A Dose-Response Study of Inactivated Low Pathogenic Avian Influenza H9N2 Virus in Specific-Pathogen-Free and Commercial Broiler Chickens.

Abstract

Since the first report of low pathogenic avian influenza (LPAI) H9N2 virus in Egypt in 2011, the Egyptian poultry industry has suffered from unexpected economic losses as a result of the wide spread of LPAI H9N2. Hence, inactivated H9N2 vaccines have been included in the vaccination programs of different poultry production sectors. The optimal antigen content of avian influenza virus vaccines is essential to reach protective antibody titers. In this study, the correlation between antigen content (based on hemagglutinating units [HAU]) and postvaccination (PV) antibody response of H9N2 inactivated vaccine was studied. Five different vaccine antigen loads (128, 200, 250, 300, and 350 HAU formulas/dose) were investigated in commercial broiler and specific-pathogen-free (SPF) chickens. Vaccine safety and PV antibody responses were monitored. At the fourth week PV only SPF vaccinated groups (128, 200, 250, and 300 HAU/dose) were challenged using LPAI H9N2 (A/Ck/EG/114940v/NLQP/11) virus with 10(6) EID50/bird. Oropharyngeal swabs were used to monitor virus shedding at 2, 4, 6, and 10 days postchallenge. Results showed that all vaccine formulations were well tolerated, and the highest antibody titers were observed in birds vaccinated with higher HAU. Vaccines containing 128 and 200 HAU/dose did not induce the required protective HI titers by 3 wk PV. Meanwhile, the challenge experiment in SPF chickens showed that 250 and 300 HAU vaccine doses were required to reduce the level and duration of virus shedding. Study results thus suggest that inactivated H9N2 vaccines containing at least 250 HAU/dose will induce the optimal protective titers and minimize virus shedding in SPF chickens.