What is Dr. Zubair Khalid's research focus?

Dr. Zubair Khalid specializes in molecular virology, mRNA vaccine development, and computational biology, with a focus on avian pathogens like IBDV and Avian Reovirus.

Where is Dr. Zubair Khalid currently working?

Dr. Zubair Khalid is a Postdoctoral Research Associate at the University of Maryland (UMD), specifically within the Department of Animal and Avian Sciences.

Staphylococcus aureus Bumblefoot and Osteomyelitis in Broilers: Etiology, Pathogenesis, and Integrated Control

Introduction

Bumblefoot (pododermatitis) and osteomyelitis are among the most economically significant infectious causes of lameness in commercial broiler production. Staphylococcus aureus is the primary bacterial agent associated with these conditions, although other staphylococci and coagulase-negative species are frequently isolated [1, 2, 3]. The disease complex, now widely referred to as bacterial chondronecrosis with osteomyelitis (BCO), involves a sequence of events starting from footpad lesions, hematogenous translocation, and eventual necrotic lesions in the proximal femur and tibia [4, 5]. This article provides a structured, evidence-based review of the etiology, diagnosis, pathogenesis, and control of S. aureus bumblefoot and osteomyelitis in broilers, drawing on the recent literature and experimental models.

Etiology and Virulence Factors

Staphylococcus aureus is a Gram-positive, coagulase-positive coccus that produces a wide array of virulence factors enabling colonization, invasion, and immune evasion. The organism expresses protein A, hemolysins, leukocidins, and multiple exotoxins. Of particular relevance to poultry osteomyelitis is the ability to form robust biofilms on bone surfaces and joint tissues [6, 5]. Biofilm formation is mediated by polysaccharide intercellular adhesin and biofilm-associated proteins, which confer resistance to phagocytosis and antimicrobial therapy.

Several studies have characterized the virulence gene repertoire of S. aureus isolates from broiler skeletal lesions. The most commonly detected genes include coa (coagulase), clfA and clfB (clumping factors), fnbA and fnbB (fibronectin-binding proteins), and icaA/icaD (biofilm synthesis) [6]. Methicillin-resistant S. aureus (MRSA) has been reported in poultry and is of concern due to potential zoonotic transmission, although the primary impact in broilers is on treatment efficacy [7]. Horizontal transfer of pathogenicity islands between S. aureus and Staphylococcus agnetis has been demonstrated, suggesting ongoing genomic plasticity that may broaden virulence potential in poultry [8].

Epidemiology and Host Susceptibility

The prevalence of S. aureus bumblefoot and osteomyelitis in broiler flocks varies widely, with reported flock-level incidence ranging from 5% to 30% depending on housing conditions, litter quality, and management practices [1, 4]. Young broilers are most susceptible during the rapid growth phase (weeks 3 to 6), as the femoral head and proximal tibia are regions of active endochondral ossification and metaphyseal vascular anastomosis, providing a niche for bacterial emboli [5]. Predisposing factors include wet litter, high stocking density, poor ventilation, and nutritional imbalances. Concurrent coccidiosis caused by Eimeria spp. can exacerbate gut barrier disruption and enhance systemic translocation of S. aureus [9]. Mycotoxin contamination of feed, particularly deoxynivalenol and fumonisins, has been shown to increase the incidence of BCO lameness in experimental aerosol challenge models [10]. The role of the cecal microbiome in modulating susceptibility is under active investigation; preliminary results indicate that dysbiosis precedes lesion development [11].

Clinical Signs and Pathology

Bumblefoot begins as hyperkeratosis and erythema on the plantar footpad, progressing to ulcerative pododermatitis with serous exudate and scab formation. In cases that progress to osteomyelitis, affected birds exhibit a characteristic lameness: they shift weight between legs, sit on their hocks, or show a stilted gait. BCO lesions are predominantly found in the proximal femur (femoral head necrosis) and the proximal tibiotarsus [4, 5]. Grossly, affected femoral heads appear misshapen with a thin or absent articular cartilage, and yellow to brown caseous necrotic material fills the medullary cavity. Histopathology reveals extensive chondronecrosis, fibrin deposition, heterophilic infiltration, and bacterial clumps [12, 13]. Systemic effects include elevated corticosterone levels, reduced growth rate, and compromised welfare [14].

