Swine Erysipelas: Erysipelothrix rhusiopathiae Infection, Arthritis, and Diamond Skin Lesions

Etiology and Taxonomy

Swine erysipelas is caused by Erysipelothrix rhusiopathiae, a Gram-positive, facultatively anaerobic, non-spore-forming, rod-shaped bacterium belonging to the family Erysipelotrichaceae. The organism exhibits a characteristic slender, pleomorphic morphology and produces alpha-hemolysis on blood agar. E. rhusiopathiae is distinguished from other species in the genus by its ability to produce hydrogen sulfide (H2S) in triple sugar iron agar and its resistance to neomycin and kanamycin. The bacterium possesses a thick peptidoglycan cell wall and expresses a variety of surface proteins that mediate adhesion, immune evasion, and virulence [1, 2].

Serotyping of E. rhusiopathiae is based on heat-stable somatic antigens. At least 28 serovars (serovars 1a, 1b, 2 through 23, and serovar N) have been described, with serovars 1a, 1b, and 2 being the most frequently isolated from clinical cases of swine erysipelas globally [3, 4]. The surface protective antigen (Spa) is a critical immunogenic protein; three Spa types (SpaA, SpaB, and SpaC) have been identified. SpaA is predominantly associated with serovars 1 and 2, which are the most virulent and clinically relevant in swine [5, 3, 6]. Recent genomic analyses have revealed that clonal lineages of E. rhusiopathiae responsible for acute outbreaks can be differentiated using genome-wide single-nucleotide polymorphism (SNP) analysis, enabling high-resolution epidemiological tracking [7].

Epidemiology

E. rhusiopathiae is a ubiquitous environmental bacterium capable of surviving for extended periods in soil, water, and organic matter. Swine are the primary reservoir, but the organism can infect a wide range of mammalian and avian species, including turkeys, sheep, and marine mammals. Transmission occurs via the fecal-oral route through ingestion of contaminated feed or water, or through direct contact with infected animals or contaminated fomites. Skin abrasions and wounds also serve as portals of entry.

Risk factors for swine erysipelas outbreaks include high stocking density, poor biosecurity, stress from transport or weaning, and sudden changes in diet or temperature. In Northeast Mainland China, potential risk factors have been identified that include the introduction of replacement gilts without adequate quarantine and the presence of wild boar populations [8]. Wild boars serve as a significant reservoir and potential source of E. rhusiopathiae infection for domestic swine, as demonstrated by genomic comparisons showing host adaptation and selective pressure from antibiotic use [9, 10]. Anthropic environmental factors, such as proximity to pig farms and the presence of carcass disposal sites, have been associated with increased seroprevalence in wild boar populations [11].

The disease exhibits a seasonal pattern, with higher incidence in warmer months when environmental survival of the bacterium is enhanced and insect vectors may contribute to mechanical transmission. Outbreaks can occur in both vaccinated and unvaccinated herds, often due to the emergence of novel SpaA-type variants that evade vaccine-induced immunity [12, 13].

Pathogenesis

Following ingestion or entry through skin abrasions, E. rhusiopathiae adheres to and invades host epithelial cells and endothelial cells. The bacterium expresses multiple adhesins, including glyceraldehyde-3-phosphate dehydrogenase (GAPDH), which binds to host fibronectin and plasminogen, facilitating attachment to porcine endothelial cells [14]. The choline-binding protein B (CbpB) functions as both an adhesin and a plasminogen-binding protein, further promoting tissue colonization [15]. Other surface proteins, such as those identified by Zhu et al. (2018), contribute to adhesion and immune evasion [16, 17].

Once systemic dissemination occurs, the bacterium resists phagocytosis and survives within macrophages. A putative transcription regulator has been implicated in the virulence attenuation of acriflavine-resistant vaccine strains, suggesting that transcriptional control mechanisms are critical for full virulence [18]. Genome-wide identification of virulence genes using transposon mutagenesis has revealed that a tagF homolog, involved in teichoic acid biosynthesis, is essential for virulence and represents a target for safe oral vaccine development [19].

The acute septicemic form results from rapid bacterial proliferation in the bloodstream, leading to widespread vascular damage, thrombosis, and multi-organ failure. The characteristic diamond skin lesions are a consequence of localized vasculitis and thrombosis in dermal capillaries, resulting in ischemic necrosis and the formation of raised, rhomboid urticarial plaques. Chronic infection is characterized by the persistence of bacteria in joint spaces and cardiac valves, where they induce an immune-mediated inflammatory response leading to proliferative synovitis and valvular endocarditis.

Clinical Signs

The clinical presentation of swine erysipelas is classified into acute, subacute, and chronic forms.

Acute Form

The acute form is characterized by sudden onset of high fever (40.5 to 42.0 degrees Celsius), depression, anorexia, and reluctance to move. Affected pigs exhibit a stiff, stilted gait due to generalized myalgia and arthralgia. Within 24 to 48 hours, pathognomonic diamond skin lesions appear. These lesions are raised, firm, rhomboid or square-shaped urticarial plaques that range from pink to deep purple in color. They are most commonly observed on the skin of the back, flanks, thighs, and ears. The lesions result from thrombotic vasculitis and can coalesce to form large areas of necrosis. In severe cases, septicemia leads to cyanosis of the extremities and sudden death.

