Molecular analysis of the hemagglutinin genes of Australian H7N7 influenza viruses: role of passerine birds in maintenance or transmission?

In 1985 a fowl plague-like disease occurred in chickens in Lockwood, Victoria, Australia and caused high mortality. An H7N7 influenza virus was isolated from the chickens (A/Chicken/Victoria/1/85); additionally, an antigenically similar virus was isolated from starlings (A/Starling/Victoria/5156/85) and serological evidence of H7N7 virus infection was found in sparrows. Antigenic analysis with monoclonal antibodies to H7, oligonucleotide mapping of total vRNA, and sequence analysis of the HA genes established that the chicken and starling influenza viruses were closely related and probably came from the same source. There was high nucleotide sequence homology (95.3%) between the HA genes of A/Chick/Vic/85 and a fowl plague-like virus isolated from chickens in Victoria 9 years earlier [A/Fowl/Vic/76 (H7N7)]. The sequence homologies indicated that the A/Chick/Vic/85 and A/Fowl/Vic/76 were derived from a common recent ancestor, while another recent H7N7 virus, Seal/Mass/1/80 originated from a different evolutionary lineage. Experimental infection of chickens and starlings with A/Chick/Vic/1/85 (H7N7) was associated with high mortality (100%), transmission to contact birds of the same species, and virus in all organs. In sparrows one-third of the birds died after infection and virus was isolated from most organs; transmission to contact sparrows did not occur. In contrast, the H7N7 virus replicated in ducks and spread to contact ducks but caused no mortality. These studies establish that the host species plays a role in determining the virulence of avian influenza viruses, and provide the first evidence for transmission of virulent influenza viruses between domestic poultry and passerine birds. They support the hypothesis that potentially virulent H7N7 influenza viruses could be maintained in ducks where they cause no apparent disease and may sometimes spread to other wild birds and domestic poultry.