The Influenza A virus which is the main pathogen of avian influenza (AI) can be isolated from various animals such as human, swine, equine, mink, marine mammals and also from avian species such as poultry and wild birds. Avian species, especially wild birds such as wild ducks are known to be asymptomatic when infected with influenza A virus. These wild birds are known to serve as a natural host and reservoir for influenza A viruses. This characteristic is one of the main factors why the viruses are capable of generating genetic diversity, consistently producing mutant viruses. These mutations to the gene contribute to the replication, pathogenicity, transmission and antiviral resistances of influenza A viruses. In this study we isolated AI viruses from fecal samples of wild birds in the wild bird habitats and analyzed the change of amino acids based on timeline and region to monitor the biological change of the AI virus. Total 38,921 fresh fecal samples were collected from major wild bird habitats from 2014 to 2016 in Korea and total 123 AI viruses were isolated.
A total of 22 amino acid sites of influenza virus were selected and analyzed for change in amino acids. These sites include 10 sites related with replication, 10 sites related with pathogenicity, 3 sites related with transmission, and 7 sites related with antiviral resistance of influenza virus. All the sites exist within the 8 gene segments of AI virus. The analysis of our study showed a substitution rate of 71.7% at the C38Y amino acid site within the polymerase basic protein 1 (PB1) gene, 66.7% at the D222G site within the hemagglutinin (HA) 1 gene, 75.6% at the A184 site within the nucleoprotein (NP) gene. The alterations of the amino acid in the PB1, HA1 and NP gene is closely related to increase of pathogenicity in chickens and mammals including mouse. The remaining sites of interest showed few or no change in amino acids. In this study we have confirmed that consistent alteration of amino acid sites related with replication and pathogenicity exists within AI viruses isolated from wild birds in Korea. This information may be very crucial to AI monitoring and surveillance programs. Frequent study of the genetic variation of AI viruses is required as domestic high pathogenic avian influenza (HPAI) outbreaks are still taking place and considering the fact that millions of wild birds are migrating to Korea.