Is there a relation between genetic or social groups of mallard ducks and the circulation of low pathogenic avian influenza viruses?

We investigated the circulation dynamics of low pathogenic avian influenza viruses (LPAIVs) in the mallard (Anas platyrhynchos) reservoir in Italy. In particular, we evaluated the temporal distribution of virologic findings by combining virus isolation data with a new population genetic-based study approach. Thus, during 11 consecutive sampling periods (wintering periods between 1993/94 and 2003/04), categorised into 40 sampling sub-periods, cloacal swab samples were collected from 996 wild and 16 captive-reared mallards, to be screened by RT-PCR before attempting influenza A virus isolation in embryonated eggs. Forty-eight LPAIVs were isolated from wild mallards and antigenically characterised by haemagglutination-inhibition and neuraminidase-inhibition assays. When considering LPAIV antigenic subtypes in which more than one mallard tested virus isolation positive (H1N1, n. 22; H2N3, n. 2; H5N3, n. 2; H6N5, n. 3; H6N8, n. 2; H7N3, n. 3; H11N6, n. 5), at least two birds infected with a specific HN subtype clustered within one same sampling sub-period. In the context of the novel population genetic approach, total DNA was extracted from a subset of 16 captive-reared and 65 wild ducks (2000/01 and 2001/02 sampling periods) to assess genetic diversity by amplified fragment length polymorphisms (AFLP) markers. Analyses of AFLP results showed that captive-reared mallards clustered together, whereas two main independent clusters characterised the distribution pattern of most wild mallards. Within this subset of samples, nearly identical H7N3 LPAIV strains were isolated from two wild mallards belonging to the same genetic cluster. Blood sera were also collected from the above subset of mallards and examined for antibodies to the homologous H7N3 virus strain. Four out of six wild mallards testing H7N3-seropositive by haemagglutination-inhibition assay (2001/02 period) belonged to the genetic cluster including H7N3 virus shedding ducks. Overall, our data raise the possibility of an enhanced transmission and circulation of LPAIVs in genetic or social groups of wild mallards, gathered in flocks possibly related by parentage and/or geographic origin.