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Definition of avian influenza
- Avian influenza (AI) is a contagious disease caused by influenza A viruses, affecting many species of birds.
- AI in domestic birds is classified into two categories according to disease severity: low pathogenic avian influenza (LPAI) and highly pathogenic avian influenza (HPAI).
- LPAI viruses are generally of low virulence producing only mild symptoms in domestic flocks, such as a transient decline in egg production or reduction in weight gain.
- HPAI viruses are highly virulent and usually result in nearly 100% mortality in infected domestic flocks.
Influenza A virus composition and subtypes
- Influenza A viruses have 10 genes that code for 10 different proteins: three transcriptases (PB1, PB2 and PA), two surface glycoproteins (haemagglutinin [HA] and neuraminidase [NA]), two matrix proteins (M1 and M2), one nucleocapsid protein (NP), and two nonstructural proteins (NS1 and NS2).
- Differences in the surface glycoproteins (HA and NA) distributed on the outside of the virus particles are used to determine virus subtypes.
- There are 16 known HA proteins (H1 to H16) and nine known NA proteins (N1 to N9), producing subtypes such as H2N7, H7N7 and H1N9.
- All known HPAI viruses have been of the H5 or H7 subtype.
- Viruses of the H5, H7, H9 and H10 subtypes have infected humans.
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