Comparative pathogenesis of aerosol-induced colibacillosis reveals greater susceptibility in broiler chicks compared to layer chicks, with systemic clearance of bacteria but persistence in the bursa of Fabricius

Avian colibacillosis presents significant challenges to the poultry industry by adversely affecting bird health and productivity and raising global public health concerns. While numerous studies on colibacillosis in broiler and layer chickens have been reported, a comparative investigation on the disease progression following respiratory infection in these two chicken types under identical conditions is lacking. This study aimed to compare the clinical, pathological, and microbiological features in broilers and layers experimentally infected with ilux2-marked E. coli PA14/17480/5-ovary strain (ilux2-APEC). Sixty-four 2-week-old specific pathogen-free chicks (32 broilers, 32 layers) were divided into four groups (n = 16), with control and infected groups for each type. Birds were necropsied at 3, 7, 10, and 14 days post-infection (dpi). In infected groups, spleen-to-body weight ratios were significantly higher compared to their respective controls. Mean maximum clinical scores, macroscopic, and microscopic lesions were significantly higher in infected broilers than in infected layers. Infected broilers showed a higher incidence of ilux2-APEC re-isolation from respiratory tissues compared to infected layers. Additionally, ilux2-APEC was re-isolated from blood, femoral head, heart, liver, and spleen exclusively in infected broilers. Unexpectedly, bioluminescence imaging identified bacterial colonization in the bursa of Fabricius in both infected groups, persisting up to 14 dpi. Immunohistochemistry detected Escherichia coli in trachea, lung, air sac, heart, liver spleen, and bursa of Fabricius, with higher detection rates in infected broilers. These findings highlight the increased susceptibility of broilers to APEC infection, suggesting a need for targeted disease control measures and potential genetic selection for improved disease resistance in broilers. © 2025 The Author(s).