Detection of highly pathogenic avian influenza virus infection in vaccinated chicken flocks by monitoring antibodies against non-structural protein 1 (NS1)

H5 and H7 highly pathogenic avian influenza virus (HPAIV) represent a major global concern in poultries and human health. Avian influenza (AI) vaccines are available but not preferred for field applications, primarily because vaccination interferes with sero-surveillances of AIV infection. To overcome the problem, ELISA systems using non-structural protein 1 (NS1) of AIV as antigens (NS1-ELISA) have been developed to measure anti-NS1 antibodies that are raised in AIV-infected but not in vaccinated chickens. However, some AI-vaccinated chickens having a weak anti-virus immune response may subsequently be infected with AIV and spread the virus. This raises a concern for the validity of NS1-ELISA to detect AIV infection in previously vaccinated chickens. In this study, we developed NS1-ELISA and assessed its feasibility to detect HPAIV infection in chickens previously immunized with H5 or H7 AI vaccines. The results indicated that the NS1-ELISA could identify HPAIV infection in both unvaccinated and vaccinated chickens at 1 week after infection in correlation with results from time-consuming virus isolation tests. Taken together, the NS1-ELISA system would be valuable tool to define HPAIV infection when AI vaccine program is in place.