Unveiling the differential progression of aerosol-induced colibacillosis in broiler and layer chicks, with the persistence of bacteria in the bursa of Fabricius

@enInfection with avian pathogenic Escherichia coli (APEC) causes colibacillosis, which is one of the major reasons for high morbidity and mortality in chickens, resulting in serious economic losses and public health concerns. This study aimed to compare the progression of experimental APEC respiratory infection in broiler and layer chicks under comparable condition, addressing a gap in the existing knowledge. Sixty-four 2-week-old specified pathogen-free chicks (32 of each type) were divided into four groups, with infected and control groups for each bird type. The infected groups were aerosolized with a bioluminescent strain, ilux2-E. coli PA14/17480/5/ovary (ilux2-APEC), while the control groups received phosphate-buffered saline. Birds were monitored daily for clinical signs and necropsies performed at 3, 7, 10, and 14 days post infection (dpi). Results showed that infected broilers exhibited significantly higher mean clinical signs scores, with two birds euthanized due to neurological symptoms. Similarly, the mean pathological lesions characterized by marked macroscopic polyserositis and microscopic fibrinoheterophilic exudates primarily in lung and air sacs. In addition, the ilux2-APEC re-isolation rates from respiratory tissues were significantly higher in infected broilers compared to infected layers. Furthermore, the ilux2-APEC was re-isolated from the blood, femoral head, heart, liver, and spleen exclusively in infected broilers. The bioluminescence imaging identified bacterial colonization in the bursa of Fabricius persisting up to 14 dpi in both bird types. Immunohistochemistry detected Escherichia coli in the trachea, lung, air sacs, heart, liver, spleen, and bursa of Fabricius, with higher detection rates in infected broilers. In conclusion, our findings showed that broiler chicks are more susceptible to aerosol APEC infection than layer chicks, emphasizing the importance of effective management and targeted infection control measures in broiler production. Additionally, unexpected yet persistent bacterial colonization in the bursa suggests it might be a target organ, although its exact role in pathogenesis requires further clarification.