Influenza virus Influenza history Historical records

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Influenza virus
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Influenza history

• Historical records indicate flu-like epidemics
  throughout recorded history.
• Epidemics frequent but irregular. Sometimes
  disappear for a period of time.
• Variation in severity but usually cause mortality
  in elderly.
• Some appear to spread across Russia from Asia.
• Influenza virus isolated in 1933
• Seroarcheology suggests antigenic variation
• Major pandemics in 1957, 1968, 1977
• Frequent epidemics between pandemics
• Average annualized influenza associated excess
  morality in the US from 1979-2001 41,400
• Bottom line Influenza virus somehow manages to
  make frequent minor changes and sudden major
  changes which permit temporary evasion of a
  population's immunity. How?

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Influenza epidemiology in humans
Fields Virology, 2nd ed, Fields Knipe, eds,
Raven Press, 1990, Fig.40-1
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Influenza mortality from 1957 to 1979
Fields Virology, 2nd ed, Fields Knipe, eds,
Raven Press, 1990, Fig.40-11
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Influenza classification

• Family orthomyxoviridae
• Three types A, B, C
• Types distinguished by antigenic differences in
  matrix and nucleoprotein antigens.
• A is more pathogenic than B. C is not a big
  problem
• Type A undergoes infects humans, swine, horses,
  seals, mink, whales, birds.
• Primary reservoir is birds
• In birds, infection is mostly asymptomatic, virus
  can replicate in lungs and intestinal mucosa
  shed in feces
• Respiratory infection in humans
• Interspecies transmission.
• Influenza B and C are human viruses do not
  infect birds.

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Influenza A reservoir
Wild aquatic birds are the main reservoir of
influenza A viruses. Virus transmission has been
reported from weild waterfowl to poultry, sea
mammals, pigs, horses, and humans. Viruses are
also transmitted between pigs and humans, and
from poultry to humans. Equine influenza viruses
have recently been transmitted to dogs. (From
Fields Vriology (2007) 5th edition, Knipe, DM
Howley, PM, eds, Wolters Kluwer/Lippincott
Williams Wilkins, Philadelphia, Fig 48.1)
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Influenza virus
Electron micrographs of influenza virus. AC The
structure of the internal components (D) the
external view. A substantial fraction (up to 50)
of influenza virions contain large helical
internal components (A, B), which may contain
individual ribonucleoprotein (RNP) segments (C)
linked together. The individual RNPs each contain
a binding site for the viral polymerase, as seen
by the immunogold labeling of the end of the RNP
segment (C). The external view of the virions (D)
illustrates the pleomorphic appearance and the
surface spikes. Bar in all figures equals 50 nm.
(From Fields Virology, 4th ed, Knipe Howley,
eds, Lippincott Williams Wilkins, 2001, Fig.
47-2)
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Influenza virus structure

• Genome Eight negative sense ssRNA molecules,
  each encoding one protein.
• Helical nucleocapsid
• Envelope glycoproteins hemagglutinin (HA) and
  neuraminidase (NA)

Structure of influenza virus. The diagram
illustrates the main structural features of the
virion. The surface of the particle contains
three kinds of spike proteins the hemagglutinin
(HA), neuraminidase (NA), and matrix (M2) protein
embedded in a lipid bilayer derived from the host
cell and covers the matrix (M1) protein that
surrounds the viral core. The ribonucleoprotein
complex making up the core consists of at least
one of each of the eight single-stranded RNA
segments associated with the nucleoprotein (NP)
and the three polymerase proteins (PB2, PB1, PA).
RNA segments have base pairing between their 3
and 5 ends forming a panhandle. Their
organization and the role of NS2 in the virion
remain unresolved. (From Fields Virology, 4th ed,
Knipe Howley, eds, Lippincott Williams
Wilkins, 2001, Fig. 47-2)
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Influenza gene functions
From Medical Microbiology, 5th ed., Murray,
Rosenthal Pfaller, Mosby Inc., 2005, Table
60-1.
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Hemagglutinin

• Required for virus binding to cell surface
  sialyloglygolipids and sialyloglygoproteins.
• Responsible for virus penetration.
• Antibodies to HA neutralize virus.
• Trimer in envelope.
• Cleavage to HA1 and HA2 required for infectivity.
• Thirteen HA subtypes
• Lowest homology is 25 (H1 and H3)
• Highest homology is 80 (H2 and H5)
• Less than 10 variation within subtype.

