Investigating the pathogenicity of novel non-pathogenic Avibacterium paragallinarum isolates and their protective potential against infectious coryza.

Infectious coryza (IC) is a chicken respiratory disease caused by Avibacterium paragallinarum (AP). Recently, multiple naïve-healthy layer (NHL) flocks tested positive for AP by both qPCR and bacterial isolation without clinical signs. Therefore, acquired AP isolates from these NHL flocks were dubbed non-pathogenic AP (npAP). The aim of this study was to investigate the pathogenicity of npAP isolates (Phase I) and their ability to confer protection against IC (Phase II). Seven groups (G1-G7) of 26-week-old laying hens were challenged via intra-sinus injection with seven npAP isolates obtained from five sites out of four states. Positive control group was challenged with Midwestern pathogenic AP (pAP) serovar C1 isolate, while the negative control received saline. No significant differences (P > 0.05) in mean clinical, infraorbital sinus gross and histopathological lesion scores were observed between G1-G7 and the negative control, supporting that these isolates are non-pathogenic, whereas the positive control had significantly higher scores (P < 0.05). Four weeks later (Phase II), G1-G7, as well as the original positive control group, were re-challenged with the same C1 isolate to assess protection. G1-G7 developed clinical signs, infraorbital sinus gross and histopathological lesions comparable to the new positive control. In conclusion, the study results match field observations and confirm the novel npAP isolates are non-pathogenic to chickens. Additionally, npAP isolates do not confer protection against circulating Midwestern pAP serovar C1. Given their prevalence in NHL flocks in the US, npAP should be ruled out before confirming IC diagnosis in flocks without clinical signs.