 1 millionth of a mm100 nm = 1 ten thousandth of a mm
100 nm
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Viri
on S
ize`
“Big”
“Medium”
“Small”
“Even Smaller”
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Viral Genomes
Nucleic Acid
DNA
RNA
Double Stranded
Positive
Negative
RNA DNA
Single Stranded
Double Stranded
Single Stranded
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Vira
l Gen
omes
Nucleic-Acid Virus Genome
ssDNADNA
dsDNA
ssRNA dsDNA ssRNA(the retroviruses)
Negative
Positive
dsRNARNA
ssRNA
One way to distinguish different types of viruses (e.g., influenza virus from HIV) is in terms of the characteristics of their nucleic-acid genomes.
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Proteins produced by viruses
• Structural proteins• Non-structural proteins
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Term
s D
escr
ibin
g V
irion
s Virion is another name for virus particle. Virions are infectious meaning that they can deliver their nucleic acid to the cytoplasm of a susceptible cell (which for phages would be a bacterium).
Capsid is the the protein coat that surrounds the nucleic acid and defines a virus as a virus.
Capsids are made up of individual proteins called capsomers.
The virion particle consists, minimally, of protein and nucleic acid which together is called a Nucleocapsid.
Many particularly animal viruses have lipid bilayers surrounding the nucleocapsid; those viruses are described as Enveloped.
Non-enveloped viruses are described as Naked. In enveloped viruses the envelope makes initial contact
with cells and subsequent interaction with the cell surface is mediated by envelope proteins (proteins found in or on the envelope lipid bilayer).
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Complex (Tailed) Phage VirionNote that this head actually is elongated top to bottom rather than isometric.
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Not-Complex Virions
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Infection of a cell
Stage Biological implications
Host defenses
Drug intervention
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Infection of the animal
• Entry - the beginning of infection– Local replication vs systemic spread
• Consequences of infection– none to illness (signs, symptoms)
• Signs and symptoms• Why some animals get sick while others do not• Patterns of disease
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Why do some infected animals get sick and others don’t?
• Viral factors - virulence• Host Factors
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Viral Factors:
Why are some isolates of a virus more likely to cause severe disease than
others?determinants of viral virulence
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Host Factors:
• Genetic Resistance– loss of receptors
• CCR5-32 mutation and resistance to HIV
– variation in immune response genes– genetic defects in defenses
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Host factors:
• age related susceptibility– greater susceptibility of new born animals– greater susceptibility of adults
• prior exposure, acquired resistance• maternal protection• concurrent infections, immuno-suppression, increase
in susceptible cells
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Release of virusRelease by lysis of cell(cytopathic)
or by budding (withoutdeath of cell, non-cytopathic)
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Incubation period
incubation period - time between infection and the appearance of clinical signs
infection
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InterferonsI
II
I
IE
Induced interferongenes
antiviraleffects
activatedfactor
infected cell
ImmuneModulation
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Acquired anti-viral immunity (antibody)
BVirus neutralized
Virus infectedcell
Antibody targetsFc receptor bearing cellTo kill virus infected cell
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Taxonomy
• What is it?• On what is it based?• Is it important?• Do I need to remember all the details?
International Committee onTaxonomy of Viruses
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Groups, types (sero-types), isolates and ‘strains’
Group
Type -A
Type - B
Type - C
Group specific antigen
Type -A specificantigen
Type - C specificantigen
isolate
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Group and type specific antigens
group specific antigen
type specific antigen
“naked”virus(eg FMDV)
enveloped virus(eg influenza, FeLV)
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Wha
t is
a P
hage
? “The agents were called filterable viruses… Virus
means ‘poison,’ a term that once had been applied to all infectious agents. With time, the adjective filterable was dropped and only the word virus was retained.”
Bacteriophage were originally described as a macroscopic phenomenon that was slightly different from the “poisoning” of a plant or animal.
Instead, what was observed was the destruction of a bacterial culture.
People weren’t all that sure what a bacterium was so the destruction was seen more as an “eating” of the culture (by an otherwise unseen agent) rather than a poisoning.
