Influenza-An Overview


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.

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