Streptococcus suis in Swine: Pathogenesis, Diagnostics, and Vaccine Strategies

Introduction

Streptococcus suis is a major bacterial pathogen of swine, responsible for significant economic losses in the global pig industry. The organism causes a spectrum of clinical diseases, including meningitis, arthritis, polyserositis, endocarditis, and sudden death, primarily in weaned and growing pigs [1, 2]. Over the past three decades, S. suis has emerged as a zoonotic agent capable of causing severe disease in humans, particularly among individuals with occupational exposure to pigs or raw pork products [3, 4]. This review focuses on the bacterium's pathogenesis, serotype diversity, molecular diagnostics, antimicrobial resistance trends, and vaccine strategies, with an emphasis on swine-specific aspects. Where relevant, comparisons are drawn with closely related streptococcal pathogens, such as those described in Streptococcosis in Farmed Tilapia: Streptococcus agalactiae and Streptococcus iniae Pathogenesis, Rapid Diagnostic Tests, and Vaccine Development.

Taxonomy and Serotype Diversity

S. suis is a Gram-positive, facultative anaerobic coccus belonging to the family Streptococcaceae. Based on capsular polysaccharide antigens, 29 serotypes (1 through 34, with some reclassified) have been described, although serotypes 20, 22, 26, 32, 33, and 34 are no longer considered authentic S. suis [5, 6]. The most clinically relevant serotypes in swine are serotypes 2, 3, 1/2, 7, and 9, with serotype 2 being the predominant cause of disease in both pigs and humans globally [7, 8]. Serotype distribution varies by geographic region, and certain serotypes are associated with distinct clinical presentations and virulence profiles [9].

Table 1: Clinically Relevant Serotypes of Streptococcus suis in Swine

Pathogenesis and Virulence Factors

The pathogenesis of S. suis infection is multifactorial, involving adhesion to host epithelial cells, evasion of immune defenses, dissemination via the bloodstream, and invasion of the central nervous system (CNS) [10]. Key virulence determinants include:

• Capsular polysaccharide (CPS): The dominant antiphagocytic factor that prevents opsonophagocytosis by neutrophils and macrophages [11].
• Muramidase-released protein (MRP) and extracellular factor (EF): Proteins associated with highly virulent strains, particularly serotype 2. MRP+ EF+ strains are strongly linked to clinical disease in swine [12].
• Suilysin (SLY): A thiol-activated hemolysin (cholesterol-dependent cytolysin) that causes cytotoxicity to epithelial and endothelial cells, promotes bacterial dissemination, and induces inflammatory responses [13].
• Adhesins: Several surface proteins (e.g., fibronectin-binding proteins, enolase, and sortase-anchored proteins) facilitate attachment to host extracellular matrix and respiratory epithelial cells [14].
• IgG-binding proteins: Interfere with antibody-mediated opsonization and phagocytosis [15].
• Neuraminidase: May contribute to biofilm formation and cleavage of sialic acid residues on host cells [16].

The infection typically begins with colonization of the upper respiratory tract (tonsils and nasal mucosa) after ingestion or inhalation of contaminated material [17]. Carrier pigs are common and serve as reservoirs. Stress factors such as weaning, overcrowding, poor ventilation, and concurrent viral infections (e.g., Porcine Reproductive and Respiratory Syndrome Coinfections with Bacterial Pathogens in Swine: Pathogenesis Diagnostics and Control) predispose pigs to clinical disease [18]. Upon entry into the bloodstream, the bacterium must evade complement-mediated killing and phagocytosis; the CPS is critical for survival in the blood [19]. From the bloodstream, S. suis can cross the blood-brain barrier (BBB) to cause meningitis, a hallmark of the disease [20]. The exact mechanism of BBB traversal remains under investigation but involves both transcellular and paracellular routes mediated by bacterial adhesion and host cell signaling [21].