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Structure of the influenza hemagglutinin monomer
HA monomer. Sites A-E are immunodominant
epitopes (From Fields Virology, 2nd ed, Fields
Knipe, eds, Raven Press, 1990, Fig.40-4)
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Structure of the influenza hemagglutinin trimer
HA trimer. (From Fields Virology, 2nd ed, Fields
Knipe, eds, Raven Press, 1990, Fig.39-6)
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Influenza A hemagglutinin and neuraminidase
subtypes
Fields Virology, 4th ed, Knipe Howley, eds,
Lippincott Williams Wilkins, 2001, Table 47-1
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Neuraminidase

• Removes sialic acid from any glycoconjugate.
• Aids virus spread
• May remove decoy receptors on irrelevant cells
  during infection
• Prevents virus clustering at cell surface upon
  release
• High concentration of anti-NA antibody are
  necessary for virus neutralization.

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Influenza replication
Replication of influenza A virus. After binding
(1) to sialic acid-containing receptors,
influenza is endocytosed and fuses (2) with the
vesicle membrane. Unlike for most other RNA
viruses, transcription (3) and replication (5) of
the genome occur in the nucleus. Viral proteins
are synthesized (4), helical nucleocapsid
segments form and associate (6) with the M1
protein-lined membranes containing M2 and the HA
and NA glycoproteins. The virus buds (7) from the
plasma membrane with 11 nucleocapsid segments.
(-), Negative sense (), positive sense ER,
endoplasmic reticulum. (From Medical
Microbiology, 5th ed., Murray, Rosenthal
Pfaller, Mosby Inc., 2005, Figure 60-2.)
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Influenza pathogenesis in humans
Six seronegative volunteers received 104.0 TCID50
of wild-type A/Bethesda/1015/68 virus
intranasally on day 0. (From Fields Virology, 4th
ed, Knipe Howley, eds, Lippincott Williams
Wilkins, 2001, Fig. 47-10.)
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Influenza pathogenesis

• Children at risk for severe disease
• Otitis media frequent in children (12)
• Reye syndrome
• Most common in children
• CNS and hepatic symptoms
• Salicylates a co-factor
• Complications
• Predominantly in high risk patients
• Elderly
• Immunocompromised
• Cardiopulmonary disease
• Primary viral pneumonia
• Secondary bacterial pneumonia
• Myositis and cardiac involvement
• Neurologic syndromes
• Guillain-Barre syndrome
• Encephalopathy, encephalitis
• Reye syndrome

Pathogenesis of influenza A virus. The symptoms
of influenza are caused by viral pathologic and
immunopathologic effects, but the infection may
promote secondary bacterial infection. CNS,
Central nervous system. (From Medical
Microbiology, 5th ed., Murray, Rosenthal
Pfaller, Mosby Inc., 2005, Figure 60-3.)
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Influenza genetics

• Mutation rate
• Higher than DNA viruses polymerase does not
  proofread.
• Similar to other RNA viruses cannot explain
  antigenic variation by high mutation rate alone.
• Reassortment (recombination)
• Individual RNA segments segregate independently
  in mixed infections.

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Influenza reassortment in an intermediate host
Natural demonstration of reassortment of
influenza variants in an intermediate host, with
subsequent interspecies transmission. A 1980
H7N7 emergent virus with high mortality for seals
is consistent with reassortment between two
concurrently circulating avian viruses. The
virus caused a conjunctivitis in humans handling
the seals, and could be experimentally
transmitted (assymptomatically) to other species.
(From Fields Virology, 2nd ed, Fields Knipe,
eds, Raven Press, 1990, Fig.40-8)
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Antigenic drift
Antigenic drift of H3N2 viruses between 1968 and
1997 as demonstrated by their cross-reactivity in
hemagglutination-inhibition tests. (From Fields
Virology, 4th ed, Fields Knipe, eds, Raven
Press, 1990,Table 47-3.)
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Mechanism and consequences of antigenic drift

• Point mutations occur in HA (and NA) in antigenic
  sites.
• New viruses escape established immunity in
  population and have a selective advantage.
• Drift accounts for frequent epidemics.

