Influenza is a collective and usual term defining infectious disease caused by highly contagious and pathogenic influenza viruses that are member of the family Orthomyxoviridae and genus influenzavirus A. Influenza A viruses are pleomorphic, enveloped, 80-120 nm in diameter, 200 to 300 nm in length with negative-sense RNA virus and eight segmented genome which is responsible to encode at least 10 viral protein. Based on genomic composition and encoded protein, influenza viruses are classified into three types A, B, and C. Among these three types, only type A virus (influenza A virus) is a most major zoonotic culprit. Influenza A viruses have been isolated from a poultry, wild and cage birds, pig, horses, dogs, sea-mammals and humans. Ducks are considered as the natural reservoirs of avian influenza (AI).
Based on antigenicity of two viral glycoproteins, hemagglutinin (HA) and neuraminidase (NA), influenza A viruses are further classified into 18 HA (H1-H18) and 11 NA (N1-N11) subtypes. New subtype (H17, H18) are isolated from bats in Guatemala. Birds like wild waterfowl are the natural reservoir for the all H1-H16 and N1-N9. According to the disease severity, AI is classified into two forms: highly pathogenic avian influenza (HPAI) and low-pathogenic avian influenza (LPAI). However, according to latest OIE Terrestrial Animal Health Code, AI is an infection of poultry caused by any influenza A virus with high pathogenicity (HPAI), and by H5 and H7 subtypes with low pathogenicity (H5/H7 LPAI). Influenza A viruses with high pathogenicity in birds other than poultry, including wild birds, are also notifiable. Low pathogenicity non-H5 and non-H7 influenza A viruses (i.e. H1–4, H6 and H8–16) are not defined as avian influenza and are not notifiable. The world Organization of Animal Health (OIE) defines highly pathogenic virus strains as follows:
  1. Any influenza virus that is lethal for six, seven, or eight of eight (>75%) 4 to 6-week old susceptible chickens within 10 days following intravenous inoculation with 0.2 ml of a 1:10 dilution of a bacteria-free, infectious allantoic fluid, or
  1. Any influenza virus that has an intravenous pathogenicity index (IVPI) greater than 1.2.
  1. Any H5 or H7 virus that does not meet the criteria above, but has an amino acid sequence at the HA cleavage site that is compatible with HPAI viruses, and
  1. 2
  1. Any influenza virus that is not an H5 or H7 subtype and that kills one to five of eight inoculated.
  1. Low Pathogenic Avian Influenza (LPAI) is any influenza A virus apart from H5, H7 and H5/H7 HPAI. LPAIs generally cause milder disease or no disease at all in poultry but can be associated with severe disease, particularly in the presence of other pathogens or when health status of infected bird is low. LPAI virus may become HPAI viruses when genetic mutation result in changes to their HA cleavage site. Initially, H5 and H7 subtypes of influenza A virus were only associated with fowl plague viruses, but in 1966 and 1968, LPAI viruses were isolated from turkeys with low mortality, that were typed as H5 subtype, that is, the first H5 LPAI viruses Since, 1971, numerous H5 and H7 LPAI viruses have been isolated and characterized, thus dispelling the myth that subtypes H5 and H7 equate with HPAI. The primary virulence characteristic that separates the LPAI and the HPAI viruses is the ability of HPAI viruses to be cleaved by ubiquitous proteases found within most cells in the host.

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