Three strains of viruses that cause influenza:
- Type A
- Isolated from animals (fowl, swine, horses, etc.)
- The cause of all pandemics
- Type B
- Isolated from humans
- Low mortality rate
- The cause of illness in the very young
· Type C
o Isolated from humans
o A strain not implicated in epidemic disease
Characteristics of the Influenza virus
Consists of:
· Surface antigens
o Hemagglutinin (H)
- Subtypes (H1 to H 12)
- Binds to red cell and to virus on the host cells
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o Neuraminidase (N)
- Subtypes (N1 to N 9)
- Releases newly grown viruses
- Allows viruses to spread
· A core
o Containing the genetic material, ribonucleic acid, RNA
- As eight separate genetic fragments
- The eight genes allow the virus to mutate easily, thereby changing the appearance of the surface antigens.
o A Minor mutation causes a surface “antigenic drift” in H1, H2 or H3 and/or in N1 and N2.
§ This type of mutated virus is transmitted person to person
§ A portion of immunity is retained from one drift to another
§ Partial immunity may be transferred year to year.
o The result is an “ordinary flu epidemic”
o A Major mutation causes a great change in surface antigens, an “antigen shift”.
§ It is caused by an individual who contracts flu from a source in which there was a viral exchange between two different species such as between a swine and fowl.
§ The result is the creation of an entirely new hybrid antigen, an antigen shift. It is a new strain that has no worldwide immunity.
§ All pandemics are caused by antigen shifts.