Section: Livestock Bacteria

Erysipelothrix rhusiopathiae: Swine Erysipelas – Arthritis, Diamond Skin Lesions, and Vaccination

Introduction

Swine erysipelas is a globally significant bacterial disease of pigs caused by the Gram-positive, rod-shaped, facultative intracellular organism Erysipelothrix rhusiopathiae. The disease manifests in acute, subacute, and chronic forms, with the chronic phase characterized by vegetative endocarditis and a nonerosive, proliferative arthritis. The hallmark cutaneous lesions, known as diamond skin lesions, are pathognomonic for the acute form. This article provides a detailed review of the etiology, epidemiology, clinical presentation, pathology, diagnostic approaches, treatment options, and vaccination strategies for E. rhusiopathiae infection in swine, with emphasis on arthritis and the characteristic diamond skin lesions. The article integrates molecular insights from genomic studies and vaccinology, citing a comprehensive set of peer-reviewed publications [1–50].

Etiology and Taxonomy

E. rhusiopathiae is a slender, pleomorphic, non-spore-forming, facultatively anaerobic bacillus that belongs to the family Erysipelotrichaceae within the phylum Firmicutes. The organism is catalase-negative, oxidase-negative, and produces hydrogen sulfide in triple sugar iron agar. It ferments a narrow range of carbohydrates, primarily glucose, lactose, and fructose, without gas production [1]. The bacterium possesses a thick cell wall that lacks teichoic acid but contains a unique peptidoglycan structure with lysine as the dibasic amino acid. The genome of E. rhusiopathiae is approximately 1.75–1.88 Mb with a G+C content of 36–37% [41, 45, 47]. Comparative genomics have revealed the presence of pathogenicity island-like regions and a set of virulence factors, including surface protective antigen (Spa) proteins, neuraminidase, and hyaluronidase [45]. The major protective antigen is SpaA (Spa for serotype 1a and 2), a surface-exposed protein that mediates adherence to host cells and elicits protective immune responses [2, 33]. Sequence variation in the hypervariable region of the spaA gene defines distinct antigenic subtypes, such as the Met-203 and Ile-257 (M203/I257) SpaA-type variant, which has been associated with chronic outbreaks in Japan [3, 43, 48].

Epidemiology

E. rhusiopathiae is ubiquitous in the environment and can persist in soil, water, and organic matter for months. Swine are the primary reservoir, but the bacterium also infects turkeys, sheep, cattle, and a wide variety of wildlife, including captive white-lipped peccaries [50]. Transmission occurs via the fecal-oral route through ingestion of contaminated feed or water, or through skin abrasions. Carrier pigs can shed the organism intermittently in feces, saliva, and urine, perpetuating herd infections. Stressors such as crowding, poor ventilation, sudden dietary changes, and concurrent diseases precipitate clinical outbreaks [4, 5]. The disease has a worldwide distribution, with serotype 1a and 2 being the most prevalent in swine [6, 30, 48]. In Japan, recent increases in acute swine erysipelas have been attributed to clonal lineages identified through genome-wide single-nucleotide polymorphism (SNP) analysis [38, 40]. Similarly, outbreaks in Eastern China have been caused by serotype 1a strains with diverse genetic backgrounds [42].

Clinical Signs

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

Acute Form

Acute erysipelas is characterized by sudden onset of high fever (40.5–42.0°C), depression, anorexia, and reluctance to move. Within 24–48 hours, pathognomonic diamond skin lesions appear as raised, erythematous, rhomboid plaques on the skin, particularly over the dorsum, flanks, and neck. These lesions result from thrombosis and ischemia in dermal capillaries, leading to a characteristic pattern that is strongly associated with E. rhusiopathiae infection. In severe cases, septicemia leads to cyanosis of the ears, snout, and extremities. Mortality is high in peracute cases, but survivors may progress to subacute or chronic disease.

Subacute and Chronic Forms

Subacute disease presents with milder fever and fewer skin lesions. Chronic erysipelas manifests weeks to months after acute infection and includes two major sequelae: vegetative endocarditis and chronic arthritis. Swine erysipelas joint arthritis is a hallmark of the chronic form, affecting approximately 10–30% of recovered pigs. The arthritis is typically polyarticular, involving the carpal, tarsal, and stifle joints. Clinically, affected pigs show lameness, stiff gait, joint swelling, and reluctance to move, leading to reduced growth rates and culling. The arthritis is nonerosive and proliferative, characterized by synovial hypertrophy and pannus formation without significant cartilage erosion [7, 8]. Diamond skin lesions may recur intermittently in chronic cases.

