Enterococcus cecorum in Broilers: Spondylitis, Vertebral Osteomyelitis, and Control
Introduction
Enterococcus cecorum is a Gram-positive, facultative anaerobic bacterium that is a member of the normal intestinal microbiota of poultry [1]. Over the past two decades, E. cecorum has emerged as a significant cause of lameness and mortality in broiler chickens worldwide, primarily through its ability to cause enterococcal spondylitis (ES) and vertebral osteomyelitis [2, 1]. The disease, colloquially termed "kinky-back," is characterized by inflammation and necrosis of the free thoracic vertebra (FTV), leading to spinal cord compression, posterior paresis, and paralysis [3, 4]. The economic impact on the broiler industry is substantial, resulting from increased mortality, culling of lame birds, reduced growth performance, and condemnation at processing [5, 6]. This review provides an exhaustive examination of the etiology, pathogenesis, clinical presentation, diagnosis, antimicrobial resistance, and control measures for E. cecorum infections in broilers, with a focus on spondylitis and vertebral osteomyelitis.
Etiology and Taxonomy
Enterococcus cecorum belongs to the genus Enterococcus within the family Enterococcaceae [1]. It is a non-spore-forming, catalase-negative coccus that typically occurs in pairs or short chains [1]. The organism was originally classified as Streptococcus cecorum but was later reclassified based on 16S rRNA gene sequencing and DNA-DNA hybridization studies [1]. E. cecorum is a commensal inhabitant of the gastrointestinal tract of chickens, but specific pathogenic lineages have emerged that possess the ability to translocate across the intestinal epithelium and cause systemic disease [7, 3]. Comparative genomic analyses have revealed that clinical isolates from broilers with ES form distinct clonal complexes, suggesting the emergence of highly pathogenic strains with enhanced virulence potential [8, 7]. Intercontinental spread of a specific lineage of clinical poultry isolates has been documented, indicating that these pathogenic clones are not geographically restricted [7].
Pathogenesis and Virulence Factors
The pathogenesis of enterococcal spondylitis involves a multistep process beginning with intestinal colonization, followed by translocation across the gut barrier, bacteremia, and finally localization to the FTV [3, 9]. The FTV is a unique anatomical structure in birds that lacks a notochordal remnant, making it a site of predilection for bacterial embolization [3]. Experimental reproduction of ES has been achieved through both oral and aerosol exposure routes, confirming that the natural route of infection is likely fecal-oral or respiratory [10, 9].
Intestinal Translocation and Bacteremia
Pathogenic E. cecorum strains must first colonize the intestinal tract and then translocate across the intestinal epithelium to enter the bloodstream [3]. Studies using chicken intestinal organoids have demonstrated that lesion-derived isolates invade organoids more efficiently than commensal cloacal isolates in a dose-dependent manner [9]. This enhanced invasive capacity is a key virulence trait distinguishing pathogenic from non-pathogenic strains [9]. Once in the bloodstream, the bacteria disseminate systemically and can localize to the FTV [3].
Colonization of the Free Thoracic Vertebra
The FTV is the primary target site for E. cecorum in broilers [3, 4]. The pathogenesis of vertebral osteomyelitis involves bacterial adhesion to the vertebral endplates, followed by inflammation, necrosis, and the formation of a caseous exudate [3]. Histologically, lesions are characterized by heterophilic and mononuclear inflammatory cell infiltration, osteolysis, and periosteal new bone formation [3, 6]. The expanding inflammatory mass compresses the spinal cord, leading to the characteristic clinical signs of posterior paresis and paralysis [3].
Role of Coinfections
The role of coinfections in potentiating E. cecorum disease has been investigated. Contrary to earlier hypotheses, coinfection with Eimeria species (coccidia) has been shown to significantly decrease the prevalence of E. cecorum bacteremia and FTV lesions in experimental models [11]. This protective effect may be due to increased intestinal epithelial turnover or enhanced immune surveillance of the intestine induced by coccidial infection [11]. Coinfections with other avian pathogens, including infectious bronchitis virus, Newcastle disease virus, reovirus, Mycoplasma synoviae, and chicken anemia virus, did not significantly increase lesion incidence in experimental settings, although some combinations resulted in higher reisolation rates of E. cecorum [9].
