Transcript Avian Influenza Virus

ORTHOMYXOVIRUSES
Introduction
• Influenza virus infections have become such a common
and integral part of our lives that any respiratory
infection that causes discomfort is typically referred to
as an episode of ’flu.
• In reality, most of these infections are probably not
caused by influenza viruses but by more benign agents.
• True influenza virus infections can pose a serious threat.
• Influenza virus infection in humans occurs in seasonal
epidemics and is estimated to result in up to 5 million
cases of serious disease with 250,000–500,000 deaths
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Introduction
• The family Orthomyxoviridae contains three genera (or
types):
• Influenzavirus A, B, and C.
• They are distinguished by their antigenically distinct
nucleoprotein (NP) and matrix (M) proteins.
• Ortho
• True or regular, distinguishes these from the
Paramyxoviruses.
• Myxo
• Mucus, refers to the ability of these viruses to attach to
mucoproteins on the cell surface.
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Influenza Viruses
• Most human infections are caused by influenza A and B.
• Type C viruses cause only minor upper respiratory illness.
• Influenza A viruses have been designated on the basis of the
antigenic relationships of the hemagglutinin “HA or H”, and
neuraminidase “NA or N” proteins.
• There are 16 types of H (H1-16) and 9 types of N (N 1-9).
• Only viruses with H1, 2, 3 and N1, 2 are known to infect
humans
• Type B strains are designated without H, N numbers since
antigenic shift in these viruses has so far not been observed.
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Morphology
• Virions are spherical, 100-200 nm in diameter, helical
symmetry, enveloped with spikes.
• About 80% of the spikes are hemagglutinin antigen and
the remainder are neuraminidase.
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Morphology
• There is a layer of matrix protein which encloses the (-) ss-RNA
genome of the virus.
• RNA is associated with nucleoprotein.
• The genome is composed of eight fragments complexed with
protein to form ribonucleoprotein arranged in helix.
• Hemagglutinin “HA” is a rod-shaped glycoprotein with triangular
cross section.
• It agglutinates erythrocytes, and plays an important role in the
attachment and entry of the virus to host cells and in determining
virulence.
• Host cell protease cleaves HA into two molecules; HA1 and HA2;
a necessary step for infection.
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Morphology
• The neuraminidase “NA” is an enzyme which destroys
neuraminic acid “sialic acid”, a component of the
specific cell receptors for the virus.
• Its main function is in the release of new virus from
cells.
• NA is a glycoprotein, its spikes look like mushrooms.
• Inside the matrix shell are the nucleoprotein and an
RNA transcriptase, that is essential for replication.
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Replication
• Attachment to specific receptors on the cell membrane.
• Endocytosis: engulfment of virion into vacuoles (endosomes),
acidic pH induces a change in the configuration of the HA.
• Uncoating.
• Virus RNA is transported to the cell nucleus
• Transcription by virus RNA transcriptase to complementary (+)
strand that can function as mRNA
• It also serves as template for the Synthesis of negative RNA
strands for new virions.
• Translation, in host cell cytoplasm, to produce viral proteins.
• The new virions are assembled at the host cell surface membrane.
• Budding and release
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1. The virion binds to a sialic acid
2. Releasing the eight viral nucleocapsids into
the cytoplasm
3. The viral nucleocapsids are transported into
the nucleus.
4. The (-) strand RNAs are copied by RNA
polymerase
5. Some of the mRNA encoding NS2/NEP
and M2 is spliced, and (6) the mRNAs are
transported to the cytoplasm
6. Translation of the viral membrane proteins
(HA, NA, and M2)
7. These proteins enter the host cell’s
secretory pathway, where HA and NA
are glycosylated.
8. & 9 All other mRNAs are translated by
ribosomes in cytoplasm
10. ,11 & 12 Some proteins are imported to
nucleus for synthesis of + strand RNA &
then – strand RNA
13. newly synthesized (-) strand RNAs enter
the pathway for mRNA synthesis.
14. Shutting down of synthesis and export of
progeny nucleocapsids to the cytoplasm.
15. The HA, NA, and M2 proteins are
transported to the cell surface
16. 17, 18 & 19 Incorporation of envelope
proteins, packaging of genome in capsid
and into envelope and then release.
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Pathogenesis
• Infection is acquired by the respiratory route causing
upper respiratory tract infection.
• Virus multiplies in the epithelium and destroys the cilia,
followed by transient viremia.
• Complications may include secondary bacterial
infection, Reye’s syndrome
• Reye’s syndrome
• A rare but potentially fatal disease of the liver), children with
fever due to influenza especially type B should avoid taking
aspirin as this may lead to Reye's syndrome.
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Pathogenesis
• Incubation period 2-3 days.
• Symptoms may include:
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shivering,
headache,
malaise
and aching in the limbs and back.
• The temperature rises rapidly to around 39 ºC.
• The severity of influenza is proportional to age.
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Immune Response
• Antibodies
• Anti-hemagglutinin, the most important, it prevents virus from
attaching to cells.
• Anti-neuraminidase, prevents the release from host cells.
