Viruses and what they do - An overview

Viruses and what they do -
An overview
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).
Viral Genomes
Nucleic Acid
DNA
RNA
Double Stranded
Positive
Negative
RNA
DNA
Single Stranded
Double Stranded
Single Stranded
SS RNA genomes
AUG GCA CGA
UAC CGU GCU
met ala arg
+ve (sense) and -ve (anti-sense) RNA genomes
Virion
capsomeres
Capsid
enveloped
virus
envelope
a Virion
nucleocapsid
Proteins produced by viruses
Structural proteins
Non-structural proteins
Some viral shapes
adenovirus
parvovirus
papillomavirus
100 nm
1 nm = 1 millionth of a mm
100 nm = 1 ten thousandth of a mm
Some viral shapes
herpesvirus
parainfluenza
virus
influenzavirus
poxvirus
1 nm = 1 millionth of a mm
100 nm = 1 ten thousandth of a mm
100 nm
Taxonomy
What is it?
On what is it based?
Is it important?
Do I need to remember all the details?
International Committee on
Taxonomy of Viruses
Viruses with ss DNA genomes
Circoviridae
Parvoviridae
porcine circovirus
canine parvovirus-2
feline panleukopenia virus
porcine parvovirus (SMEDI)
Viruses with ds DNA genomes
Poxviridae
African swine fever virus
Herpesviridae
Adenoviridae
Papovaviridae
papillomaviruses
adenoviruses
bovine
herpesvirus-1,2

porcine cytomegalovirus

equine
herpesviruses -1,4

african swine fever virus
poxviruses
malignant catarrhal fever virus
Viruses with ds RNA genomes
Reoviridae
Birnaviridae
rotaviruses
bluetongue virus
african horse sickness
infectious bursal disease (chickens)
infectious pancreatic necrosis (salmonid fish)
Viruses with +ve RNA genomes
Picornaviridae
Caliciviridae
Coronaviridae
Arteriviridae
Flaviviridae
Togaviridae
foot and mouth disease virus

porcine enteroviruses
feline calicivirus
equine arterivirus
pestiviruses (BVD)
coronaviruses
equine encephalitis viruses
flaviviruses (WNV)
Viruses with -ve RNA genomes
Paramyxoviridae
Rhabdoviridae
Orthomyxoviridae
Filoviridae
Bunyaviridae
parainfluenza virus
canine distemper virus
respiratory syncytial virus
rabies virus
vesicular stomatitis virus
influenzaviruses
Ebola virus
Haantan virus
Hendra, Nipah viruses
Viruses with reverse transcriptase
Retroviridae
Hepadnaviridae
feline leukemia virus
feline, bovine immunodeficiency

viruses
bovine, avian leukosis viruses
caprine arthritis-encephalitis virus
Groups, types (sero-types), isolates and ‘strains’
Group
Type -A
Type - B
Type - C
Group specific antigen
Type -A specific
antigen
Type - C specific
antigen
isolate
Group and type specific antigens
group specific antigen
type specific antigen
“naked”
virus
(eg FMDV)
enveloped virus
(eg influenza, FeLV)
Serotypes and neutralizing antibody (eg. FMDV)
serotypes of FMD virus
A
O
C
SAT1
SAT2
SAT3
Asia
antibodies against receptor binding protein of serotype A will neutralize viruses of serotype A but not of serotype C
receptor
receptor binding protein on viral surface
example - influenza
group specific antigen
type specific antigen
serotype H1
serotype H5
serotype H7
test based on group specific antigen will detect all three
vaccination against one serotype will not protect against others
Infection of a cell
Distribution of the CCR532 mutation in human populations
from PLoS Biology, Nov 2005
Errors in replication lead to “quasispecies”
persistent
infection
mixture of variant viruses
(quasispecies)
inclusion bodies
Release of virus
Release by lysis of cell
(cytopathic)
or by budding (without
death of cell, non-cytopathic)
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
Why do some infected animals get sick and others don’t?

Viral factors - virulence
Host Factors
Viral Factors:

Why are some isolates of a virus more likely to cause severe disease than others?
determinants of viral virulence
Host Factors:
Genetic Resistance
loss of receptors
CCR5-D32 mutation and resistance to HIV
variation in immune response genes
genetic defects in defenses
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
Patterns of disease
acute
recurrent
chronic
or persistent
slow
clinical signs
virus shedding
virus difficult to detect
Incubation period
incubation period - time between infection and the appearance of clinical signs
infection
Resistance and Recovery
Resistance and recovery
innate resistance
genetic
serum, mucous factors (complement, defensins)
induced resistance
Toll-like receptors (dendritic cells)
Pathogen Associated Molecular Patterns
interferons and inflammatory cytokines
acquired immunity
humoral
cell mediated
After the Toll Rush, LAJ O’Neill, Science 303:1481 2004
Interferons
I
I
I
I
I
E
Induced interferon
genes
antiviral
effects