Virulence-Associated Traits
Lesion-derived E. cecorum isolates exhibit reduced sensitivity to albumen and increased resistance to lysozyme compared to commensal cloacal isolates [9]. Lysozyme resistance is a potential screening method for virulence, as it reflects the bacterium's ability to survive within the host environment [9]. The chicken embryo lethality assay (ELA) has been optimized to distinguish virulent from non-virulent strains, with albumen inoculation being the optimal route for discriminating between lesion and cloacal isolates based on embryo mortality [12, 9].
Clinical Signs and Gross Pathology
Clinical Presentation
Enterococcal spondylitis typically affects broiler chickens between 5 and 7 weeks of age, although broiler breeder roosters aged 15 to 18 weeks can also be affected [10, 6]. The initial clinical sign is a subtle lameness that progresses to a characteristic hock-sitting posture, where the bird sits on its hocks with its legs extended forward [5, 4]. As the disease advances, birds develop posterior paresis and eventually complete paralysis of the legs [3, 4]. Affected birds are often alert but unable to access feed and water, leading to dehydration and emaciation [5]. Mortality can be significant, with flock-level mortality rates ranging from 2% to 15% in severe outbreaks [5, 4].
Gross Lesions
Postmortem examination reveals a characteristic nodular mass or abscess at the level of the FTV, which is located immediately anterior to the kidneys [3, 6]. The lesion is typically a firm, caseous, yellowish mass that may extend into the surrounding soft tissues and cause compression of the spinal cord [3, 4]. In chronic cases, the vertebral body may be completely destroyed, leading to pathologic fracture and kyphosis [3]. Other lesions that may be observed include pericarditis, perihepatitis, and femoral head necrosis, although these are less consistently present [1, 5].
Histopathology
Histologic examination of the FTV reveals osteomyelitis with extensive necrosis, heterophilic and mononuclear inflammatory cell infiltration, and edema in the surrounding spinal cord [3, 6]. Gram-positive cocci can often be visualized within the lesion using Gram stain [3]. The inflammatory process may extend into the adjacent intervertebral discs and paravertebral soft tissues [3].
Diagnosis
Clinical and Gross Diagnosis
A presumptive diagnosis of enterococcal spondylitis can be made based on the characteristic clinical signs (hock-sitting posture, posterior paresis) and gross lesions (caseous nodule at the FTV) [5, 4]. However, definitive diagnosis requires laboratory confirmation.
Bacteriologic Culture
E. cecorum can be isolated from the FTV lesion, spleen, liver, or bone marrow using standard bacteriologic methods [3, 5]. Samples are plated onto blood agar or selective enterococcal media (e.g., bile esculin azide agar) and incubated at 37 degrees Celsius under aerobic or microaerophilic conditions [1]. Colonies are typically small, gray, and alpha-hemolytic [1]. Identification is confirmed by Gram stain, catalase negativity, and biochemical profiling using commercial identification systems or matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) [1, 13].
Molecular Detection
Quantitative real-time PCR (qPCR) assays have been developed for the specific detection and quantification of E. cecorum in clinical samples and environmental specimens [14]. These assays target species-specific genes, such as the superoxide dismutase gene (sodA) or the 16S rRNA gene, and can detect the organism at low levels [14]. Molecular typing methods, including pulsed-field gel electrophoresis (PFGE), multilocus sequence typing (MLST), and whole-genome sequencing (WGS), are used for epidemiological investigations and to track the spread of pathogenic clones [8, 7, 13].
Serology
Commercial ELISA kits for the detection of antibodies against E. cecorum are not widely available. Research has shown that vaccination of breeder hens with a polyvalent killed vaccine does not protect offspring from ES, suggesting that maternal antibody transfer may not be sufficient for protection [15].
Antimicrobial Resistance
Resistance Profiles
Antimicrobial resistance (AMR) in E. cecorum is a growing concern, particularly given the reliance on antimicrobial therapy for disease control [16, 17]. Resistance to tetracyclines, macrolides, and aminoglycosides is common among clinical isolates [17, 1]. More alarmingly, resistance to linezolid, a critically important antimicrobial for human medicine, has been reported in E. cecorum clade F isolates from commercial broilers in China, with the optrA and fexA resistance genes identified [16].