The pathogenesis of BCO is now understood to involve a multi-step process: (1) colonization of abraded footpads with S. aureus, (2) entry into the bloodstream via the plantar venous plexus, (3) bacteremic seeding of the metaphyseal capillaries in fast-growing long bones, (4) ischemic necrosis due to bacterial thrombosis and occlusion of metaphyseal vessels, and (5) inflammatory bone resorption and osteolysis [5]. Experimental models using aerosolized S. aureus have confirmed that respiratory exposure can directly cause BCO without prior footpad lesions, indicating that hematogenous dissemination from any portal can initiate osteomyelitis [15].

Diagnostic Approaches

Clinical diagnosis relies on observation of lameness, palpation of swollen joints, and inspection of footpad lesions. Thermography has been evaluated as a screening tool for subclinical bumblefoot in poultry, identifying localized hyperthermia before visible lesions appear [16]. However, definitive diagnosis requires bacteriologic culture and molecular identification of S. aureus from synovial fluid, bone marrow, or footpad exudate. Selective media (mannitol salt agar) and coagulase testing are standard. Genotypic characterization can be performed using PCR for virulence genes and sequencing of the 16S rRNA or nuc genes. Whole-genome sequencing of outbreak isolates has provided insights into clonal dissemination and horizontal gene transfer [8, 17]. The embryo lethality assay is a useful tool for assessing isolate virulence, as highly pathogenic strains produce rapid embryo death [18]. Proteomic and metabolomic profiling of serum and bone tissue from affected birds has identified dysregulated autophagy pathways, altered bone metabolism markers, and cytokine signatures (e.g., elevated IL-6, IL-8, and chemokine levels) that might serve as biomarkers for early detection [12, 13, 19, 20].

Treatment and Antimicrobial Resistance

Treatment of established bumblefoot and osteomyelitis is challenging due to biofilm formation and poor antibiotic penetration into necrotic bone. Systemic antimicrobial therapy must be initiated early and guided by susceptibility testing. However, antimicrobial resistance in poultry-associated S. aureus is a growing problem. Methicillin resistance (MRSA) and multidrug resistance to tetracyclines, macrolides, and lincosamides have been documented in isolates from skeletal lesions [7, 21]. Coagulase-negative staphylococci (CoNS) from bone lesions also harbor resistance determinants that can be transferred to S. aureus [21]. The small therapeutic window and risk of resistance mean that prevention and control strategies are paramount.

Prevention and Control Strategies

Control of S. aureus bumblefoot and osteomyelitis requires an integrated approach targeting environmental management, nutritional optimization, and immunological enhancement. Key measures include:

Litter management: Keeping litter dry and friable reduces footpad abrasion and bacterial load. Frequent addition of fresh bedding and control of humidity are essential.
Biosecurity: Preventing introduction of S. aureus strains through contaminated equipment, personnel, or rodents.
Nutritional interventions: Supplementation with organic trace minerals (zinc, copper, manganese) has been shown to reduce BCO incidence in aerosol challenge models, possibly by improving bone integrity and immune function [22].
Probiotics: Spraying day-old chicks with Enterococcus faecium-based probiotics or administration of multi-strain probiotics throughout the rearing period can reduce the severity of BCO lesions [23, 24]. The mechanism may involve competitive exclusion, stimulation of mucosal immunity, and modulation of the cecal microbiome.
Vaccination: Inactivated vaccines using electron-beam-killed multi-strain S. aureus formulations have demonstrated efficacy in reducing lameness in experimental and field trials [25, 26]. A hybrid challenge model combining aerosol and contact transmission has been used to evaluate probiotic and vaccine efficacy [27].
Mycotoxin control: Reducing dietary levels of deoxynivalenol and fumonisins through grain screening and use of mycotoxin binders lowers BCO risk [10].

