Epidemiological Investigation of Infectious Coryza in Central China and the Effect of Enterococcus faecium on Improving Vaccine Immunity.

Infectious coryza is a major respiratory disease of chickens induced by Avibacterium paragallinarum. Understanding the prevalent serovars of the bacterium is fundamental for effective prevention and control of the disease. Moreover, improving the immune efficacy of the infectious coryza vaccine for laying hens under modern production systems is critical for disease management. Enterococcus faecium is a probiotic bacterium. However, the impact of E. faecium on infectious coryza vaccine efficacy has not been investigated. In this study, we isolated and identified Av. paragallinarum strains from chickens with suspected infectious coryza in farms in central China between 2019 and 2022. Serotyping was performed based on the haemagglutinin HMTp210 gene sequence. Seventy-two strains were isolated, including 19 strains of serovar A-1, 44 strains of serovar C-4, and six strains of serovar B. Three strains were non-serotypable. Av. paragallinarum loads in palatine clefts of the flocks were detected using qPCR targeting the recN gene which revealed increasing loads with age that stabilized at 31 weeks of age (5.59 × 10³ CFU/g). Healthy Hy-Line Grey laying hens were immunized with the infectious coryza vaccine at six weeks of age, and E. faecium TC3 was added to feed at a concentration of 10 CFU/g. Blood samples were collected 28 days post-immunization to assess antibody titers, cytokine levels, and other immune parameters. Immunoprotective efficacy was evaluated by challenge with serovar A-1 and C-4 isolates. Addition of E. faecium significantly increased infectious coryza vaccine antibody levels and enhanced protective efficacy against A-1 and C-4 strains by 6.72 % and 7.07 %, respectively. Furthermore, morbidity rates decreased by 33.53 % and 24.77 % in A-1 and C-4 strains, respectively. These findings indicate that E. faecium improves infectious coryza vaccine efficacy and strengthens the health of chicken flocks.