Epidemiological Cutoff Values
Epidemiological cutoff values (ECOFFs) for antimicrobial susceptibility testing of E. cecorum have been established to differentiate wild-type from non-wild-type populations [17, 18]. These ECOFFs are essential for monitoring the emergence and spread of AMR in poultry production systems [17, 18].
Mechanisms of Resistance
Resistance in E. cecorum is mediated by a variety of genetic determinants, including efflux pumps, target site modifications, and enzymatic inactivation [16, 1]. The optrA gene confers resistance to oxazolidinones (e.g., linezolid) and phenicols, while the fexA gene confers resistance to florfenicol [16]. The presence of these genes on mobile genetic elements facilitates their horizontal transfer among enterococcal populations [16].
Control and Prevention
Biosecurity
Strict biosecurity measures are essential for preventing the introduction and spread of pathogenic E. cecorum strains [1, 4]. These measures include all-in/all-out production, thorough cleaning and disinfection between flocks, and control of personnel and equipment movement [1]. E. cecorum has been shown to persist in the environment for extended periods, with tenacity studies demonstrating survival on various surfaces for weeks [19].
Antimicrobial Therapy
Antimicrobial therapy is often used to treat clinical outbreaks, but its efficacy is variable and complicated by AMR [1, 5]. Treatment should be guided by antimicrobial susceptibility testing of isolates from the affected flock [17, 1]. The use of metaphylactic antimicrobials in feed or water has been associated with alterations in the cecal microbiota and may select for resistant strains [20].
Probiotics and Feed Additives
Probiotic prophylaxis has been investigated as a strategy to reduce E. cecorum prevalence in the FTV [21]. The administration of specific probiotic strains may competitively exclude pathogenic E. cecorum or modulate the intestinal microbiota to reduce translocation risk [21, 20]. Feed additives, including organic acids and essential oils, have also been evaluated for their ability to control E. cecorum, but their efficacy is inconsistent [20].
Vaccination
Vaccination strategies for E. cecorum are still in the research phase. A killed polyvalent vaccine administered to breeder hens did not protect offspring from ES, indicating that alternative vaccine platforms or delivery strategies are needed [15]. Autogenous vaccines prepared from farm-specific isolates have been used empirically, but controlled efficacy data are lacking [1].
Management Practices
Management practices that reduce stress and improve gut health may help reduce the incidence of ES [1, 9]. These include optimizing stocking density, ventilation, litter quality, and nutrition [1]. Avoiding factors that compromise intestinal barrier function, such as mycotoxins or poor feed quality, is also important [1].
Diagnostic and Control Decision Tree
The following Mermaid diagram outlines a decision tree for the diagnosis and control of enterococcal spondylitis in broiler flocks.
flowchart TD
A[Broiler flock with lameness and hock-sitting posture] --> B{Clinical examination and necropsy}
B --> C[Characteristic FTV lesion present?]
C -->|Yes| D[Collect FTV lesion, spleen, and bone marrow samples]
C -->|No| E[Consider other causes of lameness]
D --> F[Bacteriologic culture and MALDI-TOF MS or PCR identification]
F --> G{E. cecorum confirmed?}
G -->|Yes| H[Perform antimicrobial susceptibility testing]
G -->|No| I[Re-evaluate differential diagnoses]
H --> J{Resistance profile determined}
J --> K[Select targeted antimicrobial therapy if indicated]
K --> L[Implement biosecurity measures and management changes]
L --> M[Consider probiotic or feed additive interventions]
M --> N[Monitor flock for recurrence and adjust control plan]
Conclusion
Enterococcus cecorum is a significant emerging pathogen in broiler production, causing enterococcal spondylitis and vertebral osteomyelitis that result in substantial economic losses and welfare concerns [2, 1]. The pathogenesis involves intestinal colonization, translocation, bacteremia, and localization to the FTV [3, 9]. Diagnosis relies on clinical signs, gross pathology, bacteriologic culture, and molecular methods [14, 5]. Antimicrobial resistance, including resistance to linezolid, is an increasing problem that complicates treatment [16, 17]. Control requires an integrated approach combining biosecurity, antimicrobial stewardship, probiotics, and management practices that support gut health [21, 20, 1]. Further research is needed to develop effective vaccines and to understand the virulence mechanisms that distinguish pathogenic from commensal E. cecorum strains [7, 15, 9].
References
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