Cross Immunity of a Sonicated Trivalent Avian Colibacillosis Vaccine to Pathogenic Escherichia coli O26 and O78 Strains in Broiler Chickens

Colibacillosis outbreaks are a global issue affecting the poultry industry. There is no cross-immunity among the strains of APEC. If a vaccine induces cross-immunity, it will play a key role in preventing colibacillosis. Herein, a sonicated trivalent colibacillosis vaccine containing O78:K80, O2:K1, and O1:K1 serotypes, with Alum as an adjuvant, was used to assess cross-immunity against E. coli O26. Ninety-six broiler chickens were randomly assigned to four groups: Group A was vaccinated and exposed to O78; Group B was unvaccinated but exposed to O78; Group C was vaccinated and exposed to O26; Group D was unvaccinated but exposed to O26. At 14 days old, chickens in groups A and C received a single dose of the vaccine, while groups B and D received normal saline subcutaneously. At 35 days old, all groups were challenged with O78 and O26 as described above. Clinical signs and lesions, isolation of the bacterium, weight gain, food intake, FCR, and antibody titers against the O antigens of the vaccine strains and O26 were recorded. The results indicated that 2 weeks post-vaccination, titers to the O antigens of the vaccine strains were significantly higher in the vaccinated groups than in the unvaccinated groups (p ≤ 0.05). Following the challenge, no significant difference was observed in food intake and FCR between the groups (p > 0.05); however, the growth rate in group A was significantly higher than in group B (p ≤ 0.05). At 42-49 days old, the vaccinated groups had the highest growth rate, which was statistically significant compared to the unvaccinated groups; and FCR in group A was significantly better than in group B (p ≤ 0.05). In conclusion, it appears that in addition to homologous immunity, the vaccine also induces cross-immunity against O26.