Subacute Form

The subacute form presents with milder fever and fewer, less pronounced skin lesions. Affected animals may show transient lameness and reduced feed intake. Recovery is often spontaneous, but some animals progress to the chronic form.

Chronic Form

The chronic form of swine erysipelas manifests weeks to months after the acute phase. The two principal manifestations are arthritis and valvular endocarditis.

Erysipelothrix rhusiopathiae swine erysipelas joint arthritis diamonds is a key clinical descriptor. Chronic arthritis results from the persistence of bacterial antigens within synovial joints, triggering a proliferative, non-suppurative synovitis. Affected joints, most commonly the carpal, tarsal, and stifle joints, become swollen, warm, and painful. The joint capsule thickens, and synovial fluid volume increases. Over time, periarticular fibrosis and cartilage erosion lead to permanent lameness, joint deformity, and reduced mobility. Pigs with chronic arthritis often exhibit poor growth and are culled due to welfare and production concerns.

Valvular endocarditis, typically affecting the mitral valve, results in vegetative lesions that impair cardiac function. Clinical signs include exercise intolerance, dyspnea, cyanosis, and signs of congestive heart failure. Embolic showers from vegetative lesions can cause infarction in the kidneys, spleen, and brain.

Pathology

Gross Pathology

In acute cases, gross findings include generalized congestion, petechial hemorrhages on serosal surfaces, and splenomegaly. The characteristic diamond skin lesions correspond to areas of dermal necrosis and hemorrhage. On cut section, these lesions extend into the subcutaneous tissue.

In chronic arthritis, affected joints show marked synovial hypertrophy, villous proliferation of the synovial membrane, and pannus formation. The joint capsule is thickened, and the articular cartilage may exhibit erosions and ulcerations. Joint effusion is common, and the synovial fluid is often turbid and contains fibrin clots.

In cases of endocarditis, the mitral valve (and less commonly the aortic valve) displays large, friable, cauliflower-like vegetative growths composed of fibrin, platelets, and bacterial colonies. These vegetations can obstruct blood flow and serve as a source of septic emboli.

Histopathology

Histologically, acute skin lesions show severe necrotizing vasculitis with fibrinoid necrosis of vessel walls, thrombosis, and perivascular infiltration of neutrophils and mononuclear cells. In chronic arthritis, the synovium is hyperplastic with infiltration of lymphocytes, plasma cells, and macrophages. The synovial villi are elongated and edematous. In endocarditis, the vegetative lesions consist of layered fibrin, degenerating neutrophils, and bacterial colonies embedded in a matrix of platelets.

Diagnostics

A definitive diagnosis of swine erysipelas requires the integration of clinical signs, gross pathology, histopathology, and laboratory confirmation.

Bacterial Isolation and Identification

E. rhusiopathiae can be isolated from blood, synovial fluid, skin lesions, heart valves, and internal organs (spleen, liver, kidney). Samples should be collected aseptically and cultured on blood agar or selective media containing antibiotics (e.g., azide and crystal violet). Colonies appear as small, transparent, alpha-hemolytic after 24 to 48 hours of incubation at 37 degrees Celsius in a 5% carbon dioxide atmosphere. The organism is catalase-negative, oxidase-negative, and produces H2S in triple sugar iron agar. Confirmatory identification can be achieved using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) or biochemical test strips.

Molecular Diagnostics

Polymerase chain reaction (PCR) assays targeting species-specific genes, such as the 16S rRNA gene or the spaA gene, provide rapid and sensitive detection of E. rhusiopathiae directly from clinical specimens. Multiplex PCR-based assays have been developed for rapid serotyping, enabling the differentiation of serovars 1a, 1b, 2, and 5 [20, 4]. These molecular tools are particularly valuable for confirming infection in cases where bacterial culture is negative due to prior antimicrobial therapy or sample degradation.

Differential diagnosis from other pathogens is critical, especially in regions where African swine fever virus (ASFV) is endemic. Molecular characterization of ASFV and differential diagnosis of Erysipelothrix in ASFV-infected pigs has been demonstrated in pig production regions in Cameroon, highlighting the need for multiplex diagnostic panels [21].

Serology

Enzyme-linked immunosorbent assays (ELISAs) and agglutination tests are available for detecting antibodies against E. rhusiopathiae. Serology is useful for herd-level surveillance and for assessing vaccine response, but it has limited utility for individual animal diagnosis due to the prevalence of subclinical infections and vaccination-induced antibodies.

