Evaluation of the efficacy of avian pathogenic Escherichia coli bivalent live vaccine (serogroups O 1 &2) against Escherichia coli infection in broilers.

Avian pathogenic Escherichia coli (APEC) leads to significant economic losses in poultry farming by causing high mortality rates, reduced productivity, and increased antibiotic use. Licensed vaccines against avian colibacillosis are available but with limitations, and the immunological basis of protection remains ill-defined. In this study, a bivalent live vaccine based on the APEC O1 and O2 serogroups was developed. The results of animal experiments showed that compared with the non-immunized group, the bivalent vaccine immunized group showed a significant decrease in lesion score and colibacillosis positivity rate (p < 0.05) after being challenged with APEC O1, O2, O78 serogroups. The mortality rate in vaccination/challenge group showed a decreasing trend without statistical significance compared with the challenge-only group. Immunological analysis confirmed that the vaccine induced a mucosal immune response mainly biased towards Th2, manifested by a significant increase in IgA antibody levels in tracheal lavage fluid and IL-6 mRNA expression in both spleen and lungs (p < 0.01). The IgA antibody levels (measured as the optical density value at 450 nm wavelength) of broilers vaccinated with the bivalent vaccine on the 28th and 29th days after vaccination were approximately 0.5 OD450 nm units higher than those of the unvaccinated group. At the same time, the proportion of CD4+TCR V β 1+cells in peripheral blood significantly increased in relation to the non-immunized group (p < 0.05). The safety assessment showed that the bivalent vaccine had no adverse effects on the growth performance and feed conversion rate of broiler chickens, did not cause pathological changes related to avian colibacillosis, and the vaccine strain survived in the body for less than 3 days without the ability of horizontal transmission. These findings provide valuable insights for the development of vaccines targeting avian colibacillosis.