Many countries have been implementing massive vaccination
against H9N2 infection in poultry. The vaccine immune pressure,
plus the immunity elicited by natural infection, however, might
have driven the antigenic evolution of H9N2 virus. The virus has
been undergoing profound antigenic changes in hemagglutinin
(HA). Since its introduction into the terrestrial poultry in Asian in
the late 1980s, H9N2 virus has evolved into
multiple HA genetic lineages, and further genetic variations within lineages are
continuously occurring.Besides the HA antigenic
evolution, H9N2 has been involved in the emergence of various
novel influenza viruses, including H5N1, H7N9 and H10N8. More
interestingly, H9N2 virus contributed its NA gene to the genesis of
highly pathogenic H5N2 virus .
This latter fact highlights the necessity of including NA in the
continuous and close surveillance on H9N2 virus.