Novel Influenza A Viruses Isolated from Canadian Feral Ducks: Including Strains Antigenically Related to Swine Influenza (HswlN1) Viruses

Twelve influenza A viruses, antigenically related to the Ho, HI and Hswl subtypes, were isolated from cloacal samples of feral ducks in Canada. Antigenic comparisons showed that these viruses were most closely related to the recent HswINI isolates from man and pigs, whereas in vivo pathogenicity tests revealed differences between the HswINl viruses from the ducks and those from humans and pigs. Antigenic haracterization f 94 additional influenza A viruses from the ducks showed four haemagglutinin subtypes (HavI, Hav4, Hav5 and Hav7), an unclas-sified haemagglutinin, and six neuraminidase ubtypes (NI, N2, Neq2, Navt, Nav2 and Nav5) in various combinations, some of which are novel and have not previously been reported. Three of these duck influenza viruses possessed a haemag-glutinin antigenically related to that of classical fowl plague virus. A much higher percentage ofvirus isolations were from juvenile ducks 08"5 %) than from adults (5 %)- All of the ducks, from which viruses were isolated, appeared healthy at the time of sampling. Serological studies on a limited number of humans and domestic birds living in close proximity to the Canadian ducks revealed no evidence of interspecies transmission. Our findings uggest that these birds serve as a substantial reservoir of antigenic-ally diverse influenza viruses, including isolates antigenically related to the current human and animal influenza viruses. This reservoir in nature may be perpetuated by a cycle involving annual infection of juvenile birds followed by transmission to the remaining susceptible birds until the next congregation during the breeding season