Thursday, November 24, 2005

Virology: A primer on Viruses and "Bird Flu"

I have recently recieved several questions concerning Viruses in general and the Bird Flu in particular so I thought I would give a short tutorial on the subject. If it's to in depth....sorry, I have a habit of doing that.
More in depth info can be found HERE.
One way to think of viruses in very small microscopic machines that have the ability to invade your cells, reproduce within them and then exit in mass numbers to infect other cells. If you picture this happening you can see why you feel so bad as this is happening. Once a virus gains access to the body (usually through the respiratory system) it attaches to the surface of the respiratory epithelium ( via the "H" protein, Hemaglutinin) and then inserts itself or is absorbed by endocytosis (cellular engulfing). Once within the cell, the virus begins replication protected from attack by the immune system. Once a significant number (crital mass) of viral particles is obtained within the cell, they are expelled (via "N" protein, Neuraminadase) and the process is repeated again. The bodies immune system is in a virtual race with the virus as anti-bodies are produced to the surface proteins and replicated for distribution throughout the body as a defense against the rapidly spreading infection. As you can imagine, if you have never been exposed to a virus before them the immune system is at a significant disadvantage and lags way behind the shear number of viruses produced. If the immune system can't get the upper hand in this fight then you lose to many respiratory cells and you can no longer over. From this you can see why there is such interest in the current "bird flu" and the recent human to human transfer everyone has feared (New Scientist breaking news)Here is a chart on the basic facts about the Orthomyxoviruses of which Influenza is a member.

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 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.

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