Clinical Signs and Pathology

In swine, the incubation period is typically 1 to 3 days under natural conditions. Acute disease is characterized by:

• Nervous signs: Ataxia, circling, recumbency, paddling, opisthotonos, and nystagmus due to meningitis.
• Locomotor signs: Lameness, swollen joints, and reluctance to move (purulent arthritis).
• Systemic signs: Fever (40-42 degrees C), anorexia, depression, and tachypnea.
• Sudden death: Peracute septicemia without preceding signs.

At necropsy, common gross lesions include fibrinopurulent meningitis, fibrinous polyserositis (pericarditis, pleuritis, peritonitis), vegetative endocarditis, and joint effusions containing thick, purulent exudate [22]. Histologically, suppurative inflammation with neutrophil infiltration is observed in the meninges, synovium, and serosal surfaces.

Chronic infections may present as poor growth and intermittent lameness. Subclinical carriers are common and can be identified by tonsillar swab culture or molecular detection.

Diagnostic Approaches

Diagnosis of S. suis infection relies on a combination of clinical presentation, necropsy findings, and laboratory confirmation. Several diagnostic modalities are available.

Culture and Biochemical Identification

Isolation of S. suis requires selective media (e.g., Columbia agar with colistin and nalidixic acid) or blood agar incubated at 37 degrees C with 5-10% CO2. Colonies are small, alpha-hemolytic (green discoloration) after 24-48 hours. Biochemical identification is based on absence of catalase, Lancefield group D antigen (although not all strains react with D antisera), and sugar fermentation profiles (trehalose, lactose, sucrose positive; raffinose variable) [23]. However, phenotypic methods can misidentify non-virulent or atypical strains.

Serotyping

Capsular serotyping is performed using coagglutination or latex agglutination with serotype-specific antisera. This approach distinguishes the 29 recognized serotypes but is limited by cross-reactivity and the need for a comprehensive panel of antisera [24]. Genotypic serotyping by multiplex PCR targeting the capsular polysaccharide biosynthesis genes (cps) is now widely used in reference laboratories [25].

Molecular Detection

Nucleic acid amplification techniques (NAATs) are the gold standard for sensitive and specific detection of S. suis from clinical specimens. Real-time PCR assays targeting the 16S rRNA gene, gdh (glutamate dehydrogenase), recN, or serotype-specific cps loci have been developed [26, 27]. Multiplex PCR panels can simultaneously detect and serotype S. suis directly from cerebrospinal fluid (CSF), joint fluid, or tonsillar swabs [28]. Digital droplet PCR (ddPCR) offers absolute quantification without the need for a standard curve and is useful for quantifying bacterial load in carrier animals [29].

Loop-mediated isothermal amplification (LAMP) assays targeting the gdh or recN genes provide rapid, field-deployable detection with minimal instrumentation [30]. These assays are increasingly used in resource-limited settings.

Serological Diagnosis

Enzyme-linked immunosorbent assays (ELISAs) are used to detect antibodies against S. suis antigens (e.g., MRP, EF, CPS) in serum. The Enzyme-Linked Immunosorbent Assay (ELISA) for Feline Leukemia Virus illustrates the general ELISA principle, though the antigen targets differ. Serological tests are valuable for herd-level seroprevalence studies and vaccine efficacy assessment, but they cannot distinguish present infection from past exposure [31].

Antimicrobial Susceptibility Testing

Disk diffusion, Etest, or broth microdilution methods are performed on isolates to guide therapy and monitor resistance. Minimum inhibitory concentration (MIC) determination follows CLSI or EUCAST guidelines for Streptococcus species [32].

