Influenza
Strains of viruses causing 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
Isolated from humans
A strain not implicated in epidemic disease
Characteristics of the Influenza virus:
Surface antigens
Hemagglutinin (H)
Subtypes (H1 to H 12)
Binds to red cell and to virus on the host cells
Neuraminidase (N)
Subtypes (N1 to N 9)
Releases newly grown viruses
Allows viruses to spread
A core
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.
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.
The result is an “ordinary flu epidemic”
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.