Virulent strains of Enterococcus cecorum are known to escape the gastrointestinal tract of broilers and cause septicemia followed by spinal lesions (enterococcal spondylitis), which result in paralysis and mortality late in the production cycle. In contrast to this traditional clinical presentation, there are increasing reports of spikes in 2-week mortality due to E. cecorum pericarditis, said Grayson Walker, at the 2023 International Poultry Scientific Forum.

Walker, a graduate student, and colleagues at North Carolina State University College of Veterinary Medicine, investigated the potential causes of this change in the clinical disease course of E. cecorum. The genetic relationships among 54 E. cecorum isolates from spinal lesions (SA), pericarditis lesions (PC) and hatchery residue were obtained from multiple locations in the southeastern United States and genotyped using pulsed-field gel electrophoresis, he said.

Band-based cluster analysis with an 80% identity threshold revealed 3 distinct groups that were predominantly associated with spinal or pericarditis lesion types or hatchery residue. A subset of 36 isolates representing each group was selected for whole-genome sequencing and SNP-based phylogenetic analysis, he explained.

Walker noted that pericarditis lesion strains were genetically dissimilar to spinal lesion strains and clustered with hatchery residue isolates in the phylogenetic analysis. In an embryo lethality assay, 6 pericarditis-associated strains were moderately virulent (63-73% survival) relative to SA3, a well-characterized strain of known virulence isolated from a spinal lesion (50% survival), he added.

In an experimental bird infection model, 3 groups of 120, 1-day-old broiler chicks were randomly distributed among 6 replicate pens per group and orally inoculated with either pericarditis-associated strains or SA3. Spleen cultures were collected at 14 and 35 days to monitor septicemia, and pericardial fluid was cultured for all dead or euthanized broilers with gross pericarditis, Walker said.

Surprisingly, he noted, challenge with SA3 resulted in either similar or significantly greater (P ≤ 0.05) prevalence of sepsis and pericarditis than did challenge with pericarditis-associated strains.

These data indicate that strains of pathogenic E. cecorum associated with pericarditis and early mortality are genetically distinct from strains isolated from spinal lesions, yet predisposing factors and alterations in virulence mechanisms resulting in acute septicemia remain unclear, Walker concluded.