Establishment of a reproducible Enterococcus cecorum infection model under farm conditions

E. cecorum (EC) has emerged as a major pathogen in fast-growing broiler chickens, causing clinical signs that impact animal welfare, performance, economic outcomes and antibiotic use. Developing alternative control strategies requires a robust experimental infection model. This study aimed to select one EC strain, among five isolated in 2023 from Ross 308 broiler lesions, based on its ability to reproduce clinical signs under experimental farm conditions. An authorized experimental farm received 1,536 male Ross 308 chicks on day (D)0, randomly allocated into six groups (four replicates of 64 birds). Five groups were inoculated by intra-crop route with different EC strains (same dose and volume), and one group received saline solution. Infected and non-infected areas were physically separated, each with dedicated feed and water systems. Each group was separated by one-meter walkways and high biosecurity level was maintained. At D7, intestinal colonization was monitored by EC PCR on cloacal swabs. Clinical signs (lameness scored by 3-point gait scoring system, mortality, underweight) and performance were recorded at D21 and D31. Necropsies were performed on lame or underweight birds. EC isolates from lesions were compared with inoculated strains using MALDI-TOF. Two groups reproduced disease in a repeatable way with their inoculated strains. Infection dynamics differed: one strain caused earlier clinical signs (≈20% lameness at D21). This strain was selected for model establishment and reproducibility was confirmed in a second trial with the same protocols. For the 3 other groups, inconsistent colonization at D7 and mismatches between inoculated and re-isolated strains suggested aero diffusion and ability of strains to outcompete others in a defined environment. This EC infection model with the chosen strain reproduced what could be observed on the field in commercial farms. This tool could allow testing alternative solutions to prevent EC clinical signs and reduce the use of antibiotic treatment.