Enterococcus cecorum in Broilers: Spondylitis, Vertebral Osteomyelitis, and Control

Introduction

Enterococcus cecorum is a Gram positive, facultative anaerobic coccus that has emerged as a significant cause of lameness and mortality in commercial broiler flocks. Unlike other enterococcal species commonly considered commensals of the poultry gastrointestinal tract, E. cecorum possesses pathogenic potential linked to specific clonal lineages that produce vertebral osteomyelitis and spondylitis. These lesions typically develop at the free thoracic vertebra (T4) and adjacent intervertebral spaces, leading to spinal cord compression, paresis, and paralysis. The economic impact arises from culling, reduced slaughter weight, and increased labor for removal of affected birds.

Etiology and Taxonomy

Enterococcus cecorum belongs to the family Enterococcaceae, order Lactobacillales. The organism is a Lancefield group D streptococcus-like coccus, catalase negative, and able to grow in 6.5% sodium chloride and at 45 degrees Celsius. Its natural habitat is the intestinal tract of chickens and other birds. However, pathogenic isolates often share a common genetic background (sequence type ST82 or related STs) that is strongly associated with invasive disease. The distinction between commensal and pathogenic E. cecorum is based on the presence of virulence factors including gelatinase, enterococcal surface protein (Esp), and cytolysin, though exact molecular correlates of pathogenicity remain under investigation.

Epidemiology

Enterococcus cecorum is transmitted horizontally through the fecal-oral route. The organism is shed in high numbers from clinically affected birds and from asymptomatic carriers. Vertical transmission via the egg is considered unlikely but has not been entirely excluded. Broiler flocks between 2 and 6 weeks of age are most commonly affected. Incidence can reach 5% to 15% of birds within a house, with mortality typically lower than morbidity. Risk factors include high stocking density, poor litter quality, concurrent immunosuppressive conditions (e.g., infectious bursal disease), and feed-related factors such as high calcium or phosphorus imbalances that may alter bone metabolism.

Pathogenesis

The pathogenesis of E. cecorum vertebral osteomyelitis begins with colonization of the intestinal epithelium, followed by translocation across the gut barrier. Bacteremia occurs and organisms lodge in the metaphyseal capillaries of the vertebrae, particularly at the free thoracic vertebra (T4). The low oxygen tension and high nutrient availability in the growth plate region favor bacterial proliferation. Inflammatory mediators attract heterophils and macrophages, leading to necrosis, abscess formation, and osteolysis. Extension into the spinal canal causes compression of the synsacral spinal cord and nerve roots, resulting in unilateral or bilateral leg paresis.

Host Cell Interactions

Enterococcus cecorum adheres to chicken intestinal epithelial cells via surface adhesins. Once in the bloodstream, the bacteria interact with endothelial cells of the vertebral sinusoids. The polysaccharide capsule and biofilm formation protect the organism from phagocytosis. The production of gelatinase and serine protease degrades host connective tissue, facilitating spread along bone trabeculae. The host response includes a robust acute phase reaction, with increased heterophil counts and fibrin deposition.

Clinical Signs

Clinical signs in broilers typically appear between 28 and 42 days of age. Affected birds show:

• Unilateral or bilateral lameness, progressing to inability to stand.
• Sitting on hocks with legs extended (penguin-like posture).
• Reluctance to move toward feeders and drinkers, leading to dehydration and weight loss.
• In severe cases, complete paralysis of the legs with normal wing and head movement.
• Palpable swelling over the caudal thoracic and synsacral region.

Mortality is low, but morbidity leads to culling. Secondary septicemia can develop with lesions in the liver, spleen, and heart valves (endocarditis).

Pathology

Gross Lesions

On necropsy, the hallmark lesion is a focal to locally extensive osteomyelitis and spondylitis at the free thoracic vertebra (T4). The vertebral body shows enlargement, deformation, and a caseous or purulent exudate within the bone marrow cavity. Transverse sectioning reveals necrosis and abscess formation measuring 2 to 10 mm in diameter. The adjacent intervertebral disc may be narrowed or destroyed. In chronic cases, periosteal new bone formation and ankylosis of adjacent vertebrae occur. Spinal cord compression is visible as flattening of the cord at the lesion site.

Other lesions include:

• Hepatomegaly with scattered white foci.
• Splenomegaly.
• Endocarditis on the mitral and aortic valves (less common).
• Osteomyelitis of the femoral head and tibiotarsal bone in some cases.

Histopathology

Histological examination shows central liquefactive necrosis with clusters of Gram positive cocci, surrounded by a zone of degenerate heterophils and macrophages. Osteoclasts are activated, leading to bone resorption. Fibroplasia and osteoid deposition occur in chronic lesions. The spinal cord shows axonal swelling, Wallerian degeneration, and gliosis due to compression.

Diagnosis

Diagnosis of Enterococcus cecorum infection is based on clinical signs, postmortem examination, and laboratory confirmation.

Culture

Samples for culture include vertebral lesions, synovial fluid, and liver or spleen. Swabs or tissue samples are inoculated onto blood agar or selective Enterococcus agar (e.g., bile esculin azide agar) and incubated at 37 degrees Celsius for 24 to 48 hours. Colonies are small, alpha hemolytic or non-hemolytic, and appear as Gram positive cocci in pairs or short chains. Biochemical identification is performed using commercial strip systems (e.g., API 20 Strep) which confirm E. cecorum based on esculin hydrolysis, L-arabinose fermentation, and lack of acid from sorbitol.

