Experimental study of the efficacy of three inactivated H9N2 influenza vaccines on broiler flocks

Abstract

This study was carried out to evaluate the effect of different Low Pathogenic Avian Influenza (LPAI) H9N2 vaccines containing different virus strains (vaccine A, vaccine B, and vaccine C) on the productivity and immunity of 10 days-old broiler chickens. Two hundred day-old Cobb broiler chicks were divided into eight groups, 25 chicks in each group. Six groups were vaccinated with Vaccine A, Vaccine B, and Vaccine C (2 groups/each vaccine) at 10 day-old, respectively. Chickens of groups 7 and 8 were kept as control groups. One group from each vaccine was challenged at 28 days old with 106 EID50/0.2ml of A/chicken/Morocco/SF1/2016 (H9N2) virus via the oculonasal route. The remaining groups were kept unchallenged to evaluate the immune response. Chicks were sampled each week individually until 42 days old for zootechnical traits and serological evaluation. Two necropsies were done at 5 and 10 days post-challenge (DPC). Lungs and tracheas were collected for histopathology, and the virus shedding was monitored at 5, 7, 9, and 11 DPC by real-time RT-PCR. Results indicated that vaccine B provides significantly better growth performances (P < 0.05), final body weight gain (2689.6 g ± 73.2), and feed conversion ratio (2.10 ± 0.06) when compared to other vaccinated groups. During the challenge (28th -35th days), vaccine B showed a significant increase in antibody titer (26180 ± 1129.1) than other vaccines. In contrast, the vaccine C group had a similar immune response to that of the control group. Both vaccines A and B were able to stop virus shedding by 11 DPC with higher mean Cq values. However, the vaccine C group continued to shed the virus until 11 DPC. Pathological examination of challenged birds revealed lesions predominantly in the respiratory tract. At 5 DPC, fibrinous sinusitis, tracheitis with fibrin plug, pneumonia, and fibrinous airsacculitis were observed. By 10 DPC, the fibrinous inflammation inceased, and only congestion in the trachea, lungs, and sinuses with thickening of air sacs were observed. Histopathology revealed lymphoplasmacytic tracheitis and congestive pneumonia. Scoring of lesions generally revealed more severe lesions at 5 DPC. Statistical analysis of both macroscopic and microscopic scores showed no significant differences between groups in both necropsies. In conclusion, vaccine B has significantly better seroconversion, better growth performance parameters, and a relatively early stop of viral shedding compared to other vaccines.