(INFLUENZA) as an example of VIRAL INFECTIONS :immunity and infections

This post is an extension to viral infections under immunity and infections.  Click here for the main content http://biomaniac9.blogspot.com/2016/03/immunity-and-infections-1-viral.html

INFLUENZA (AN EXAMPLE):

Influenza infects the upper respiratory tract and major central airways in humans,horses,birds,pigs and even seals.

STRUCTURE

Envelope:

• They are spherical or ovoid and surrounded by an outer envelope which is a lipid bilayer acquired from the plasma membrane of the infected host.
• 2 types of glycoproteins are inserted into the envelope: hemagglutinin, neuraminidase
• Hemagglutinin: they are in the form of trimers and help virus to attach to the host cells.They are approx 1000 in numbers .
• Neuraminidase:They facilitate viral budding from the host cell by cleaving N-acetylneuraminic acid(sialic acid) present in nascent viral glycoproteins and host cell membrane glycoproteins.

Matrix protein:

It is present beneath the envelope and it surrounds the nucleocapsid.

Nucleocapsid:

The nucleocapsid consists of 8 different ssRNA which encodes protein and RNA polymerase. Each RNA strand encodes one or more different influenza proteins.

TYPES:
3 types of influenza viruses are there:
Type A-It is the most common type and responsible for the major human pandemics.
Type B- It causes human diseases but not animal diseases and has caused epidemics.

Type C-It causes mild human illness.

NOMENCLATURE:
As we know, antigenic variation gives rise to a number of subtypes of infulenza.
A/C to the nomenclature of WHO, each virus strain is defined by
1)The animal from which it was first isolated
2)The place from where it was isolated

3)Strain number
4)Year of isolation
5)Antigenic description of HA and NA.

e.g.A/Sw/Lowa/15/30(H1N1) designates strain A isolate 15 that arose in swine in lowa in 1930 and has antigenic subtypes 1 of H1 and N1.

NOTE:There are 13 different hemagglutinin and 9 neuraminidase spikes protruding from the viral envelope in case of type A influenza virus.
On the other hand, neither type B nor C is classified by H and N subtypes.

2 different mechanisms generate antigenic variability in HA and NA.
1)antigenic drift
2)antigenic shift

Antigenic drift:

• It involves a series of spontaneous point mutations that result in minor changes in HA and NA.

Antigenic shift:

• It involves sudden emergence of a new subtype of influenza whose HA and NA are different from that virus which was present in a preceding epidemic.It is thought to occur through genetic reassortment between influenza virions from humans and from various animals like pigs, horses, ducks.
• E.g.In a pig infected with human H3N2 and swine H1N1, H3N1 was recovered.
• Antigenic shift results in antigenic variation for which the immune system lacks memory and is unprepared. This results in outbreaks of influenza, sometimes reaching pandemic proportions.
• Among all these, virus undergoes Ag-drift generating minor variations, which account for strain differences within a subtype.
• An individual infected by a given strain mount an immune response against it so that the strain is eliminated. But, if too many point mutations are accumulated; then antigenicity is altered to an extent that they are able to escape immune elimination.
• These variants become a new strain of influenza; causing another local epidemic cycle.

Jumping viruses:

• Besides, a virus with even little genetic change can become capable of causing infections or epidemics in a new species.e.g. virus H3N8 caused infulenza epidemics in racing dog.This virus was 96 %identical to the virus known to have existed in horses for 30 years.

Humoural responses to influena:

• Antibody specific for the HA molecule is produced during an influenza infection.
• This antibody is strain-specific and protects against a given strain.
• The titers of these serum antibodies peak within few days of infection and then decrease over the next 6 months but then it plateaus and remain stable for several years.
• This Antibody is not needed to treat influenza but to protect against reinfection by the same strain.
• This is supported by the fact that patients with agammaglobulinemia  recover from the disease. And, experimentally it has been proved that if mice are infected with influenza virus and Ab production is experimentally suppressed; the mice, though, recover from the infection but can be reinfected with similar strain.