Field application of the H9M2e enzyme-linked immunosorbent assay for differentiation of H9N2 avian influenza virus-infected chickens from vaccinated chickens.

Vaccination for control of H9N2 low-pathogenicity avian influenza (LPAI) in chickens began in 2007 in South Korea where the H9N2 virus is prevalent. Recently, an enzyme-linked immunosorbent assay (ELISA) using the extracellular domain of the M2 protein (M2e ELISA) was developed as another strategy to differentiate between vaccinated and infected chickens. Here, an ELISA using the extracellular domain of the M2 protein of H9N2 LPAI virus (H9M2e ELISA) was applied to differentiate infected from vaccinated chickens using the H9N2 LPAI virus M2 peptide. The specificity and sensitivity of the optimized H9M2e ELISA were 96.1% and 83.8% (the absorbance of the sample to the absorbance for the positive control [S/P ratio] ≥ 0.6), respectively, with the cutoff value (S/P ratio = 0.6), and the criterion of avian influenza (AI) infection in a chicken house was established as >20% reactivity of anti-M2e antibody per house with this cutoff value. After infection in naïve chickens and once-vaccinated chickens with a hemagglutination inhibition (HI) assay titer of 9.25 ± 0.75 log(2) units, the sera from infected chickens were confirmed as AI infected when the chickens were 1 week old in both groups, and AI infection lasted for 24 weeks and 9 weeks in naïve and once-vaccinated chickens, respectively, although in twice-vaccinated chickens with a higher HI titer of 11.17 ± 0.37 log(2) units, anti-M2e antibody in infected sera did not reach a level indicating AI infection. In field application, anti-M2e antibody produced in infected chickens after vaccination or in reinfected chickens could be identified as AI infection, although HI test could not distinguish infected from vaccinated sera. These results indicate the utility of H9M2e ELISA as a surveillance tool in control of H9N2 LPAI infections.