Pathology

Gross Pathology

At necropsy, acute cases show generalized congestion, petechiae on serosal surfaces, and enlarged, friable spleens. Diamond skin lesions correspond to areas of cutaneous infarction. In chronic arthritis, affected joints have thickened, hyperemic synovial membranes, increased synovial fluid volume, and fibrous adhesions. The synovium contains villous projections (pannus) that can extend over the articular cartilage. Vegetative endocarditis, if present, typically involves the mitral or aortic valves, with large, friable, cauliflower-like vegetations.

Histopathology

Microscopically, acute skin lesions show diffuse dermal edema, thrombosis of capillaries and venules, and infiltration of neutrophils and macrophages. In chronic arthritis, the synovium exhibits hyperplasia of synoviocytes, infiltration of lymphocytes and plasma cells, and deposition of fibrin on the synovial surface [8]. Timoney [9] reported that complement concentrations in synovial fluid from arthritic joints were increased compared with normal joints, but conversion products of the third component of complement were absent, suggesting a less prominent role for immune complexes in erysipelas arthritis than in human rheumatoid arthritis. Lysosomal enzyme levels, particularly lysozyme and acid phosphatase, are elevated in synovial fluids, with lactic dehydrogenase levels indicating cell death as a major mechanism of enzyme release [10]. Hypersensitivity mechanisms have also been implicated, as sterile Erysipelothrix antigens can induce synovitis in sensitized animals [11, 44, 46].

Pathogenesis

After oral or cutaneous entry, E. rhusiopathiae adheres to and invades endothelial cells and macrophages via SpaA-mediated interactions. The bacterium resists phagocytosis and can survive within macrophages, promoting dissemination to joints and heart valves. Hyaluronidase and neuraminidase facilitate tissue spread. In the joint, persistent bacterial antigen stimulates a chronic inflammatory response that is driven by both innate and adaptive immunity. The presence of the bacterium in synovial tissues triggers infiltration of T lymphocytes and macrophages, with release of pro-inflammatory cytokines. Chronic arthritis results from a delayed-type hypersensitivity reaction to bacterial antigens, as demonstrated by experimental inoculation of killed organisms in immunized animals [11, 46]. The tagF homolog (ERH_0432) encoding a CDP-glycerol glycerophosphotransferase has been identified as a critical virulence gene; its deletion markedly attenuates the bacterium and prevents arthritis [12]. Similarly, a mutation in the XRE family transcriptional regulator ERH_0661 partially accounts for attenuation of the Koganei live vaccine strain [36].

Diagnostics

Clinical and Gross Diagnosis

The presence of diamond skin lesions in febrile pigs is highly suggestive of acute erysipelas. Chronic arthritis may be suspected in animals with progressive lameness and joint swelling, but differentiation from other causes of polyarthritis (e.g., Mycoplasma hyosynoviae, Streptococcus suis) requires laboratory confirmation.

Bacteriological Methods

E. rhusiopathiae can be isolated from blood, synovial fluid, or affected tissues using selective media such as citrate azide Tween carbonate (CATC) agar or modified blood agar containing antibiotics [13]. Colonies appear small, transparent, and alpha-hemolytic after 24–48 hours at 37°C. Identification is confirmed by Gram stain (slender Gram-positive rods), catalase negativity, and production of hydrogen sulfide in triple sugar iron agar. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) offers rapid identification from culture [34, 50].

Serological Methods

Several serological assays have been developed for swine erysipelas diagnosis and surveillance. The enzyme-linked immunosorbent assay (ELISA) using recombinant SpaA (rSpaA415) or a 65 kDa immunodominant antigen demonstrates high sensitivity and specificity [14, 15, 37]. Commercial ELISA kits are available. The gel diffusion precipitin test is less sensitive but can be used for herd-level screening [16]. Serology is primarily used to assess vaccine response rather than for acute diagnosis, because antibodies may cross-react with other bacteria.

Molecular Methods

Polymerase chain reaction (PCR) targeting the spaA gene or 16S rRNA is the gold standard for molecular detection. Real-time PCR assays provide rapid quantification and can detect the organism directly in synovial fluid or tissue homogenates [13]. Sequence analysis of the 432-bp hypervariable region of spaA enables genotyping and differentiation of antigenic variants, such as the M203/I257 SpaA-type [3, 42]. Genome-wide SNP analysis has been employed for epidemiological tracing of outbreak strains [38, 40].

Treatment

Acute erysipelas responds well to antimicrobial therapy. Beta-lactam antibiotics, particularly penicillin G, are the drugs of choice. E. rhusiopathiae is also susceptible to tetracyclines, macrolides, and lincomycin [42]. Early treatment is critical to prevent progression to chronic disease. For chronic arthritis, antimicrobial therapy is less effective because the joint environment limits drug penetration and the inflammatory process becomes self-perpetuating. Nonsteroidal anti-inflammatory drugs (NSAIDs) may be used adjunctively to reduce pain and inflammation. No specific antivirulence therapies are commercially available, but experimental probiotics such as Bacillus direct-fed microbials have shown in vitro inhibitory activity against E. rhusiopathiae [34].

