Can ducks with subtype-specific protective immunity and without enteric shedding, allow influenza infection of other ducks?

Recent studies showed that low-pathogenic (LP) avian influenza viruses (AIVs) stuck on wild ducks' feathers during migration can be infectious. In addition, experimental reproduction of AIV infection by allopreening route, showed that viruses stuck on ducks’ feathers could be infectious several weeks after the end of enteric viral shedding. In this work we investigated whether infectious AIVs covering feathers may evade the duck’s immune system. Cloacal swabs, feather swabs and serum samples were collected from 202 wild mallards trapped in Central Italy during the wintering and spring migration (December 2006-March 2007). Both feather and cloacal swabs were screened by reverse transcription (RT)-PCR, specific for virus matrix gene, and then tested by virus isolation (VI) assays. Sera were tested for the presence of antibodies against influenza A virus nucleoprotein by NP-ELISA. Seroprevalence for influenza A virus was 89% (179/202); 51.4% (92/179) and 19.5% (35/179) of these seropositive mallards, tested RT-PCR positive for AIVs on feather and in cloaca respectively. LPAIVs were isolated from 12 feather swabs and from 11 cloacal swabs. NP-ELISA positive sera from 10 ducks found to be H10N7 VI-positive on feathers and VI-negative from cloaca were also investigated by hemagglutination inhibition assay (HI), using as antigen an LPAIV H10N7 strain isolated from feather swabs in this study. HI results showed that one of 10 mallards was immune against homologous H10N7, showing an HI titre of 1:32-64. Our results show evidences suggesting that birds carrying viruses on their feathers might have a post-infection immunity and play an active role in spreading AIVs infection in nature.