Molecular Diagnostics

PCR-based methods are preferred for rapid and specific detection. Real-time PCR targeting the superoxide dismutase gene (sodA) or the 16S ribosomal RNA gene can distinguish E. cecorum from other enterococci. High-resolution melt analysis and sequencing provide clonal typing when needed.

Serology

No commercial serological tests are widely available. Research ELISA methods using whole-cell antigens have been described but are not standardized or validated for routine use.

Differential Diagnosis

The differential diagnosis for lameness and vertebral lesions in broilers includes:

• Staphylococcus aureus bacterial osteomyelitis and bumblefoot (see article Staphylococcus aureus Bumblefoot and Osteomyelitis in Broilers).
• Mycoplasma synoviae infectious synovitis (see Mycoplasma synoviae).
• Escherichia coli colibacillosis and articular involvement (see Escherichia coli in Chickens).
• Gallibacterium anatis salpingitis and systemic infection (see Gallibacterium anatis).
• Ornithobacterium rhinotracheale respiratory disease and secondary arthritis.
• Clostridium perfringens necrotic enteritis (see Necrotic Enteritis in Broiler Chickens).
• Non-infectious causes: tibial dyschondroplasia, rickets, vitamin E/selenium deficiency, and slipped tendon (perosis).

The following Mermaid diagram outlines a diagnostic decision tree for lameness with vertebral focus.

flowchart TD
    A[Lameness in broilers 4-6 weeks], > B[Physical exam: vertebral swelling?]
    B, >|Yes| C[Necropsy: cut free thoracic vertebra]
    C, > D[Caseous/purulent lesion?]
    D, >|Yes| E[Gram stain: Gram positive cocci]
    D, >|No| F[Consider other causes]
    E, > G[Culture on blood agar and Enterococcus agar]
    G, > H[Colonies: alpha/non-hemolytic, esculin positive]
    H, > I[Biochemical identification: E. cecorum]
    I, > J[PCR sodA for confirmation]
    J, > K[Diagnosis confirmed: E. cecorum spondylitis]
    F, > L[Staphylococcus aureus, Mycoplasma synoviae, E. coli, trauma]

Table 1 summarizes key clinical and pathological features.

Treatment

Antimicrobial therapy is difficult in broiler flocks due to limited approved drugs and the challenge of delivering effective concentrations to bone lesions. The choice of agent should be guided by culture and susceptibility testing. E. cecorum isolates often show resistance to tetracyclines and macrolides. Susceptibility to amoxicillin, penicillin, and enrofloxacin (where licensed) is variable but sometimes present. However, antimicrobial treatment after clinical signs appear rarely reverses paralysis and is not cost effective. Medicated feed or water application at the first signs of lameness in a group may reduce new case incidence, but evidence for efficacy is weak.

Control and Prevention

Control strategies focus on reducing environmental bacterial load, improving host resistance, and managing flock husbandry.

Biosecurity and Hygiene

• All-in/all-out management with thorough cleaning and disinfection between flocks.
• Remove affected and culled birds promptly to reduce shedding.
• Maintain dry litter through adequate ventilation and drinker management.
• Reduce stocking density to limits recommended for good welfare.

Nutritional Strategies

• Optimize calcium and phosphorus levels in the finisher feed to avoid excessive bone mineralization that may predispose to metaphysical infection.
• Supplementation with organic acids (e.g., formic acid, propionic acid) in water or feed may reduce enterococcal load in the gut.
• Probiotics containing Lactobacillus or Bacillus spp. have been investigated for competitive exclusion of E. cecorum, though consistent results are lacking.

Vaccination

No commercial vaccine is currently available for E. cecorum. Autogenous bacterins prepared from field isolates have been used in some integrated operations, but published efficacy data are limited. Research into subunit vaccines based on Esp or other surface proteins is ongoing.

Genetic Selection

Some broiler lines appear to have lower susceptibility to bacterial osteomyelitis. Breeding programs that select for improved skeletal integrity and immune response may reduce incidence over time.

Public Health Considerations

Enterococcus cecorum is not considered a zoonotic pathogen. It is rarely isolated from humans and does not pose a food safety risk comparable to other enterococci such as E. faecalis or E. faecium. However, the presence of antimicrobial resistance genes in poultry isolates is relevant to the broader One Health context of resistance dissemination.

Conclusion

Enterococcus cecorum is an emergent pathogen in broiler production, causing vertebral osteomyelitis and spondylitis that results in significant welfare and economic loss. Diagnosis relies on careful postmortem examination, culture, and molecular confirmation. Control is based on improved biosecurity, litter management, and prudent antimicrobial use. Further research is needed to define virulence mechanisms and develop effective prevention strategies including vaccines.

References

[1] Barnes HJ, Vaillancourt JP, Gross WB. Enterococcal infections. In: Diseases of Poultry. 14th ed. Wiley-Blackwell; 2020.

[2] Kense MJ, Landman WJ. Enterococcus cecorum infections in broiler chickens: a review. Avian Pathology. 2011;40(4):341-350.

[3] Merck Veterinary Manual. Enterococcal Infections in Poultry. Kenilworth, NJ: Merck & Co.; 2023.

[4] Jung A, Rautenschlein S. Virulence factors of Enterococcus cecorum strains isolated from chickens with vertebral osteomyelitis. Veterinary Microbiology. 2014;168(2-4):389-395.