Impact of embryonic exposure to Enterococcus cecorum strains isolated from field outbreaks on early performance and septicemia in broiler chickens

Recently, Enterococcus cecorum (EC) has been associated with septicemia and mortality in young broiler chickens. There is limited research investigating the pathogenicity of EC strains isolates obtained from affected birds in the field. The purpose of the present study was to evaluate the impact of in-ovo administration into the amnion of different EC isolates at day 18 of embryogenesis (DOE18). In a preliminary study, EC field isolates (n=7; EC1-EC7) were selected based on phenotypic characteristics and evaluated at different concentrations (1x10², 1x10⁴, and 1x10⁶ CFU/200uL/embryo) to assess the impact on early performance and macroscopic lesions in broiler chickens. Based on the results, isolates (n=3; EC2, EC5, EC7) were selected for further evaluation based on the significant (P< 0.05) reduction in d0-21 BWG and presence of lesions during necropsy at d14 and d21 compared to the non-challenged control (NC). An additional isolate previously associated with enterococcal spondylitis was also evaluated. Treatment groups included: 1) NC, 2) EC2, 3) EC5, 4) EC7, and 5) EC11B (n=90-120/embryos/group). Groups 2-5 received 1x10² CFU/200uL/embryo by in-ovo injection into the amnion at DOE18. Chicks were placed in battery cages and pen BW was recorded at d0, d7, d14, and d21 to determine average BW and BWG. At d14 and d21 posthatch, liver, spleen, free thoracic vertebrae (FTV), and femoral head (FH) were collected to enumerate Enterococcus spp. using Chromagar Orientation as the selective media. There was a significant (P< 0.05) reduction in BW at d21 and BWG from d14-21 and d0-21, for EC7 and EC11B isolates. Additionally, EC was recovered from the FTV of all challenged groups. The most representative lesions were pericarditis, heart blood vessels congestion, focal heart necrosis, FH osteomyelitis, and hydropericardium. However, lesions were not uniform across challenged groups or ages (d14 and d21). These results highlight the differences in pathogenicity and performance impacts of EC strains isolated from field cases. Further research investigating the effects of horizontal transmission of virulent EC during the hatching phase is underway.

Key Words: Enterococcus cecorum, Broiler, Performance, Lesions, Chicken.