The flow of diagnostic and control decisions can be summarized as follows:

flowchart TD
    A[Clinical lameness in broilers], > B[Footpad inspection]
    B, > C{Ulcerative pododermatitis?}
    C, >|Yes| D[Swab footpad for culture & PCR]
    C, >|No| E[Palpate joints & proximal femur]
    E, > F{Swollen joint / femoral head necrosis?}
    F, >|Yes| G[Aspirate synovial fluid or bone marrow]
    F, >|No| H[Monitor flock, assess litter & management]
    D, > I[Bacterial isolation & identification]
    G, > I
    I, > J{S. aureus confirmed?}
    J, >|Yes| K[Perform antimicrobial susceptibility testing]
    K, > L[Consider targeted therapy if early]
    L, > M[Implement control measures: litter management, probiotics, vaccination]
    J, >|No| N[Identify other staphylococci or pathogens]
    N, > M
    M, > O[Reduce lameness incidence in subsequent flocks]

Conclusion

Staphylococcus aureus bumblefoot and osteomyelitis represent a persistent challenge in broiler production, driven by the pathogen's versatility, biofilm formation, and antibiotic resistance. The pathogenesis is rooted in the bird's rapid skeletal growth and vascular anatomy, which create a permissive environment for bacterial seeding. Current evidence supports a comprehensive control model that combines environmental management, nutritional support, probiotics, and vaccination. Continued surveillance of virulence determinants and resistance profiles, alongside ongoing research into host-pathogen interactions, will be essential to mitigate the welfare and economic impacts of this disease.

References

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[23] Perera R, Alharbi K, Hasan A, et al. Evaluating the Impact of the PoultryStar Bro Probiotic on the Incidence of Bacterial Chondronecrosis with Osteomyelitis Using the Aerosol Transmission Challenge Model. Microorganisms. 2024. https://pubmed.ncbi.nlm.nih.gov/39203472/

[24] Do ADT, Anthney A, Alharbi K, et al. Assessing the Impact of Spraying an Enterococcus faecium-Based Probiotic on Day-Old Broiler Chicks at Hatch on the Incidence of Bacterial Chondronecrosis with Osteomyelitis Lameness Using a Staphylococcus Challenge Model. Animals (Basel). 2024. https://pubmed.ncbi.nlm.nih.gov/38731373/

[25] Perera R, Asnayanti A, Alharbi KS, et al. Leveraging Electron Beam-Inactivated Multi-Strain Staphylococcus Vaccine for Preventing BCO Lameness in Broiler Chickens. Vaccines (Basel). 2025. https://pubmed.ncbi.nlm.nih.gov/41012149/

[26] Assumpcao ALFV, Arsi K, Asnayanti A, et al. Electron-Beam-Killed Staphylococcus Vaccine Reduced Lameness in Broiler Chickens. Vaccines (Basel). 2024. https://pubmed.ncbi.nlm.nih.gov/39591106/

[27] Anthney A, Alharbi K, Perera R, et al. Evaluating the Effectiveness of Probiotic and Multivalent Vaccination Strategies in Mitigating Bacterial Chondronecrosis with Osteomyelitis Lameness Using a Hybrid Challenge Model. Animals (Basel). 2025. https://pubmed.ncbi.nlm.nih.gov/40003050/

[28] Cooper JE, Needham JR. The starling (Sturnus vulgaris) as an experimental model for staphylococcal infection of the avian foot. Avian Pathol. 1981. https://pubmed.ncbi.nlm.nih.gov/18770141/

[29] Do ADT, Assetova G, Asnayanti A, et al. Survey of Bacterial Chondronecrosis with Osteomyelitis Lesion Incidence in Broiler Farms in Kazakhstan Regions. Animals (Basel). 2026. https://pubmed.ncbi.nlm.nih.gov/42278019/

[30] Matos M, Mitsch P, Liebhart D, et al. Co-Infection of Chickens with Staphylococcus lentus and Staphylococcus aureus from an Outbreak of Arthritis, Synovitis, and Osteomyelitis Argues for Detailed Characterisation of Isolates. Animals (Basel). 2024. https://pubmed.ncbi.nlm.nih.gov/39272359/