• IgA antibodies prevent infection, they act at the mucus surfaces
of the respiratory tract “local antibodies”
• Interferon
• Non-specific inhibitors of influenza
• Cell-mediated immunity
• Cytotoxic T cells
• Alveolar macrophages
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Genetic variation in influenza viruses
Antigenic shift “gene reassortment”
• Occurs in influenza A virus.
• In a cell infected with two different
viruses, the progeny virions may
contain mixtures of each parent’s
genes.
• Influenza A viruses of human and
animal origin may recombine to
form a new subtype.
• New reassortment subtypes will have
different HA or NA or both.
• New subtypes cause devastating
epidemics in non-immune
population.
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Antigenic drift
• Caused by minor changes in HA, NA or both due to
mutations
• It occurs in both influenza A and B.
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Difference between Antigenic Shift &
Drift
Antigenic Shift
Antigenic Drift
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Major Antigenic Change
Minor Antigenic Change
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Forming new sub-type
Forming new strain of virus
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Large change in nucleotides of RNA
Small mutation of RNA
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Occurs as a results of genome
reassortment between difference
subtypes.
Occurs as a result of the
accumulation of point mutations in
the gene.
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An antigenic change which results in
severe alternation in HA
(hemagglutinin) or NA
(neuraminidase) subtypes.
An antigenic change can alter
antigenic sites on the molecule such
that a virion can escape recognition
by the host’s immune system.
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Large and sudden mutation
Random and Spontaneous Mutation
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Influenza Epidemics
• New epidemics of influenza are caused by a virus
antigenically different from that which was present in
earlier epidemics.
• The pandemic of 1918-1919 was especially terrible in
its effects in Europe.
• Worldwide, it killed about 40-50 million people.
• Since 1977 the H3N2 and H1N1 strains have circulated
side by side.
• H1N1, H1N2, H3N2 are the only known Flu A virus
subtypes currently Circulating among humans.
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Laboratory Diagnosis
• Immunofluorescence staining of cells in nasopharyngeal
aspirates.
• Virus isolation from throat or nasal swabs, using chick
embryo or cell culture.
• Hemagglutination test or hemagglutination inhibition.
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Prophylaxis- Chemoprophylaxis
• Amantadine
• Inhibits virus replication by blocking a viral ion channel (M2
protein) and prevents the virus from infecting cells.
• Resistant strains have developed against amantadine.
• If amantadine is given during the first day of illness, it shortens
the average duration of pyrexia.
• Oseltamivir (Tamiflue) and Zanamivir (Relenza)
• They are neuraminidase inhibitors
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Prophylaxis- Vaccination
• Types:
1. Live attenuated vaccine (in chick embryo)
2. Inactivated vaccine- in chick embryo, harvested, purified by
ultracentrifugation, inactivated with formalin or βpropiolactone.
3. Split vaccines: Whole virus extracted with ether to reduce
side effects of whole virus vaccines.
4. Subunit or surface antigen: Purified HA and NA extracted
with detergent.
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Prophylaxis- Vaccination
• Who should be immunized?
• Individuals at special risk, e.g., old people and those with
chronic diseases, infants.
• People in closed institutions
• Groups in community service, e.g., healthcare Staff.
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Avian Influenza Virus (Bird flu), H5N1
• Avian influenza virus occurs naturally among birds.
• Wild birds carry viruses in their intestine but usually do
not get sick of them.
• The virus is very contagious among birds and can make
some domesticated birds including chickens, ducks and
turkeys very sick and kill them.
• Infected birds shed the virus in their saliva, nasal
secretions and feces.
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Avian Influenza Virus (Bird flu), H5N1
• The highly pathogenic form of the virus spreads more
rapidly through flocks of poultry, it may affect multiple
internal organs and has a mortality rate that can reach
90-100% often within 48 hours.
• Infection with H5N1 influenza can occur in humans.
• Infection results from contact with infected poultry e.g
domesticated chickens, ducks and turkey.
• Infection from one ill person to another is rare.
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Avian Influenza Virus (Bird flu), H5N1
• Symptoms in humans have ranged:
• influenza-like symptoms (fever, cough, sore throat and muscle
aches)
• eye infections,
• pneumonia and severe respiratory diseases
• and other life threatening complications
• e.g respiratory failure.
• Some virulent influenza strains can escape innate and
adaptive immunity via NS1 protein (nonstructural
protein).
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Control
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Elimination of infected birds.
Careful dealing with birds.
Restriction of poultry and egg import.
Administration of antiviral drugs.
Vaccine is available.
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Swine Flu (H1N1)
• The causative agent of 2009 pandemic.
• This strain is a reassortment of several strains of H1N1
that are usually found separately in humans, birds and
pigs.
• The virus can spread from person to person
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Control
• Personal hygiene.
• Aeration of living and work places.
• Sneezing and coughing using tissue paper and disposing
it immediately.
• Bed rest of infected persons
• Avoid contact with infected persons.
• Antiviral drugs (e.g Neuraminidase inhibitors)
• Vaccine is available.
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Laboratory diagnosis
• RT-PCR
• Serological testing for specific antibodies.
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