# Commission Decision of 4 August 2006 approving a Diagnostic Manual for avian influenza as provided for in Council Directive 2005/94/EC (notified under document number C(2006) 3477) (Text with EEA relevance) (2006/437/EC), Division 1: Aetiology and virulence

Extent: E+W+S+N.I.

AI is a highly contagious viral infection caused by viruses of the family Orthomyxoviridae, genus influenzavirus A. Influenza A viruses are the only orthomyxoviruses known to infect birds. Many species of birds have been shown to be susceptible to infection with influenza A viruses; aquatic birds form a major reservoir of such viruses, but the overwhelming majority of isolates have been of low pathogenicity in chickens and turkeys, the main birds of economic importance to be affected by the disease.
Influenza A viruses have antigenically related nucleoproteins and antigenically related matrix proteins, but are classified into subtypes on the basis of the antigenic relatedness of the surface glycoproteins haemagglutinin (HA) and neuraminidase (NA). At present, 16 HA subtypes (H1–H16) and 9 NA subtypes (N1–N9) are recognised. Each influenza A virus has one HA and one NA antigen, apparently in any combination.
Influenza A viruses are divided into two groups on the bases of their ability to cause disease in susceptible poultry:
(a) highly pathogenic avian influenza (HPAI) viruses, which cause an extremely serious disease characterised by generalised infection of the infected poultry, where they may induce very high flock mortality (up to 100 %); and
(b) low pathogenic avian influenza (LPAI) viruses, which cause a mild, primarily respiratory, disease in poultry, unless there is exacerbation by other co-infections or factors.
Wild birds, especially migratory waterfowl, play a very important role as the influenza A virus reservoir, as shown by the isolation of nearly all possible combinations of HA and NA subtypes from wild birds. Generally, except when there has been spill-over of HPAI from infected poultry, only LPAI viruses are detected in wild birds.
Primary introductions of AI viruses to poultry farms most likely originate from direct or indirect contact with wild birds.
In domestic poultry, there is a possibility that such LPAI viruses introduced from a wild reservoir may circulate undetected, as clinical signs are often mild or absent.
Once introduced into poultry, LPAI virus strains of H5 and H7 subtypes may then mutate into HPAI strains. To date, only viruses of the H5 and H7 subtypes have been shown to cause HPAI.
Although it appears that several mechanisms may be responsible for mutation from the LPAI to the HPAI virus, the factors that bring about this mutation are not known. In some instances mutation seems to have taken place rapidly at the primary site after introduction from wild birds, in other instances the LPAI virus has circulated in poultry for months before mutating. Therefore, it is impossible to predict if and when such mutation will occur. However, it may reasonably be assumed that the more widespread the circulation of LPAI in poultry, the higher the chances that mutation to HPAI will occur.
The incubation period is difficult to estimate and probably varies with the strain of virus and host, usually five to six days is quoted, but the range for individual birds is probably from a few hours to about seven days.

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Source: https://www.legislation.gov.uk/eudn/2006/437/annex/chapter/II/division/1 (the official version)
Page: https://openlegislation.uk/eu-decision-2006-437/annex/chapter-II/division-1
Contains public sector information licensed under the Open Government Licence v3.0. Source: legislation.gov.uk, The National Archives.