Diagnostic Workflow

The following Mermaid diagram illustrates a diagnostic decision tree for swine erysipelas.

flowchart TD
    A[Clinical Signs: Fever, Diamond Skin Lesions, Lameness], > B{Acute or Chronic?}
    B, >|Acute| C[Collect Blood, Skin Lesion Biopsy, Spleen]
    B, >|Chronic| D[Collect Synovial Fluid, Joint Tissue, Heart Valve]
    C, > E[Gram Stain: Gram-positive rods]
    C, > F[Bacterial Culture on Blood Agar]
    C, > G[PCR: spaA or 16S rRNA]
    D, > H[Gram Stain and Culture]
    D, > I[PCR: spaA or 16S rRNA]
    E, > J{Positive?}
    F, > J
    G, > J
    H, > J
    I, > J
    J, >|Yes| K[Confirm E. rhusiopathiae]
    J, >|No| L[Consider Differential Diagnoses: ASFV, Streptococcus suis, Mycoplasma hyosynoviae]
    K, > M[Serotyping by Multiplex PCR or Agglutination]
    M, > N[Antimicrobial Susceptibility Testing]
    N, > O[Implement Treatment and Control Measures]

Treatment

Antimicrobial therapy is most effective when initiated early in the acute phase of infection. E. rhusiopathiae is susceptible to a range of beta-lactam antibiotics, including penicillin, amoxicillin, and ceftiofur. Penicillin G administered intramuscularly at a dose of 20,000 to 30,000 IU per kg body weight for 3 to 5 days is considered the treatment of choice. Amoxicillin and ceftiofur are also highly effective [22]. Tetracyclines, macrolides, and tiamulin have variable activity, and susceptibility testing is recommended to guide therapy.

The emergence of multidrug-resistant strains is a growing concern. A multidrug-resistant E. rhusiopathiae strain (ZJ) carrying several acquired antimicrobial resistance genes, including the pleuromutilin-lincosamide-streptogramin A resistance gene lsa(E), has been characterized [23, 37]. This underscores the importance of prudent antimicrobial use and routine susceptibility surveillance.

Supportive care, including non-steroidal anti-inflammatory drugs (NSAIDs) for fever and joint pain, and fluid therapy for dehydrated animals, is beneficial. Chronically affected animals with severe arthritis or endocarditis are unlikely to respond to therapy and should be culled.

Control and Prevention

Vaccination

Vaccination is the cornerstone of swine erysipelas control. Both modified-live vaccines (MLVs) and inactivated (killed) vaccines are commercially available. MLVs, such as the acriflavine-resistant strain, provide rapid onset of immunity and are often used in outbreak situations. Inactivated vaccines, typically administered as two doses to gilts and annual boosters to sows, induce a strong humoral immune response.

The efficacy of vaccination can be compromised by the emergence of SpaA-type variants. The vaccine SER-ME has been shown to effectively protect pigs against challenge with the E. rhusiopathiae M203/I257 SpaA-type variant [12]. A field comparison study in Spain evaluated two vaccine protocols in two types of swine breeds, demonstrating that vaccine efficacy can vary by breed and management system [24].

Novel vaccine strategies are under investigation. Recombinant Bacillus subtilis strains expressing SpaA and CbpB have been constructed and evaluated for immunogenicity in a mouse model [25]. Recombinant DnaK and SpaA proteins have also shown immunogenicity and protection in a murine model [26]. These approaches aim to provide broader cross-protection against multiple serovars and Spa types.

Biosecurity and Management

Biosecurity measures to prevent the introduction and spread of E. rhusiopathiae include:

• Quarantine and testing of incoming stock.
• All-in/all-out production systems.
• Effective cleaning and disinfection of facilities. E. rhusiopathiae is susceptible to common disinfectants, including sodium hypochlorite, quaternary ammonium compounds, and glutaraldehyde.
• Rodent and bird control to reduce mechanical transmission.
• Minimizing stress through proper nutrition, ventilation, and stocking density.

Antimicrobial Prophylaxis

In herds with a history of recurrent outbreaks, in-feed or in-water antimicrobial prophylaxis (e.g., penicillin or tetracyclines) may be used during high-risk periods. However, this practice should be carefully managed to avoid selection for antimicrobial resistance.

Public Health Considerations

E. rhusiopathiae is a zoonotic pathogen capable of causing erysipeloid in humans, a localized cellulitis typically acquired through occupational exposure (e.g., butchers, veterinarians, fish handlers). Human infection is usually self-limiting but can rarely lead to septicemia and endocarditis. This article focuses on swine infection; for human clinical details, readers are directed to appropriate medical references.

Conclusion

Swine erysipelas remains a significant cause of economic loss and animal welfare concern in pig production systems worldwide. The pathognomonic diamond skin lesions and chronic arthritis are hallmarks of infection. Advances in molecular diagnostics, including multiplex PCR for serotyping and genome-wide SNP analysis, have improved our understanding of the epidemiology and evolution of E. rhusiopathiae. The emergence of multidrug-resistant strains and SpaA-type variants that evade vaccine-induced immunity presents ongoing challenges. Integrated control strategies combining vaccination, biosecurity, and prudent antimicrobial use are essential for effective disease management.

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