INTRODUCTION
Influenza in birds is caused by infection with viruses of the family Orthomyxoviridae placed in the genus
influenzavirus A. Influenza A viruses are the only orthomyxoviruses known to naturally affect birds. Many species
of birds have been shown to be susceptible to infection with influenza A viruses; aquatic birds form a major
reservoir of these viruses, and the overwhelming majority of isolates have been of low pathogenicity (low
virulence) for chickens and turkeys. Influenza A viruses have antigenically related nucleocapsid and matrix
proteins, but are classified into subtypes on the basis of their haemagglutinin (H) and neuraminidase (N) antigens
(World Health Organization Expert Committee, 1980). At present, 16 H subtypes (H1–H16) and 9 N subtypes
(N1–N9) are recognised with proposed new subtypes (H17, H18) for influenza A viruses from bats in Guatemala
(Swayne et al., 2013; Tong et al., 2012; 2013). To date, naturally occurring highly pathogenic influenza A viruses
that produce acute clinical disease in chickens, turkeys and other birds of economic importance have been
associated only with the H5 and H7 subtypes. Most viruses of the H5 and H7 subtype isolated from birds have
been of low pathogenicity for poultry. As there is the risk of a H5 or H7 virus of low pathogenicity (H5/H7 low
pathogenicity avian influenza [LPAI]) becoming highly pathogenic by mutation, all H5/H7 LPAI viruses from
poultry are notifiable to OIE. In addition, all high pathogenicity viruses from poultry and other birds, including wild
birds, are notifiable to the OIE.