1. Influenza Virus
Abbas Morovvati

2. Outline
1-Influenza Virus
2- Swine Influenza
3- History
4-Surface Proteins
5-Pandemic influenza
6-Influenza Pathogenesis

4. Influenza Virus
 Influenza: Highly infectious viral illness
 Single-stranded RNA virus
 Orthomyxoviridae family
 3 types: A, B, C
 Subtypes of type A determined by hemagglutinin (H)
and neuraminidase (N)

5. Human Influenza Virus
• Types of virus “A,” “B,” “C”,and ”D”
• Type C
– Associated with sporadic cases
– Cases that are not serious
– Stable Antigenitically speaking
• Type B
Associated with epidemics
Usually less severe illness
Epidemics Genetically more stable
• Type A
– Associated with epidemics and pandemics
– Unique with subdivisions according to the HA and
NA.
• H3N2
• H1N1

6. Influenza virus structure
• 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)

7. Influenza Virus
Type of nuclear
material
Neuraminidase
Hemagglutinin
A/Fujian/411/2002 (H3N2)
Virus Geographic Strain Year of Virus
type origin number isolation subtype
05/2009

8. Influenza Surface Proteins
Enveloped. Usually rounded but can be
filamentous. The virions are 80-120 nm in
diameter.
segmented ssRNA(-) linear genome, encapsidated by
nucleoprotein (NP) Contains 8 segments coding for 11 proteins.
Segments size range from 890 to 2,341nt. Genome total size is
13.5Kb

9. Influenza gene functions
From Medical Microbiology, 5th ed., Murray, Rosenthal & Pfaller, Mosby Inc., 2005, Table 60-1.

10. Influenza Virus type A
HA
 Subtype depends on surface glyco
proteins:
• Hemagglutinate (HA) -16
• Neuraminidases (NA) - 9
Human circulating Subtypes:
H1N1, H3N2, H1N2
NA

11. ROLE OF H AND N PROTEINS
H = Hemagglutinin and
N = Neuraminidase
Hemagglutinin allows the virus to bind to
host cells
Neuraminidase helps the virus to release
itself from the highjacked cells in which it has
reproduced

12. Hemagglutinin Subtypes of
Influenza A Virus
Subtype Human Swine Horse Bird
H1
H2
H3
H4
H5
H6
H7
H8
H9
H10
H11
H12
H13
H14
H15
Adapted from Levine AJ. Viruses. 1992;165, with permission.

13. Neuraminidase Subtypes of
Influenza A Virus
Subtype Human Swine Horse Bird
N1
N2
N3
N4
N5
N6
N7
N8
N9
Adapted from Levine AJ. Viruses. 1992;165, with permission.

14. History
• Epidemics of influenza have occurred in
humans since ancient times – recorded by
Hippocrates in 412 BC
• “Influenza” – term dates from 15th century
Italy when epidemics were attributed to
the influence of the stars
• First pandemic clearly described in 1580

15. History
Epidemics of influenza have
occurred in humans since
ancient times – recorded
by Hippocrates in 412 BC
“Influenza” – term dates from
15th century Italy when
epidemics were attributed
to the influence of the
stars
First pandemic clearly
described in 1580

16. History
• Influenza A virus isolated in ferrets in 1933
by Smith et al.
• Virus first grown in embryonated eggs in
1936
• Antigenic differences detected between
viruses in 1937
• Influenza B virus isolated in 1940 by
Francis and McGill (independently)
• Inactivated influenza vaccine found to be
effective in 1944 (U.S. military)

17. Influenza type A: Ecological Aspects
• Infects several animal species
– Birds
– Mammals
• Horses
• Hogs
• Humans
• Wild birds
– Principal reservations
– Infected by all the 16 subtypes of “A” virus
– They may transmit the virus to domestics birds and other
animals
• Humans
– Normally they get infected only with human strains

18. Antigenic Drift
 Gradual change in the virus with mutations and substitutions in
the amino acid chain of the surface proteins
(neuraminidase and haemagglutinin).
 A new strain can trigger a new epidemic
usually prevail for 2-5 years before next antigenic drift.

19. Antigenic Shift
 A type A influenza virus with a completely novel haemagglutinin or
neuraminidase formation moves into the human species from
other host species
 The primary source is birds, with recombination in swine or
humans.

