Research Note: Effects of Escherichia coli co-infection on the protective efficacy assessment of two common infectious bronchitis vaccines.

Avian infectious bronchitis (IB), a highly contagious disease hazardous to the poultry industry, is caused by an etiological agent called the infectious bronchitis virus (IBV). Some IBV strains (IBVs) alone usually do not cause high mortality in field conditions if not with secondary pathogens including Escherichia coli (E. coli). Herein, we established an IBV and E. coli co-infection model to evaluate the protective efficacy of two IBV vaccine strains against a new emerging genotype GVI-1 with mild virulence in experimental conditions. Chickens were inoculated with IBV field isolate ZQX (genotype GVI-1) and challenged 4 dlater with the E. coli strain MS160427 (serotype O8). Subsequently, these chickens were euthanized at seven days postchallenge (d.p.c.) with E. coli. An autopsy revealed that lesions in the IBV plus E. coli co-infection group were more severe than those in the IBV-infected group. This pathological model was used to assess the protective effect of two commonly used vaccine strains (H120 and 4/91) against the IBV ZQX strain, and a significantly better protective efficacy was observed for 4/91 compared with H120. Thus, IBV and E. coli co-infection could be employed in assessing the protective efficacy of IBV vaccines.