Since 2012, H7N3 highly pathogenic
avian influenza (HPAI) has produced negative economic and animal welfare impacts on poultry in central Mexico. In the present study, chickens were vaccinated with two different recombinant fowlpox virus
vaccines (rFPV-H7/3002 with 2015
H7 hemagglutinin [HA] gene insert, and rFPV-H7/2155 with 2002 H7 HA gene insert), and were then challenged three weeks later with H7N3 HPAI virus (A/chicken/Jalisco/CPA-37905/2015). The rFPV-H7/3002
vaccine conferred 100% protection against mortality and morbidity, and significantly reduced virus shed titers from the respiratory and gastrointestinal tracts. In contrast, 100% of
sham and rFPV-H7/2155 vaccinated birds shed virus at higher titers and died within 4 days. Pre- (15/20) and post- (20/20) challenge serum of birds vaccinated with rFPV-H7/3002 had
antibodies detectable by hemagglutination inhibition (HI) assay using challenge virus
antigen. However, only a few birds (3/20) in the rFPV-H7/2155 vaccinated group had
antibodies that reacted against the challenge strain but all birds had
antibodies that reacted against the homologous
vaccine antigen (A/turkey/Virginia/SEP-66/2002) (20/20). One possible explanation for differences in
vaccines efficacy is the antigenic drift between circulating viruses and
vaccines. Molecular analysis demonstrated that the Mexican H7N3 strains have continued to rapidly evolve since 2012. In addition, we identified in silico three potential new N-glycosylation sites on the globular head of the H7 HA of A/chicken/Jalisco/CPA-37905/2015 challenge virus, which were absent in 2012 H7N3 outbreak virus. Our results suggested that mutations in the HA antigenic sites including increased glycosylation sites, accumulated in the new circulating Mexican H7 HPAIV strains, altered the recognition of
neutralizing antibodies from the older
vaccine strain rFPV-H7/2155. Therefore, the protective efficacy of novel rFPV-H7/3002 against recent outbreak Mexican H7N3 HPAIV confirms the importance of frequent updating of
vaccines seed strains for long-term effective control of H7 HPAI virus.