SWINE FLU

Swine influenza, or “Swine Flu” or “Hog Flu” or “H1N1 Influenza” is a highly contagious acute respiratory disease caused by any strain of the influenza virus endemic in pigs (swine) that regularly cause outbreaks of influenza among pigs.

Strains endemic in swine are called swine influenza virus (SIV)
Swine Influenza Virus:
The classical swine flu virus (an influenza type A H1N1 virus) was first isolated from a pig in 1930.
It causes high levels of illness and low death rates in pigs.

Swine influenza viruses may circulate among swine throughout the year,but most outbreaks occur during the late fall & winter months similar to outbreaks in humans.
Classification:
Influenza virus A (common): Influenza type A is antigenically highly variable and is responsible for most cases of epidemic influenza.

Various subtypes H1N1, H1N2,H3N1,H3N2, & H2N3 .

In pigs, three influenza A virus subtypes (H1N1, H3N2, & H1N2) are the most common strains worldwide.
Influenza virus B :
Due to its limited host range and the lack of genetic diversity, this form of influenza does not cause pandemics in humans

Influneza virus C (rare):
Influenza type C is antigenically stable and causes only mild illness in immunocompetent individuals.
Antigenic differences exhibited by two of the internal structural proteins, the nucleocapsid (NP) and matrix (M) proteins, are used to divide influenza viruses into types A, B, and C.
Antigenic variations in the surface glycoproteins, HA and NA, are used to subtype the viruses.
Only type A has designated subtypes.
Four HA (H1–H3, H5) and two NA (N1, N2) subtypes have been recovered from humans.
The standard nomenclature system for influenza virus isolates includes the following information: type, host of origin, geographic origin, strain number, and year of isolation.
The host of origin is not indicated for human isolates, eg, A/Hong Kong/03/68(H3N2), but it is indicated for others, eg, A/swine/Iowa/15/30(H1N1).
Structure & Function of Hemagglutinin
The HA protein of influenza virus binds virus particles to susceptible cells and is the major antigen against which neutralizing (protective) antibodies are directed.
Structure & Function of Neuraminidase:
The NA functions at the end of the viral replication cycle.
It is a sialidase enzyme that removes sialic acid from glycoconjugates.
It facilitates release of virus particles from infected cell surfaces during the budding process and helps prevent self-aggregation of virions by removing sialic acid residues from viral glycoproteins.
Antigenic Drift & Antigenic Shift
The two surface antigens of influenza undergo antigenic variation independent of each other.
Minor antigenic changes are termed antigenic drift.
Antigenic drift is a gradual change in antigenicity due to point mutations that affect major antigenic sites on the glycoprotein.
Major antigenic changes in HA or NA, called antigenic shift, result in the appearance of a new subtype.
Antigenic shift is most likely to result in major epidemics or pandemics.
Antigenic shift is an abrupt change due to genetic reassortment with an unrelated strain.
- Important Properties of Orthomyxoviruses:
Virion: Spherical, pleomorphic, 80–120 nm in diameter (helical nucleocapsid, 9 nm) 
Composition: RNA (1%), protein (73%), lipid (20%), carbohydrate (6%) 
Genome: Single-stranded RNA, segmented (eight molecules), negative-sense, 13.6 kb overall size 
Proteins: Nine structural proteins, one nonstructural