Characterizing the impact of Enterococcus cecorum infection during late embryogenesis on disease progression, cecal microbiome composition, and early performance in broiler chickens

Abstract

Enterococcus cecorum (EC) has been associated with septicemia and early mortality in broiler chickens. There is limited research investigating the pathogenicity of EC field strains obtained from affected birds. The purpose of this study is to evaluate the impact of in-ovo administration into the amnion of different EC field isolates at day 18 of embryogenesis (DOE18). In Exp 1, seven EC field isolates alone or in combination (EC1-EC3, EC4-EC5, EC6, and EC7) were selected based on phenotypic characteristics and evaluated at different concentrations (1 × 102, 1 × 104, and 1 × 106 CFU/200uL/embryo) to assess the impact on early performance and macroscopic lesions. Three isolates (n=3; EC2, EC5, EC7) were selected for additional evaluation based on the significant (P<0.05) BWG reduction (d0-21) compared to the negative control (NC) and the presence of macroscopic lesions observed during posting sessions at d14 and d21. An additional isolate associated with enterococcal spondylitis was included in Exp 2 (EC11B). Treatment groups for Exp 2 include: 1) NC, 2) EC2, 3) EC5, 4) EC7, and 5) EC11B (n=90-120/embryos/group). Groups 2-5 were challenged at 1 × 102 CFU/200uL/embryo by in-ovo injection into the amnion at DOE18. Chicks were placed in battery cages for the duration of the study (21 days), and pen weights were recorded at d0, d7, d14, and d21 to calculate average BW and BWG. At d14 and d21 post-hatch, liver, spleen, free thoracic vertebrae (FTV), and femoral head (FH) were aseptically collected to enumerate Enterococcus spp. using Chromagar Orientation as the selective media. Cecal contents were collected at d21 to evaluate the effect of EC challenge on the cecal microbiome composition. There was a significant (P<0.05) reduction in BW at d21, and BWG from d14-21 and d0-21, for EC7 and EC11B. EC was recovered from the FTV of all challenged groups at d14 and d21. The most representative lesions were pericarditis, hydropericardium, focal heart necrosis, and FH osteomyelitis. However, lesions were not uniform across challenged groups or ages (d14 and d21). Alpha diversity of the cecal contents was markedly lower in EC5 and EC11B compared to all treatment groups suggesting that EC exposure during late embryogenesis affect the cecal microbiome up to 21 days post-hatch. Additionally, these results highlight the differences in pathogenicity and performance impacts of EC strains isolated from field cases and suggests that hatchery exposure to EC during late embryogenesis is a potential route of introduction into a flock.

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