Download General Virology(contin.)

‫بسم‬
‫هللا‬
‫الرحمن‬
‫الرحيم‬
PATHOGENESIS OF VIRAL DISEASES
It is the interaction of viral and host factors
that lead to disease production.
1. Entry into the host
2. Local or primary replication
3. Viral spread to target organs and cell
tropism
4. Effect of the virus on the host cell
5. Host immune response and recovery from
infection
6. Fate of the virus after clinical recovery
7. Virus shedding into the environment
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Through inhalation, ingestion, sexual contact
or vertical transmission from a mother to her
baby through the placenta, birth canal or
breast feeding.
Some viruses can enter the body through skin
abrasions, injections or bite of an arthropod
vector or an animal.
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Viruses usually replicate at the primary site of
entry.
Some viruses produce disease at the site of
entry where they spread locally over the
epithelial surfaces with no invasion of the
underlying tissues or spread to distant sites,
e.g. Influenza virus causes respiratory tract
infection and Rotavirus causes
gastrointestinal tract (GIT) infection.
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Many viruses produce disease at sites distant
from site of entry, e.g. enteroviruses enter via
the GIT and after primary replication, they
spread within the host to produce central
nervous system (CNS) disease.
Routes of spread:
a. Through blood (viremia) or lymphatics.
b. In few cases neuronal spread occurs, as in
rabies.
a. Presence of specific cell surface receptors for
that virus.
 Poliovirus selectively infects a nerve cell with
a high concentration of surface receptors for
poliovirus.
 HIV infects cells with CD4 receptors.
b. Presence of certain proteolytic enzymes
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Certain paramyxoviruses are not infectious
until their envelope glycoprotein undergoes
proteolytic cleavage.
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c. Local temperature, pH and oxygen tension
e.g. rhinoviruses multiply exclusively in the
upper respiratory tract, as they are adapted
to multiply at low temperature and high
oxygen tension.
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a. Cell death
The infection is lethal and kills the cell
causing a cytopathic effect (CPE).
The virally infected cells may also be killed
by cytotoxic T lymphocytes (CTL) and NK
cells.
Cell injury produced by the virus causes
lesions and dysfunction (symptoms and
signs) in organs concerned.
Some tissues rapidly regenerate e.g. intestinal
epithelium, while others do not e.g. brain.
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b. Cell transformation
The cell is not killed but is changed from a
normal cell to a cell with the properties of
malignant or cancerous cell.
This involves humoral and cell mediated
immune response, interferons and other host
defense factors.
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It is either one of two outcomes
i-Complete resolution
Most viruses do not remain in the host after
clinical recovery being completely eliminated
from the host body (viral clearance).
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Sometimes the virus persists in the host producing
either chronic infection or latent infection.
a. Chronic infection: Virus is continuously detected
with mild or no clinical symptoms, e.g. HBV
b. Latent infection: Virus persists in an occult form
inside the infected cell.
During latency, no viral markers could be detected.
Intermittent flare up (reactivation) may occur where
the virus can be recovered, e.g. Herpes viruses become
latent in sensory ganglia.
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It represents the time at which the infected
person becomes infectious to contacts.
Shedding usually occurs from body surfaces
involved in viral entry.
In some viral infections, the human
represents dead-end host and shedding does
not occur i.e. human is not infectious to
others e.g. Rabies.
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1. Systemic Diseases :
The virus spreads widely and invades many
tissues and organs because of viremia.
Usually associated with long incubation period.
In most cases, it produces long lasting
immunity.
2. Localized Diseases :
The virus invades only tissues adjacent to the
site of entry.
Associated with short incubation period.
It produces transient immunity.
LABORATORY DIAGNOSIS OF
VIRAL INFECTION
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I-Direct Demonstration of the Virus or Its
constituents in Clinical Specimens
II-Virus Isolation
III- Serological Detection of Specific
Antibodies
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A. Detection of viral particles by Electron
Microscopy (E/M):
It is required for detection of viruses of
special morphology e.g. Herpes simplex
viruses and Rota viruses.