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Antigenic shift

• Viruses may reassort in a non-human species,
  shielded from human immunity.
• Reassortment may involve interspecies
  transmission.
• Reassorted viruses may enter the human population
  through interspecies transmission.
• Reassorted viruses express new HA, for which
  population has no immunity.
• Shift accounts for major pandemics.

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Influenza A evolution antigenic shift
Postulated evolution of the influenza A viruses
currently circulating in humans. Seroarcheology
suggests that H2N2 and H3N8 influenza viruses
circulated in humans in 1889 and 1900,
respectively. Phylogenetic evidence suggests that
an influenza virus possessing eight gene segments
from avian influenza reservoirs was transmitted
to humans and pigs before 1918 and replaced the
1900 strain. This virus was probably carried from
North America to Europe by American troops and
caused the catastrophic Spanish influenza
pandemic of 1918. In 1957 the Asian pandemic
virus acquired three genes (PB1, HA, and NA) from
the avian influenza gene pool in wild ducks by
genetic reassortment and kept five other genes
from the circulating human strain. After the
Asian strain appeared, the H1N1 strains
disappeared from humans. In 1968 the Hong Kong
pandemic virus acquired two genes (PB1 and HA)
from the duck reservoir by reassortment and kept
six genes from the virus circulating in humans.
After the appearance of the Hong Kong strain, the
H2N2 Asian strains were no longer detectable in
humans. In 1977 the Russian H1N1 influenza virus
that had circulated in humans in 1950 reappeared
and spread in children and young adults. This
virus probably escaped from a laboratory and has
continued to cocirculate with the H3N2 influenza
viruses in the human population. (From Fields
Virology, 4th ed, Knipe Howley, eds, Lippincott
Williams Wilkins, 2001, Fig. 47-1.)
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Diagnosis and treatment

• Diagnosis
• Culture, hemadsorbtion, viral antigen detection
• Treatment
• Amantidine and rimantidine target M2
• Zanamivir and oseltamivir target neuraminidase

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Vaccination

• Trivalent two current A strains and one current
  B strain.
• For 2010 season
• A/California/7/2009 (H1N1)like virus
• A/Perth/16/2009 (H3N2)like virus
• B/Brisbane/60/2008like virus
• Formalin fixed wild type virus approved for
  parenterally administered vaccination.
• Live attenuated vaccine (Flumist)
• Temperature sensitive recombinant bearing
  relevant HA and NA genes.
• Must anticipate shift and drift in order to
  identify appropriate vaccine strain.

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Recombinant live attenuated vaccine
Production of live attenuated viruses by genetic
reassortment between two influenza A viruses by
transfer of genes from an attenuated donor virus
to a virulent wild-type virus. The attenuated
reassortant live vaccine virus contains the genes
that code for the hemagglutinin and neuraminidase
from the virulent wild-type virus and the genes
that confer attenuation from the attenuated donor
virus. (From Fields Vriology (2007) 5th edition,
Knipe, DM Howley, PM, eds, Wolters
Kluwer/Lippincott Williams Wilkins,
Philadelphia, Fig 48.11)
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Summary influenza

• Structure
• Negative sense segmented ssRNA genome, helical
  nucleocapsid, enveloped
• Pathogenesis
• respiratory transmission
• replication in nucleus budding
• no spread (usually)
• innate and antibody response important antigenic
  shift and drift
• local symptoms from cell killing systemic
  symptoms from immune response exaggerated
  disease in young and elderly viral and bacterial
  pneumonia complications
• Diagnosis
• culture, hemadsorbtion, viral antigen detection
• Treatment/prevention
• amantidine and rimantidine target matrix
  zanamivir and oseltamivir target NA
• killed and live vaccines need constant updating