The novel avian influenza A H7N9 virus originated from multiple reassortment events. The HA gene might have originated from avian influenza viruses of duck origin, and the NA gene might have transferred from migratory birds infected with avian influenza viruses along the east Asian flyway. The 6 internal genes of this virus probably originated from 2 different groups of H9N2 avian influenza viruses, which were isolated from chickens. Detailed analyses also showed that ducks and chickens probably acted as the intermediate hosts leading to the emergence of this virulent H7N9 virus. Genotypic and potential phenotypic differences imply that the isolates causing this outbreak form 2 separate subclasses.
The novel avian influenza A H7N9 virus might have evolved from at least 4 origins. Diversity among isolates implies that the H7N9 virus has evolved into at least 2 different lineages. Unknown intermediate hosts involved might be implicated; extensive global surveillance is needed, and domestic-poultry-to-person transmission should be closely watched in the future.
Right…as I said…it’s complicated!