The filterable agent in this case was described as an “eater” of bacteria, or “Phage” from Greek: Bacteriophage = “Bacteria Eater.”
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Wha
t is
a P
hage
? “The agents were called filterable viruses… Virus
means ‘poison,’ a term that once had been applied to all infectious agents. With time, the adjective filterable was dropped and only the word virus was retained.”
Bacteriophage were originally described as a macroscopic phenomenon that was slightly different from the “poisoning” of a plant or animal.
Instead, what was observed was the destruction of a bacterial culture.
People weren’t all that sure what a bacterium was so the destruction was seen more as an “eating” of the culture (by an otherwise unseen agent) rather than a poisoning.
The filterable agent in this case was described as an “eater” of bacteria, or “Phage” from Greek: Bacteriophage = “Bacteria Eater.”
“The word bacteriophage or phage that infect bacteria is both singular and plural when referring to one type of virus. The word phages is used when different types of phages are being referenced.”
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Viri
on A
ttach
men
t Org
ans Some virions are isometric: they have a fully symmetrical
capsids, almost spherical.
Isometric virions attach to cells via Attachment Proteins, a.k.a., Spikes which are proteins that symmetrically project from their virions.
Spikes project from capsids in Naked viruses.
Spikes project from envelopes in Enveloped viruses.
Some virions are helical with attachment proteins at end or ends.
Most phages have tailed virions which sort of combine the morphology of isometric and helical virions, with the isometric part called a head and the helical part called a tail.
Attachment is made at the end of the tail opposite the head, and often is mediated by thin “feelers” called tail fibers.
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Some Important PhagesHere complex means “tailed”
Note various genome architectures.
An example of Lysogenic Conversion.
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Lytic
Infe
ctio
n S
trate
gy
This is the productive-infection strategy followed by most phages including all tailed phages.
Phages that are obligately lytic are called Virulent (vs. chronic or temperate).
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Chr
onic
Infe
ctio
n S
trate
gy
This is the strategy followed by filamentous phages.
This is a process called Extrusion.
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Late
nt In
fect
ion
Stra
tegy
For phages this is called Lysogeny.
Only Temperate phages are able to display lysogeny.
For phages this is called Lysogenic Conversion.
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Vira
l Inf
ectio
n S
trate
gies
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Viru
lent
Pha
ge L
ife C
ycle
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Chr
onic
Pha
ge L
ife C
ycle
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Tem
pera
te P
hage
Life
Cyc
le(s
)
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Tem
pera
te P
hage
Life
Cyc
le(s
)
The prophage DNA can remain integrated indefinitely.
This is a process called Induction.
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Ads
orpt
ion
& H
ost R
ange
Adsorption describes the virion’s attachment process. The virion contains proteins that recognize molecules
found on the surface of cells (much as antibodies bind to antigens).
Receptor Molecules (typically proteins) are molecules that cells make for various reasons (e.g., transport proteins) which viruses coopt for adsorption.
Typically the Host Range of a virus is determined, at least in part, by the ability of the virus adsorption proteins to bind to host Receptor Molecules.
Restriction-Modification systems can also limit phage host range (as well as biochemical incompatbilities).
Through mutation (and other means) phages can overcome these host-range barriers, resulting, typically in a changed host range.
Because of transduction, these changes in host range can allow gene exchange between even only distantly related bacteria.
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Vertebrate RNA Viruses
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Vertebrate DNA Viruses
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Routes of Transmission
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Routes of Transmission
Some viruses are acquired via a respiratory route but cause symptoms elsewhere so aren’t considered “respiratory” viruses; example: measles virus
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Ani
mal
Viru
s Li
fe C
ycle Attachment to susceptible cell
Uptake of nucleocapsid into cell
Targeting of nucleocapsid to site of reproduction
Uncoating of nucleic acid from capsid
Synthesis of viral proteins
Replication of nucleic acid
Maturation of virion particles
Virion release from cell
Virus spread within host
Exit of virus from host
Transmission of virus to new host
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Ste
ps n
ot s
een
with
Pha
ge Attachment to susceptible cell Uptake of nucleocapsid into cell
Targeting of nucleocapsid to site of reproduction
Uncoating of nucleic acid from capsid
Synthesis of viral proteins
Replication of nucleic acid
Maturation of virion particles
Virion release from cell
Virus spread within host
Exit of virus from host
Transmission of virus to new host
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Ent
ry o
f Env
elop
ed V
iruse
s
Note that viral proteins remain on outside of cell and therefore exposed to host immunity.
