Migrating waterfowl, particularly mallards (Anas platyrhynchos), are implicated in the global spread of influenza A viruses (IAVs). Experimental infection studies are used to assess a bird's ability to maintain IAV, but these studies have not represented the natural variation in physiological condition observed in migrating waterfowl and have only used captive-bred birds. We assessed how body condition affects susceptibility, viral shedding, and antibody production in wild-caught and captive-bred juvenile mallards challenged with IAV H5N9. We found wild mallards fed ad libitum were more susceptible to IAV infection, shed higher virus loads, and shed virus more frequently compared to birds in poor condition. No difference in viral load was detected between wild and captive-bred mallards. Antibody production did not vary according to condition. We conclude captive-bred mallards represent wild mallards during IAV infection when fed ad libitum, and IAV infection in mallards is largely influenced by body condition.Waterfowl are frequently exposed to the same or different IAV subtypes during migration. We examined whether primary H5N9 infection in juvenile mallards provides long-term immunity towards reinoculation with the same or different IAV subtype. Our findings demonstrate mallards can mount long-term protective immunological responses against reinfection with the same H5N9 virus, but were susceptible to reinfection with a different H7N3 subtype 49 days after primary inoculation. Additional IAV immunity studies are needed using different virus subtypes and different challenge timings.
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