Background: Egyptian poultry industry has suffered from high economic losses as a result of the wide spread of highly pathogenic H5N1. Determination of the optimal antigen content of avian influenza virus vaccines is urgent to reach protective antibody titers and reduce virus shedding.
Material and method: Groups of one day old commercial broiler chicks were divided in to 8 groups 1, 2 and 3 were vaccinated with a prepared vaccine contain 500HAU of H5N1 reassortant antigen; while group 4, 5 and 6 were vaccinated with an imported reassortant vaccine with 500HAU antigen content of H5N1 at 1, 5 and 10 days of age; respectively. Groups 7 was positive challenged control and group 8 negative challenged groups. All chicken groups were maintained at isolators along the experiment study. Blood samples were collected for weekly for 4 weeks and antibody titers were determined by HI test. All vaccinated groups were challenged 4 weeks post vaccination and tracheal and cloacal swabs were taken at 3, 5, 7, and 10 days post challenge and tested by real time RT-PCR (rRT-PCR) and virus isolation and titration in SPF ECE.
Results: Results of HI demonstrated significant difference between groups in relation of age of vaccination where the groups vaccinated at 10 days of age were significantly higher compared to others with maximum titers at 4 weeks post vaccination. The protection % post challenge revealed 0, 20, 86 % and 0, 20 and 86 % in groups 1, 2, 3 and groups 4, 5, and 6; respectively. Results of rRT-PCR and virus isolation revealed that all chicken groups vaccinated at 1 and 5 days of age revealed 100% shedding at 3rd, 5th, 7th and 10th days post challenge. However, groups 3 and 6 which were vaccinated at 10 days of age demonstrated different shedding pattern where group 3 (vaccinated with local prepared 500HAU vaccine) showed at the 3rd and 5th days shedding by rRT-PCR and 80% and 20 % of the chickens in tracheal swabs and 80% and 40% in cloacal swabs when tested by virus isolation in eggs at 3 and 5 days post challenge; respectively. Whereas swabs of 7 and 10 days post challenge of group 3 were negative by rRT-PCR and virus isolation. In the other hand , group 6 (vaccinated with imported 500HAU vaccine) demonstrated shedding % at 3 and 5 days post challenge by rRT-PCR and for virus isolation were positive in 60% for tracheal swabs 3 day post challenge and no shedding at 5th post challenge and 60% and 20% for cloacal swabs ; respectively. At 7th and 10th days post challenge shedding of all chickens in group 6 were negative by both rRT-PCR and virus isolation.
Conclusion: Vaccination against H5N1 AIV is greatly affected by both antigen content of vaccine and level of maternal immunity in vaccinated chicks.
Key words: Avian influenza; antigen content; maternal immunity; challenge; rRT-PCR