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Ent
ry o
f Env
elop
ed V
iruse
s
Note that viral proteins don’t remain on outside of cell.
Fusion occurs with vesicle membrane rather than plasma membrane
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Many Transcription Strategies
Don’t worry about virus names (in red).
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Dependence on Host Enzymes
In other words, the greater the size of a virus’ genome, the less dependent that virus is on the host metabolism.
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Bud
ding
of E
nvel
oped
Viru
ses
Not all enveloped viruses bud through the plasma membrane.
Note though that naked animal viruses generally lyse their host cell to effect virion release.
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Cyt
opat
hic
Effe
ctsCytopathic Effects are changes virus infection makes on host-cell morphologies.
Cells infected with adenovirus.
Cells infected with HSV.
Uninfected cells.
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Various Impacts of Animal Viruses on Cells
Impact is virus-type and also host-cell type specific.
Concentrate on the non-tumorgenic aspects of viral infection.
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HIV
Life
Cyc
le(th
roug
h re
vers
e tra
nscr
iptio
n)
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HIV
Life
Cyc
le(th
roug
h vi
rion
mat
urat
ion)
Note that maturation actually takes place via virion budding!!!
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HIV
Life
Cyc
le(b
uddi
ng th
roug
h pl
asm
a m
embr
ane)Reverse transcriptase enzyme already incorporated into capsid.
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Tim
e C
ours
e of
Infe
ctio
ns
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But note that “the diversity of influenza sequences worldwide in any given year appears to be roughly comparable to the diversity of HIV sequences found within a single individual at one time point.” (Bette Korder as cited in Microbe, 2006 1(3):111-112)H
ow th
e Fl
u C
hang
es it
s “S
pots
”E.g., swapping of coat proteins between avian and human strains of influenza virus.
Particularly important is the protein virus protein, hemagglutinin.
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Prion-Associated Disease
These are all Transmissible Spongiform Encephalopathies and all appear to be caused by proteinaceous infectious agents called Prions.
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Hyp
othe
sis
for P
rion
Act
ion
Accumulated abnormal proteins kill neuron, with prions spreading to adjacent neurons.
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Prevention of infection and/or disease
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Protection of the new born animal
antibodies (CMI) in colostrum– maternal immunization– colostrum replacers– implications for immunization of young animals
• caution when using modified-live vaccines• interference by maternal antibodies
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Prevention of virus infections/disease
• vaccination– inactivated vaccines– attenuated vaccines– subunit– vectored– DNA vaccines
• management
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Diagnosis of viral diseases
• clinical signs• virus detection• detection of exposure Laboratory}
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Detection of virus
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Detection of virus
• isolation (isolation +immunological detection)• quantitation (plaque assay, TCID50)• PCR• haemagglutination (or HAI)• ELISA (in clinic or lab)• immunological detection (IH or IF)• electron microscopy
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Virus isolation (tissue culture, experimental animals)
cultured cells(two dimensionalanimals) cytopathic effect
immunofluorescence(anti-herpesvirus antibody)
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Virus quantitation (plaques)
plaquescount plaques(plaque forming unit/ml)
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Haemagglutination (HA)
virus No virus
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Enzyme linked immunabsorbant assay (ELISA)
Sample to be tested
virus
capturing antibody
Detecting antibody
Enzyme -> colour
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Viruses can be useful too
• biological control of pests• cancer therapy• gene therapy• nanotechnology• symbiotic virus-host relationships
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Thank You