Abstract:

In order to study the immune effect of attenuated strain of subtype B avian metapneumovirus (LN16-A strain) on commercial broilers, 3-week-old commercial broilers were selected from large-scale farms and immunized with attenuated strain of subtype B avian metapneumovirus (LN16-A strain) by dropping nose (200 μL, virus content≥10³ TCID₅₀/piece), and the blank control group was given the same dose of PBS. Twenty-one days after immunization, blood serum was collected under the wings and corresponding antibodies were detected using ELISA and neutralization tests. At 6-week-old, the B subtype avian metapneumonia virus (LN16-V strain) was administered via nasal drip (200 μL, 5000 TCID₅₀ per chicken) to chickens for virus challenge. After the challenge, the virus copy number of nasal swabs from each group of chickens was continuously observed for 7 days. RT-qPCR was used to detect the virus copy number. On the 9th day after the challenge, 2 chickens from each group were randomly selected for dissection. Nasal turbinate, tracheal, and lung tissues were collected, and pathological sections were made using HE staining to observe the histopathological changes of the tissues. The results of antibody showed that 21 days after immunization, the positive rate of ELISA antibody and neutralizing antibody in the immunization group was 100%, with an average ELISA antibody titer of 1 860 and an average neutralizing antibody titer of 7.44 log2. The results of clinical symptoms showed that the commercial broilers in the control group had obvious clinical symptoms, such as cloudy, viscous, and filamentous nasal juice, and the incidence rate was 100%. While the broilers in the immunization group had no clinical symptoms. In addition, the results of RT-qPCR showed that the virus copy number of nasal swab in the immunization group decreased by 78% to 96%, compared to the control group. The results of pathological sections showed that the turbinate bone, trachea and lung of broilers in the control group had different degrees of pathological damage, while no obvious pathological changes were observed in the organs of broilers in the immunization group. The above results indicate that the attenuated strain of subtype B avian metapneumovirus (LN16-A strain) could induce the production of specific antibodies in commercial broilers, which could significantly reduce the virus shedding level of broilers after challenge, and had a good protective effect on commercial broilers. This study may provide important theoretical guidance and technical support for the prevention and control of avian metapneumovirus disease in large-scale broiler farms.

Key words: subtype B avian metapneumovirus, attenuated strain, commercial broilers, immune effect