activated
factor
infected

cell
Immune
Modulation
Acquired anti-viral immunity (antibody)
B
Virus neutralized
Virus infected
cell
Antibody targets
Fc receptor bearing cell
To kill virus infected cell
Acquired immunity (CMI)
CD8
CD4
perforins
Apoptosis
trigger
cytokines
Virus purged
necrosis
apoptosis
Prevention of infection and/or disease
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
Interference by maternal antibodies
2
4
6
8
10
12
*
*
*
*
*
*
passive antibody
weeks after birth
window of
susceptibility
interferes with vaccination
minimum amount needed for protection
can
vaccinate
in this
range
Prevention of virus infections/disease
vaccination
inactivated vaccines
attenuated vaccines
subunit
vectored
DNA vaccines
management
Why vaccination sometimes fails to protect
improper use
genetic differences between animals
antigenic differences
blocking by maternal antibodies
administration following infection (exceptions - rabies)

Diagnosis of viral diseases
Why??
Sensitivity and Specificity
Diagnosis of viral diseases
clinical signs
virus detection
detection of exposure
Laboratory
}
Detection of virus
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
Virus isolation (tissue culture, experimental animals)
cultured cells
(two dimensional
animals)
cytopathic effect
immunofluorescence
(anti-herpesvirus antibody)
Virus quantitation (plaques)
plaques
count plaques
(plaque forming unit/ml)
Virus quantitation (TCID50, LD50)
10-2
10-3
10-4
10-5
10-6
TCID50 = 104
50% Tissue culture infectious
dose
PCR
Viral DNA
primers
First cycle
Second cycle
Twenty cycles
million
BVDV-2
BVDV-1a
BVDV-1b
PCR tests offered by PDS
bov. resp. coronavirus $30
rotavirus genotyping $65
bov. RSV $20
BVD genotyping $62
porc. circovirus $27
influenza $30
swine influenza geotyping $50
equine papillomavirus $20
malignant catarrhal fever virus $27
pan-herpesvirus
West Nile virus $30
Haemagglutination (HA)
virus
No virus
Haemagglutination
Dilution
2
4
8
16
32
64
128
256
512
prozone
titre
1024
No
virus
Enzyme linked immunabsorbant assay (ELISA)
Sample to be tested
virus
capturing antibody
Detecting antibody
Enzyme -> colour
In-clinic ELISAs
Feline leukemia
And
Immunodeficiency
viruses
Others:
canine parvovirus
bovine herpesvirus - 1
bovine viral diarrhoea virus
equine infectious anemia virus
influenza-A
porcine respiratory reproductive syndrome virus
Immunological detection
Immunohistochemistry
Immunofluorescence
bovine herpesvirus antigens
in endothelial cells
BHV-1 antigens in neuron
In trigeminal ganglion
Detection of exposure
Detection of exposure (serology)

virus neutralization
haemagglutination inhibition
ELISA
Virus neutralization
Serially dilute serum
1/2 1/4 1/8 1/16……….1/512
Add equal amount of virus (100 plaque forming units) to
each tube
Infect cultured cells
Last dilution that can prevent plaque formation is titer
No serum
1/2
1/4
1/256
1/512
1/128
--
HAI
Serum
dilution
1/20
1/40
1/80
1/160
1/320
1/640
1/1280
-ve control
1 wk
8 wks
Virus
HA
Limitations of serology
detects exposure and not when exposure occurred
for correlation with disease
Paired sera
IgM
CSF
Virology Diagnostic Laboratories
PDS, Saskatoon
Veterinary Services Branch,
Manitoba Agriculture, Winnipeg
Animal Health Monitoring Lab, Abbotsford
Central Laboratories for Veterinarians Ltd. Calgary
Sample collection and submission
Choosing samples - considerations:
alive or dead
suspected pathogen(s), tropism, pathogenesis
phase of disease
virus detection or exposure
Live animals
nasal swabs, transtracheal aspirates, respiratory secretions, scrapings
vesicular fluid, covering epithelium, biopsy from margin of lesion
feces or fecal swabs
clotted and unclotted blood
samples from unaffected animals

dead animals:
collect samples as soon as possible after death
affected organs
gut loops
Shipping the samples
transport medium (from lab or buffered saline with 50 µg/ml gentamycin)
10% buffered formalin - < 1cm thick
plastic, sealed containers, labeled with water proof ink
ice packs vs frozen samples
Samples should be accompanied by
case history and suspected pathogen(s)
treatment, vaccinations, numbers involved
list of specimens
Viruses can be useful too
biological control of pests
cancer therapy
gene therapy
nanotechnology
symbiotic virus-host relationships