influenza

• 2 causes of influenza

  respiratory and non-respiratory
• cough and sore throat are symptoms
• virus

  • simple, miniscule, infectious
  • obligate intracellular parasite
  • genetic material is surrounded by protein coat (and or envelope)
• host range


  how many animal can be infected
• antagonize


  virus weaken response to be able to cause disease
• virus is

  • mainly spherical
  • enveloped
  • ssRNA sense so has to be converted to positive first
  • replication in nucleus
  • segmented genome (n=8) for influenza
• genome is spliced
• influenza A structure of surface proteins

  • Haemagglutinin -agglutinates RBCs, antigenic, binds sialic acid receptors
  • Neuraminidase - cleaves sialic acid to release virus, degrades mucin, antigenic
  • Matrix protein 2- forms protein channel, stabilise virus budding, antigenic
• influenza A envelope and interior structure

  • outer lipid envelope- from plasma membrane of host, supported by M1 protein
  • nucleocapsid- ssRNA (nucleic acid), RNA polymerase (PBq, PB2 and PA)
• Epidemic: 


  rapid spread of infection in city, state or entire country over short period of time
• Pandemic: 


  an epidemic that spreads across boarders, even worldwide, affecting large numbers
• 2 types of glycoproteins

  HA and NA
• 18 HA types
• 11 NA types
• viruses have both N and H subtype, specific antigen e.g. H1N2
• influenza has caused 4 historical pandemics (recorded)
• influenza pandemics

  • Spanish flu
  • Asian flu
  • Hong Kong flu
  • Swine flu
• HA protein of influenza binds to sialic acid receptor
• NA cleaves receptor virus interaction
• seasonal flu vaccines protect against type A and B
• virus replication cycle steps

  • attachment
  • uncoating
  • transcription
  • replication
  • assembly
  • budding
• one replication cycle takes 8 hours to happen
• HA is the major determinant of host specificity and pathogenicity
• receptor binding site confers host-specificity
• cleavage site is where the single chain is cut into 2 chains
• Human viruses preferentially bind to N-acetylneuraminic acid- a 2,6 linked galactose
• Avian viruses preferentially bind to N-acetylneuraminic acid- a 2,3 linked galactose
• avian influenza replicates less efficiently in humans
• human influenza replicates less efficiently in birds
• swine can act as a mixed vessel
• different forms of sialic acid are preferred by different viruses
• animals especially birds play important roles in influenza virus emergence
• virus keeps changing structure due to

  • antigenic drift- minor changes in antigenic sites of HA and NA- error prone replication, no proofreading, selective advantage, flu A B and C
  • antigenic shift- major changes due to re-assortment of genes, only in A
• how strains are selected for vaccine

  • year round surveillance
  • 5 WHO collaborating centres
  • recommend 2 times a year
• antivirals

  • adamantanes
  • neuraminidase inhibitors
• adamantanes are M2 ion channel inhibitors
• neuraminidase inhibitors 


  prevent virus budding step
• nucleocapsid of influenza virus contains 8 single strands of RNA and RNA polymerase
• HA and NA are surface glycoproteins of influenza A
• influenza virus replicates in the nucleus