Vaccination

Vaccination is the cornerstone of swine erysipelas control. Both live attenuated and inactivated (bacterin) vaccines are widely used.

Live Attenuated Vaccines

The most commonly used live vaccine in Japan is the E. rhusiopathiae Koganei 65-0.15 strain (Koganei), an acriflavine-resistant derivative of serotype 1a. However, this vaccine has been associated with adverse reactions, including arthritis and endocarditis in some vaccinated pigs [12]. To improve safety, a mutant lacking the tagF gene (Δ432) was developed; this mutant is highly attenuated and provides protective immunity when administered orally or subcutaneously in pigs [12]. Another live vaccine candidate is an avirulent E. rhusiopathiae strain that can be self-administered via environmental enrichment devices, inducing oral and serum antibody responses comparable to hand-vaccination in pigs [35]. Self-vaccination reduces labor stress and improves animal welfare but is currently limited to live vaccines; killed vaccines do not elicit sufficient responses via oral mucosal delivery [35].

Inactivated Vaccines (Bacterins)

Inactivated vaccines containing serotype 1a and/or 2a are widely available. They are typically administered parenterally (intramuscularly) to sows and growing pigs. Field studies in Spain demonstrated that pre-farrowing vaccination of sows significantly increases maternally derived antibody titers in piglets, prolonging seropositivity through the post-weaning period [37]. In Japan, the inactivated vaccine SER-ME (serovar 2a immunogen) was shown to protect pigs against challenge with both the Fujisawa reference strain and the M203/I257 SpaA-type variant [33]. However, vaccine-induced immunity may not be equally protective against all serotypes; Wood et al. [17] reported that a serotype 2 bacterin failed to protect swine against serotype 10 challenge, while arthritis severity was reduced but not eliminated.

Recombinant and Subunit Vaccines

Recombinant SpaA protein administered with adjuvant induces protective immunity in pigs against challenge with serotypes 1a and 2b [2, 33]. Truncated SpaA lacking the C-terminal region retains immunogenicity and is a candidate for commercial subunit vaccines [2]. The efficacy of such vaccines depends on matching the SpaA sequence to circulating field strains. The emergence of M203/I257 variants in Japan highlights the need for periodic surveillance of spaA genotypes to ensure continued vaccine efficacy [3, 43]. Oral vaccination with subunit antigens is an area of active research, but currently, only live vaccines are suitable for non-parenteral administration [18].

Vaccination Strategies and Herd Immunity

The decision to vaccinate depends on herd prevalence, production system (indoor vs. outdoor), and economic considerations. In naive herds, whole-herd vaccination is recommended during an outbreak, followed by routine booster vaccination of sows pre-farrowing and growing pigs at 10–12 weeks of age. Outdoor production systems may require more frequent vaccination because of increased environmental exposure [19]. Vaccination against erysipelas has also been associated with improved reproductive performance in sows, possibly by reducing subclinical urogenital infections [29].

Control and Prevention

In addition to vaccination, biosecurity measures are essential. These include all-in/all-out management, proper cleaning and disinfection of pens, control of rodents and birds, and quarantine of replacement stock. E. rhusiopathiae is susceptible to common disinfectants, including glutaraldehyde, chlorhexidine, and sodium hypochlorite. Early detection and treatment of acute cases reduce the risk of chronic sequelae. In herds with endemic arthritis, vaccination must be combined with culling of chronically affected animals to reduce the environmental bacterial load.

Diagnostic and Decision-Making Workflow

A systematic approach to diagnosing and managing swine erysipelas is depicted in the following Mermaid flowchart.

flowchart TD
    A[Clinical suspicion: fever, lameness, diamond skin lesions], > B{Acute or chronic?}
    B, >|Acute| C[Collect blood, skin biopsy, synovial fluid]
    B, >|Chronic| D[Collect synovial fluid, joint tissue]
    C, > E[Culture on selective agar / PCR for spaA]
    D, > E
    E, > F[Positive?]
    F, >|Yes - treat with penicillin G| G[Clinical improvement?]
    G, >|Yes| H[Continue treatment, monitor for chronic signs]
    G, >|No| I[Consider antimicrobial resistance testing]
    F, >|No| J[Consider differential diagnoses: Mycoplasma, Streptococcus, E. coli]
    H, > K[Vaccination strategy: live or inactivated vaccine]
    K, > L[Herd-level control: biosecurity, culling chronic cases]

Cross-Links to Related Topics

References

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