Which influenza virus subtypes are most likely to cause highly pathogenic avian influenza?

• A. H5N1 and H7N7
• B. H5N3 and H3N2
• C. H3N5 and H1N7
• D. H1N1 and H3N2

Answer: (A)

In avian influenza, high pathogenicity in poultry is most strongly linked to the hemagglutinin subtypes H5 and H7. Changes that create a polybasic cleavage site in the HA protein allow the virus to be activated by a wider range of host proteases, enabling systemic replication and severe disease with high mortality in birds. H5N1 is the classic example of a highly pathogenic avian influenza, and H7 subtypes such as H7N7 have also caused major HPAI outbreaks in poultry. The other subtypes listed are less typically associated with highly pathogenic disease in birds, and are more often linked to low-pathogenic avian influenza or infections in humans. Therefore, H5N1 and H7N7 are the best-fit pair for highly pathogenic avian influenza.