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)
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 BAssociated with epidemicsUsually less severe illnessEpidemics Genetically more stable• Type A – Associated with epidemics and pandemics – Unique with subdivisions according to the HA and NA. • H3N2 • H1N1
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)
Influenza Virus Type of nuclear material Neuraminidase Hemagglutinin A/Fujian/411/2002 (H3N2) Virus Geographic Strain Year of Virus type origin number isolation subtype05/2009
Influenza Surface Proteins Enveloped. Usually rounded but can be filamentous. The virions are 80-120 nm in diameter.segmented ssRNA(-) linear genome, encapsidated bynucleoprotein (NP) Contains 8 segments coding for 11 proteins.Segments size range from 890 to 2,341nt. Genome total size is13.5Kb
Influenza gene functions From Medical Microbiology, 5th ed., Murray, Rosenthal & Pfaller, Mosby Inc., 2005, Table 60-1.
Influenza Virus type A HA Subtype depends on surface glyco proteins: • Hemagglutinate (HA) -16 • Neuraminidases (NA) - 9Human circulating Subtypes: H1N1, H3N2, H1N2 NA
ROLE OF H AND N PROTEINSH = Hemagglutinin andN = Neuraminidase Hemagglutinin allows the virus to bind tohost cells Neuraminidase helps the virus to releaseitself from the highjacked cells in which it hasreproduced
Hemagglutinin Subtypes of Influenza A VirusSubtype Human Swine Horse BirdH1H2H3H4H5H6H7H8H9H10H11H12H13H14H15 Adapted from Levine AJ. Viruses. 1992;165, with permission.
Neuraminidase Subtypes of Influenza A VirusSubtype Human Swine Horse BirdN1N2N3N4N5N6N7N8N9 Adapted from Levine AJ. Viruses. 1992;165, with permission.
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
HistoryEpidemics 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 starsFirst pandemic clearly described in 1580
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)
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
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.
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.
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
Genetic Mechanisms associated with the occurance (surgimiento) of pandemics: Shift• Genetic Re-associations• Adaptive Mutations of an avian virus – Pandemic of 1918
Terminology• Seasonal InfluenzaAvian Influenza In migratory jungle birds Infection among domestic birds Enzootic Status (Asia, Africa???)• Pandemic Influenza
Influenza Pandemics in the XX CenturyCredit: US National Museum of Health andMedicine 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 ofdeaths deaths deaths
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
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
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
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
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
Influenza mortality from 1957 to 1979 Fields Virology, 2nd ed, Fields & Knipe, eds, Raven Press, 1990, Fig.40-11
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-typing12/2/2004 33
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
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.
Current Pandemic Influenza Phase (as of June 15, 2009): Phase 6
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