ORTHOMYXOVIRIDAE: The Influenza Viruses

• 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 VARIATION:
  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.

• 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