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<big><center>[[Viruses|'''BACK TO VIRUSES''']]</center></big>
<big><center>[[Viruses|'''BACK TO VIRUSES''']]</center></big>
=Introduction=
Influenza is a largely species-specific infection, but the threat of zoonotic potential is real. Virulence varies between strains, but all are spread by aerosol routes. They are typified by rhinitis, pyrexia, and anemia. Flu presents a particular challenge to vaccination as antigenic drift within the virus means that strains can vary from one year to the next. As such, vaccine produced from this year's virus may not give protection against next year's virus.
Influenza is a largely species-specific infection, but the threat of zoonotic potential is real. Virulence varies between strains, but all are spread by aerosol routes. They are typified by rhinitis, pyrexia, and anemia. Flu presents a particular challenge to vaccination as antigenic drift within the virus means that strains can vary from one year to the next. As such, vaccine produced from this year's virus may not give protection against next year's virus.
=Morphology=
*Enveloped, single-stranded RNA negative sense virus with a different gene on each of its 8 segments
*Enveloped, single-stranded RNA negative sense virus with a different gene on each of its 8 segments
*Each gene codes for one protein:
*Each gene codes for one protein:
**Neuraminidase is a sialidase enzyme that prevents reattachment to the same host cell upon detachment
**Neuraminidase is a sialidase enzyme that prevents reattachment to the same host cell upon detachment
=Virulence=
Virulence depends on the tropism of the H molecule involved:
Virulence depends on the tropism of the H molecule involved:
*H is formed by the cleavage of a precursor protein H0
*H is formed by the cleavage of a precursor protein H0
*However, a virulent virus has more basic amino acids at it's H0 cleavage site, leaving it open for cleavage by other cells, such as neurons
*However, a virulent virus has more basic amino acids at it's H0 cleavage site, leaving it open for cleavage by other cells, such as neurons
=Types and Subtypes=
*Three genuses of Flu exist: A, B, and C, of which only A is of veterinary interest
*Three genuses of Flu exist: A, B, and C, of which only A is of veterinary interest
*The subtype of a flu virus is described in terms of H (of H1-H15) and N (of N1 to N9)
*The subtype of a flu virus is described in terms of H (of H1-H15) and N (of N1 to N9)
*Vaccines cannot cross protect against different subtypes within the same species
*Vaccines cannot cross protect against different subtypes within the same species
=Antigenic Shift and Drift=
Flu continues to be a feared disease because of it's ability to evolve, both within a particular strain and into new and undefined strains. The flu epidemics of history show that antigenic variation of influenza should not be taken lightly, hence the great concern over the recombination of the H5N1 virus in asia through infection of a pig with human and avian strains of flu.
Flu continues to be a feared disease because of it's ability to evolve, both within a particular strain and into new and undefined strains. The flu epidemics of history show that antigenic variation of influenza should not be taken lightly, hence the great concern over the recombination of the H5N1 virus in asia through infection of a pig with human and avian strains of flu.
*'''Shift''' is the more serious of antigenic movements and is definied by:
*'''Shift''' is the more serious of antigenic movements and is definied by:
**Partial protection can be provided against strains within the same ''subtype'', hence the continuation of vaccination shemes
**Partial protection can be provided against strains within the same ''subtype'', hence the continuation of vaccination shemes
=Influenza by Species=
==Equine Influenza==
===Subtypes===
*Two subtypes of Equine Influenza have been described:
*Two subtypes of Equine Influenza have been described:
**H7N7 (Equine 1), which was prevalent in the UK between 1963-1977
**H7N7 (Equine 1), which was prevalent in the UK between 1963-1977
**H3N8 (Equine 2), or the European strains, have been circulating since 1965
**H3N8 (Equine 2), or the European strains, have been circulating since 1965
*Some drift has occurred, causing outbreaks of North-American-like strains of H3N8 in 1998 (EU) and 2003 (South America)
*Some drift has occurred, causing outbreaks of North-American-like strains of H3N8 in 1998 (EU) and 2003 (South America)
===Pathogenesis===
*Aerosol transmission infects the epithelium of the upper respiratory tract, resulting in cell necrosis
*Aerosol transmission infects the epithelium of the upper respiratory tract, resulting in cell necrosis
*This manifests a bronchiolitis and serous exudation
*This manifests a bronchiolitis and serous exudation