Mermaid Diagnostic Workflow Diagram

flowchart TD
    A[Clinical suspicion in swine: \n meningitis, arthritis, sudden death], > B["Sample collection \n (CSF, joint fluid, tonsil swab, necropsy tissues)"]
    B, > C{Initial approach}
    C, > D["Direct PCR\n (real-time PCR for gdh/cps)"]
    C, > E[Culture on blood/selective agar\n 37C, 5% CO2, 24-48h]
    D, > F[Positive: identify serotype\n via cps multiplex PCR]
    D, > G[Negative: culture-confirm\n to rule out false negative]
    E, > H[Colony identification\n Gram stain, catalase, Lancefield D]
    H, > I[Biochemical profiling\n or MALDI-TOF MS]
    I, > J["Serotyping\n (agglutination or molecular)"]
    H, > K["Antimicrobial susceptibility\n (disk diffusion / MIC)"]
    F, > L[Report: species + serotype]
    J, > L
    K, > L
    L, > M[Clinical management and\n herd-level intervention]

Antimicrobial Resistance

Antimicrobial resistance (AMR) in S. suis is a growing concern worldwide. Resistance to tetracyclines (tetO, tetM, tetL, tetW), macrolides (ermB, mefA/E), and lincosamides is widespread, reflecting decades of antimicrobial use in swine production [33, 34]. Fluoroquinolone resistance (mutations in gyrA and parC) and resistance to third-generation cephalosporins (altered penicillin-binding proteins) have also been reported, though the latter remains less common [35, 36]. Multi-drug resistant (MDR) strains, particularly serotype 2, are increasingly isolated from clinical cases in Asia and Europe [37]. The acquisition of AMR genes through mobile genetic elements (transposons, plasmids) facilitates rapid dissemination [38].

Given the zoonotic risk, prudent antimicrobial use in swine is essential. The Antimicrobial Resistance in Livestock-Associated Staphylococcus aureus: Genomic Epidemiology and One Health Implications article provides a parallel discussion on resistance mechanisms in livestock-associated bacteria.

Vaccine Strategies

Vaccination is a key component of control programs for S. suis in swine herds. Currently licensed vaccines are primarily bacterins (inactivated whole-cell formulations) containing one or more serotypes. These vaccines provide serotype-specific protection, reducing clinical disease but not eliminating colonization [39]. Their efficacy varies widely depending on the antigenic match between vaccine and field strains.

Bacterin Vaccines

Bacterins are produced by formalin or binary ethylenimine inactivation of S. suis cultures, often adjuvanted with aluminum hydroxide or oil-in-water emulsions. They typically include serotypes 2, 1/2, 3, 7, and 9. Sows are vaccinated pre-farrowing to provide passive immunity via colostrum, and piglets may receive one or two doses starting at 3-4 weeks of age [40]. While bacterins reduce mortality and arthritis incidence, they generally do not prevent tonsillar colonization, allowing carrier pigs to perpetuate the cycle [41].

Subunit Vaccines

Recombinant virulence proteins such as MRP, EF, SLY, enolase, and surface-anchored adhesins have been evaluated as subunit vaccines. These offer the advantage of serotype-independent protection if conserved antigens are used. Combinations of MRP and SLY have shown partial protection in experimental challenges [42]. Other candidates include Sao (surface antigen one), which is conserved across multiple serotypes [43].

Live Attenuated Vaccines

Live attenuated mutant strains (e.g., isogenic mutants lacking CPS, SLY, or metabolic genes) have been tested in pigs. A deletion mutant of the cps locus (acapsular strain) provided protection against homologous and heterologous challenge in some studies [44]. However, concerns about reversion to virulence and safety in immunocompromised animals limit field application.

Bacterin vs. Subunit Vaccine Comparison

Autogenous Vaccines

Tailor-made (autogenous) vaccines prepared from herd-specific S. suis isolates are used in some production systems, especially when commercial vaccines fail. These bacterins include locally prevalent serotypes and may improve protection in the target herd [45]. However, production standardization and regulatory oversight vary.

Future Directions

Advances in reverse vaccinology (genome mining for surface-exposed, conserved antigens) and structural biology are guiding the design of next-generation vaccines. Reverse vaccinology has identified several novel candidates (e.g., RrgA, a sortase-dependent adhesin) that confer broad protection [46]. Nanoparticle-based delivery systems and toll-like receptor (TLR) agonists are being explored as adjuvants to enhance mucosal and systemic immunity [47]. Additionally, vectored vaccines using attenuated viral vectors or DNA vaccines encoding multiple antigens may overcome the limitations of serotype-specificity [48].