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These are eosinophilic or basophilic bodies
stained with hematoxylin and eosin (H&E).
They may be intracytoplasmic (e.g. rabies
virus), intranuclear (e.g. herpes viruses), or
both (e.g. measles virus).
Inclusion bodies represent:
aggregation of mature virions
or areas of altered staining at sites of viral
synthesis
or degenerative changes in the host cell.
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Viral antigens are detected by enzyme linked
immunosorbant assay (ELISA),
immunofluorescence (IF) tests or
radioimmunoassay (RIA) e.g. hepatitis B virus.
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1. Polymerase chain reaction (PCR):
It is a highly specific test. It detects minimal
amounts of viral nucleic acid "DNA or RNA" by
amplification of the specific nucleic acid
sequences of the virus.
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2. Hybridization reaction:
Labeled nucleic acid probes are used to
detect virus genome in tissues.
A. Tissue culture
Virus isolation needs living cells:
On inoculation of the virus on a living
susceptible cell culture, the virus infects the
cell and replicates.
Viral growth in infected cells is recognized
by:
Cytopathic effects induced by virus growth
in tissue culture.
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a. The virus kills the cells:
which detach from the wall of the tissue
culture flasks.
b. The virus causes fusion of tissue culture
cells producing syncytia (multinucleated giant
cell), e.g. respiratory syncytial virus.
c. The virus causes rounding of cells, e.g.
herpes viruses, or grape like cells e.g.
adenoviruses.
Cytopathic effects induced by virus
growth in tissue culture
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2. Hemadsorption:
It is the adsorption of erythrocytes to the
infected cells due to the presence of viralencoded hemagglutinins (influenza,
parainfluenza) in cellular membrane.
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4. Inclusion body formation:
Detection of inclusion bodies is performed by
L/M examination of virus-infected cells
stained with H&E, e.g. Negri bodies in rabies
virus infection.
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3. Transformation:
This is produced by oncogenic viruses that
may lead to uncontrolled cell growth, loss of
contact inhibition between infected cells and
piling up of cells with formation of discrete
foci.
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5. Detection of viral antigens:
Viral antigens can be detected on the surface
of virus-infected cells by direct
immunofluorescence.
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6. Detection of viral nucleic acid:
Molecular-based assays such as PCR or
hybridization techniques provide rapid,
sensitive and specific methods for detection.
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Interference:
 Non-cytopathic viruses (e.g.
rubella) interfere with replication
and appearance of CPE produced
by cytopathic-viruses, e.g.
echovirus, which is added to the
cell culture as an indicator.
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Identification
The isolated virus is identified by:
1. Neutralization of CPE with standard
antiviral serum
2. Inhibition of hemagglutination or
hemadsorption with standard antiviral serum.
3. Immunofluorescence with standard
monoclonal antibody.
4. Electron microscopy.
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Rarely used for diagnosis.
It is used mainly for multiplication of viruses
for production of vaccines.
The virus is inoculated on:
yolk sac
or the chorioallantoic membrane
or the amniotic sac.
Identification: Viral growth causes death of
the embryo, production of hemagglutinin or
formation pocks.
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Some viruses can be only detected by
inoculation in laboratory animals (e.g. mice,
rabbits or monkeys).
Virus growth results either in disease
development or death of the laboratory
animal.
Identification:
Neutralization of viral pathogenicity by
standard antisera.
 Detection
of antibody in patient's
serum is the most widely used
method of diagnosis.
 The following tests can be used:
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1. ELISA.
2. Indirect immunofluorescence (IIF) test.
3. Radioimmunoassay (RIA).
4. Complement fixation test (CFT).
5. Hemagglutination inhibition (HI) test.
6. Neutralization test (Nt).
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1. Detection of IgM:
It appears early in infection and is therefore
used for diagnosis of recent or acute
infection.
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2.Detection of IgG:
A four-fold rise in IgG titer in paired serum
samples with an interval of 10-14 days
between the first and second sample, is
diagnostic of acute infection.
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If paired sera are not available, demonstration
of IgM antibodies to the virus even in the first
sample taken is diagnostic.
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