ORTHOMYXOVIRIDAE: The Influenza Viruses

on 30.1.09 with 0 comments

  • STRUCTURE: ss (-) RNA, segmented, helical nucleocapsid, enveloped.
      Cell envelope is acquired by budding through plasma membrane.
    • Hemagglutinin (HA) on viral envelope attaches to sialic acid receptor on host cells. One source of antigenic types.
      • There are 15 subtypes of HA. H1, H2, H3 exist in humans.
    • Neuraminidase (NA) is released from infected cells. Another source of antigenic types.
      • There are 9 subtypes of NA. Only N1, N2 are found in humans.
  • REPLICATION: In host nucleus. Cap-snatching transcription. Viral RNA's utilize portions of host-cell RNA to make their own 5' Cap in the host cell nucleus.

      Antigenic Shift: Major source of antigenic variation, due to re-assortment of the RNA genome segments. It leads to changes of subtype of the envelope glycoproteins.
    • Antigenic Drift: Minor changes in antigenic variation, due to point mutations in the genome.

  • INFLUENZA VIRUS-A: 8 segments.
    • DISTRIBUTION: Found in humans, aquatic birds, swine, horses, seals, whales.
    • EPIDEMIOLOGY: This is the major player in flu epidemics, because of its antigenic variation. Highly contagious, spread by person-to-person contact.
    • MANIFESTATIONS: It targets the epithelial cells of the respiratory tract, upper and lower.
      • Epithelial cells become ciliastatic as a result of infection, which can predispose to more serious bacterial infections.
      • Incubation Period: 1-4 days.
      • Symptoms: Soar throat, fever, chills, myalgia, headache.
      • Normally Self-Limiting infection, lasting 3-7 days. Cough may last 1-2 weeks.
  • INFLUENZA VIRUS-B: 8 segments
    • DISTRIBUTION: Found only in humans
    • EPIDEMIOLOGY: Less serious infection than Type-A. Generally found in children or adolescents.
      • Influenza-B does not undergo reassortments or antigenic shift.
  • INFLUENZA VIRUS-C: 7 segments.
  • DISTRIBUTION: Found in humans and swine.
  • EPIDEMIOLOGY: Rarely causes diseases. Ubiquitous, and we all generally have antibodies by early childhood.

INFLUENZA VACCINE: Constantly updated, as CDC keeps track of antigenic types of latest strains.
  • In the past they've used inactivated whole viruses.
  • This year they are using a trivalent subunit vaccine consisting of purified viral HA antigen: (1) Type-A H1N1 and (2) H3N2, and Type-B antigen.
  • Vaccine administered during the fall. Breakouts are in winter.

Category: Microbiology Notes



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