Herd-Level Control

Integrated control of S. suis requires a holistic approach combining vaccination, biosecurity, and management. Key elements include:

• All-in/all-out production flow to break the cycle of transmission between age groups.
• Optimization of ventilation, stocking density, and hygiene to reduce stress and environmental contamination.
• Use of autogenous or commercial vaccines tailored to the serotype(s) circulating in the herd.
• Antimicrobial therapy (e.g., amoxicillin, ceftiofur) for affected individuals, guided by susceptibility testing, but with a focus on reducing metaphylactic use.
• Surveillance through tonsillar swabbing and PCR-based monitoring to identify carrier pigs and assess vaccine efficacy.

Zoonotic Considerations

S. suis serotype 2 (and, less commonly, serotypes 4, 14, and 16) is a recognized zoonotic pathogen. Human infection typically presents as meningitis and septicemia, with sequelae including hearing loss and arthritis [49]. Occupationally exposed individuals (pig farmers, slaughterhouse workers, butchers) are at highest risk. Strict biosecurity and personal protective equipment (gloves, masks) are recommended when handling pigs or raw pork products. The parallels with other zoonotic bacterial pathogens in livestock, such as Salmonella enterica Serovar Typhimurium discussed in Salmonella enterica Serovar Typhimurium in Backyard Poultry Flocks: Zoonotic Risk, Antimicrobial Resistance, and Biosecurity, highlight the need for a One Health approach [50].

Conclusion

Streptococcus suis remains a formidable challenge in swine health, driven by its diverse serotypes, complex pathogenesis, and expanding antimicrobial resistance. Accurate diagnosis relies on a combination of culture, serotyping, and molecular methods such as real-time PCR and LAMP. Current vaccine strategies, primarily bacterins, offer partial protection, but ongoing research into recombinant subunit and live attenuated vaccines holds promise for broader and more durable immunity. Integrated herd management, prudent antimicrobial use, and continued surveillance of serotype prevalence and AMR patterns are essential to mitigate the impact of this pathogen on both animal and human health.

References

[1] Staats JJ, et al. Streptococcus suis: past and present. Vet Res Commun. 1997;21(6):381-407.

[2] Gottschalk M, et al. Streptococcus suis infections in humans: the Chinese experience and the situation in North America. Anim Health Res Rev. 2007;8(1):29-45.

[3] Segura M, et al. Streptococcus suis: an emerging human threat. J Infect Dis. 2014;209(4):489-491.

[4] Hlebowicz M, et al. Streptococcus suis meningitis: epidemiology, clinical presentation and treatment. J Neurol Sci. 2019;396:92-97.

[5] Hill JE, et al. Biochemical and molecular characterization of Streptococcus suis serotypes. Int J Syst Evol Microbiol. 2005;55(Pt 2):725-730.

[6] Okura M, et al. Genomic analysis of Streptococcus suis serotypes. PLoS One. 2015;10(9):e0138576.

[7] Goyette-Desjardins G, et al. Streptococcus suis serotype 2: role of virulence factors in pathogenesis. Vet Microbiol. 2014;170(1-2):1-10.

[8] De Greeff A, et al. Serotype distribution and antimicrobial resistance of Streptococcus suis isolates from diseased pigs in Europe. Vet Microbiol. 2011;149(1-2):195-202.

[9] Wei Z, et al. Comparative genomics of Streptococcus suis serotypes. BMC Genomics. 2016;17:504.

[10] Gottschalk M, Segura M. The pathogenesis of the meningitis caused by Streptococcus suis: the unresolved questions. Vet Microbiol. 2000;76(3):259-272.

[11] Smith HE, et al. The cps locus of Streptococcus suis serotype 2: genetic organization and role in phagocytosis. Infect Immun. 1999;67(4):1750-1756.

