As the general term “highly pathogenic avian influenza” and the historical term ‘fowl plague’ refer to
infection with highly pathogenic strains of influenza A virus, it is necessary to assess the
pathogenicity of Influenza A virus isolates for domestic poultry. Although all naturally occurring
“highly pathogenic avian influenza” strains isolated to date have been either of the H5 or H7
subtype, most H5 or H7 isolates have been of low pathogenicity. The methods used for the
determination of strain virulence for birds have evolved over recent years with a greater
understanding of the molecular basis of pathogenicity, but still primarily involve the intravenous
inoculation of a minimum of eight susceptible 4- to 8-week-old chickens with infectious virus; strains
are considered to be highly pathogenic if they cause more than 75% mortality within 10 days, or
inoculation of 10 susceptible 6-week-old chickens resulting in an intravenous pathogenicity index
(IVPI) of greater than 1.2. Characterisation of suspected highly pathogenic strains of the virus
should be conducted in a virus-secure biocontainment laboratory. Regardless of their pathogenicity
for chickens, H5 or H7 viruses with a HA0 cleavage site amino acid sequence similar to any of
those that have been observed in highly pathogenic viruses are considered influenza A viruses with
high pathogenicity. H5 and H7 isolates that are not highly pathogenic for chickens and do not have
an HA0 cleavage site amino acid sequence similar to any of those that have been observed in
highly pathogenic viruses are considered to have low pathogenicity. For the purposes of the OIE
Terrestrial Animal Health Code, avian influenza is notifiable to OIE and is defined as 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
As the general term “highly pathogenic avian influenza” and the historical term ‘fowl plague’ refer to
infection with highly pathogenic strains of influenza A virus, it is necessary to assess the
pathogenicity of Influenza A virus isolates for domestic poultry. Although all naturally occurring
“highly pathogenic avian influenza” strains isolated to date have been either of the H5 or H7
subtype, most H5 or H7 isolates have been of low pathogenicity. The methods used for the
determination of strain virulence for birds have evolved over recent years with a greater
understanding of the molecular basis of pathogenicity, but still primarily involve the intravenous
inoculation of a minimum of eight susceptible 4- to 8-week-old chickens with infectious virus; strains
are considered to be highly pathogenic if they cause more than 75% mortality within 10 days, or
inoculation of 10 susceptible 6-week-old chickens resulting in an intravenous pathogenicity index
(IVPI) of greater than 1.2. Characterisation of suspected highly pathogenic strains of the virus
should be conducted in a virus-secure biocontainment laboratory. Regardless of their pathogenicity
for chickens, H5 or H7 viruses with a HA0 cleavage site amino acid sequence similar to any of
those that have been observed in highly pathogenic viruses are considered influenza A viruses with
high pathogenicity. H5 and H7 isolates that are not highly pathogenic for chickens and do not have
an HA0 cleavage site amino acid sequence similar to any of those that have been observed in
highly pathogenic viruses are considered to have low pathogenicity. For the purposes of the OIE
Terrestrial Animal Health Code, avian influenza is notifiable to OIE and is defined as 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
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