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Background: National surveillance of avian influenza virus (AIV) in South Korea has been annually conducted for the early detection of AIV and responses to the introduction of highly pathogenic avian influenza (HPAI) virus. In this study, we report on a nationwide surveillance study of AIV in domestic poultry and wild birds in South Korea between 2012 and 2014. Methods: During the surveillance programs between 2012 and 2014, 141,560 samples were collected. Of these, 102,199 were from poultry farms, 8215 were from LBMs, and 31,146 were from wild bird habitats. The virus isolation was performed by inoculation of embryonated chicken eggs and AIV isolates were detected using hemagglutination assay. For subtying of AIV, the hemagglutinin and neuraminidase genes were confirmed by sequencing. Phylogenetic analysis of the H5 subtypes was performed using 28 H5 AIV isolates. Results: Between 2012 and 2014, a total of 819 AIV were isolated from 141,560 samples. Virus isolation rates for AIV were 0.6, 0.4, 0.1, and 2.7% in wild birds (n = 202), domestic ducks (n = 387), minor poultry (n = 11), and the live bird market (LBM) (n = 219), respectively. In wild birds, various subtypes were found including H1-H7 and H9-H13. The major subtypes were H5 (n = 48, 23.9%: N3 (n = 4) and N8 (n = 44)), H4 (n = 39, 19.4%), and H1 (n = 29, 14.4%). In domestic poultry, mainly ducks, the H5N8 (n = 275, 59.3%), H3 (n = 30, 17.2%), and H6 (n = 53, 11.4%) subtypes were predominantly found. The most frequently detected subtypes in LBM, primarily Korean native chicken, were H9 (n = 169, 77.2%). H3 (n = 10, 4%) and H6 (n = 30, 13.7%) were also isolated in LBM. Overall, the prevalence of AIV was found to be higher between winter and spring and in western parts of South Korea. The unusual high prevalence of the H5 subtype of AIV was due to the large scale outbreak of H5N8 HPAI in wild birds and domestic poultry in 2014. Conclusions: Enhanced surveillance and application of effective control measures in wild birds and domestic poultry, including LBM, should be implemented to control AI and eradicate HPAI. [ABSTRACT FROM AUTHOR]