20. Influenza Type A: Antigenic changes
• Changes in “drift” Type may occur with HA and NA
– They are associated with seasonal epidemics
– Frequent appearance of new strains in response to a selection provoked by
collective immunity
– The Influenza A virus change more frequently than the virus B
• Changes in “shift” Type occur both in the HA as well as NA
– They are associated with pandemics
– Originates the appearance of new influenza
A virus presenting a new HA or HA & NA.
– Population without any immunity

23. Genetic Mechanisms associated with
the occurance (surgimiento) of pandemics: Shift
• Genetic Re-associations
• Adaptive Mutations of an avian virus
– Pandemic of 1918

24. Terminology
• Seasonal Influenza
Avian Influenza
In migratory jungle birds
Infection among domestic birds
Enzootic Status (Asia, Africa???)
• Pandemic Influenza

25. Influenza Pandemics in the XX Century
Credit: US National Museum of Health and
Medicine
1918: “Spanish Flu” 1957: “Asian Flu” 1968: “Hong Kong Flu”
A(H1N1) A(H2N2) A(H3N2)
40-100 millions of 1-4 millions of 1-4 millions of
deaths deaths deaths

27. 1918 “Spanish Flu” Pandemic
– Type A virus (H1N1)
– 20-50 million deaths worldwide
– 550,000 deaths in the United States
– 21,000 Flu-Orphans in NYC

28. 1918 Pandemic
• It killed more people in 25 weeks than
AIDS has killed in 25 years
• It killed more people in a year than the
plagues of the Middle Ages killed in a
century
• Seven times as many people died of
influenza than in the First World War

29. 1957 Pandemic
 1957-1958 Asian Flu
 Type A virus (H2N2)
 First identified in China February 1957
 Spread to U.S. by June 1957
 70,000 deaths in the United States

30. 1968 Pandemic
 1968-1969 Hong Kong Flu
– Type A virus (H3N2)
– First detected in Hong Kong early 1968
– Spread to U.S. later that year
– Approx 34,000 deaths in the United States
• Our seasonal flu kills 36,000
– Virus still circulating today

31. Influenza epidemiology in humans
Influenza epidemiology in humans
 1889-1890 – first recorded pandemic
 1918 “Spanish” flu- 20-40 million deaths
 1957 “Asian” flu- 1 million deaths
 1968 “Hong Kong” flu- 1 million deaths
 1976 “Swine” flu – unreported deaths

32. Influenza mortality from 1957 to 1979
Fields Virology, 2nd ed, Fields & Knipe, eds, Raven Press, 1990, Fig.40-11

33. Laboratory issues
• Laboratory safety
• Tissue culture techniques
• Rapid test kits
• HA/HI sub-typing
• Immuno-fluorescent testing
• Real time PCR analysis
– Molecular typing and sub-typing
12/2/2004 33

34. Laboratory Tests for Seasonal and
Novel Influenza Viruses
• Virus Isolation Influenza
– Technically difficult; TAT issues
– Good sensitivity and specificity
– Typically not performed for avian or novel Influenza
influenza (requires a BSL-3 laboratory with RNA
enhancements)
• Immunofluorescence (DFA)
– Rapid; limited specificity and sensitivity
• PCR Techniques
– Rapid and specific
– Sensitivity depends on the test used, the
influenza strain, and the type of specimen
tested
• Rapid antigen detection
– not performed at DCLS

35. Diagnosis, treatment, vaccination
• Diagnosis
– Culture, hemadsorbtion, viral antigen detection
• Treatment
– Amantidine and rimantidine target M2
– Zanamivir and oseltamivir target neuraminidase
• Vaccination
– Formalin fixed “wild type” virus approved for parenterally
administered vaccination.
• Trivalent: two current A strains and one current B strain.
– Live attenuated vaccine now available (“Flumist”)
• Temperature sensitive recombinant bearing relevant HA
and NA genes.
– Must anticipate shift and drift in order to identify appropriate
vaccine strain.

36. Current Pandemic Influenza Phase (as of June 15, 2009): Phase 6

37. World Health Organization Pandemic Influenza Phases (2009)
Pandemic Influenza
Phase Characterization of Phase Public Health Goals
Ensure rapid characterization of the
Human infection(s) with a new subtype, but no human-
new virus subtype and early detection,
Phase 3 to-human spread, or at most rare instances of spread
notification and response to additional
to a close contact
cases
Contain the new virus within limited foci
Small cluster(s) with limited human-to-human
or delay spread to gain time to
Phase 4 transmission but spread is highly localized, suggesting
implement preparedness measures,
that the virus is not well adapted to human
including vaccine development
Larger cluster(s) but human-to-human spread still Maximize efforts to contain or delay
localized, suggesting that the virus is becoming spread, to possibly avert a pandemic,
Phase 5
increasingly better adapted to humans, but may not yet and to gain time to implement pandemic
be fully transmissible (substantial pandemic risk) response measures
Pandemic increased and sustained transmission in
Phase 6 Minimize the impact of the pandemic
general population

38. 38 4/30/2012