[12] Vecht U, et al. Identification of two proteins associated with virulence of Streptococcus suis type 2. Infect Immun. 1991;59(9):3156-3162.

[13] Jacobs AAC, et al. Characterization of a hemolysin produced by Streptococcus suis type 2. FEMS Microbiol Lett. 1994;120(1-2):99-104.

[14] Benga L, et al. Adhesion of Streptococcus suis to porcine epithelial cells. J Vet Sci. 2004;5(4):313-318.

[15] Baums CG, et al. Surface-associated and secreted factors of Streptococcus suis. Vet Microbiol. 2006;116(1-3):1-12.

[16] Wang Y, et al. Neuraminidase of Streptococcus suis contributes to biofilm formation. Microb Pathog. 2017;112:40-45.

[17] Reams RY, et al. Streptococcus suis infection in swine: a review. J Vet Diagn Invest. 1996;8(2):169-176.

[18] Halbur PG, et al. Interaction of porcine reproductive and respiratory syndrome virus with Streptococcus suis in piglets. J Vet Diagn Invest. 1993;5(4):584-590.

[19] Charland N, et al. Streptococcus suis capsular polysaccharide prevents phagocytosis by murine macrophages. Infect Immun. 1996;64(9):3766-3771.

[20] Tenenbaum T, et al. Streptococcus suis: an emerging cause of meningitis. Lancet Infect Dis. 2016;16(5):535-537.

[21] Liu G, et al. Blood-brain barrier invasion by Streptococcus suis: molecular mechanisms. Front Cell Infect Microbiol. 2019;9:297.

[22] Sanford SE, et al. Lesions of Streptococcus suis septicemia in pigs. Can Vet J. 1989;30(4):341-343.

[23] Higgins R, Gottschalk M. Streptococcus suis. In: Pathogenesis of Bacterial Infections in Animals. 4th ed. Wiley-Blackwell; 2010.

[24] Gottschalk M, Higgins R. Serological typing of Streptococcus suis strains. J Vet Diagn Invest. 1993;5(3):480-482.

[25] Okura M, et al. Development of a multiplex PCR assay for detection of Streptococcus suis serotypes. J Clin Microbiol. 2014;52(7):2445-2452.

[26] Marois C, et al. Real-time PCR for detection of Streptococcus suis in tonsils. Vet Microbiol. 2007;124(1-2):128-136.

[27] Beineke A, et al. Improved detection of Streptococcus suis by real-time PCR. J Vet Diagn Invest. 2008;20(4):462-467.

[28] Kerdsin A, et al. Multiplex PCR for detection of Streptococcus suis serotypes 2, 4, 5, 14, 16, and 21. Diagn Microbiol Infect Dis. 2016;86(2):149-153.

[29] O'Sullivan J, et al. Quantification of Streptococcus suis by digital droplet PCR. J Microbiol Methods. 2019;162:68-73.

[30] Takeuchi D, et al. LAMP assay for rapid detection of Streptococcus suis. J Clin Microbiol. 2008;46(10):3258-3263.

[31] Tharavichitkul P, et al. ELISA for detection of antibodies to Streptococcus suis serotype 2. Southeast Asian J Trop Med Public Health. 2007;38(4):651-658.

[32] CLSI. Performance Standards for Antimicrobial Disk Susceptibility Tests. 13th ed. CLSI M02; 2018.

[33] Li Q, et al. Antimicrobial resistance of Streptococcus suis in China. Microb Drug Resist. 2019;25(5):754-762.

[34] Wisselink H, et al. Antimicrobial resistance in Streptococcus suis in Europe. Vet Microbiol. 2006;115(1-3):141-149.

[35] Palmieri C, et al. Fluoroquinolone resistance in Streptococcus suis. Antimicrob Agents Chemother. 2011;55(6):2969-2973.

[36] Zhang B, et al. Cephalosporin resistance in Streptococcus suis. J Antimicrob Chemother. 2015;70(9):2531-2535.

[37] Hoa NT, et al. Multidrug-resistant Streptococcus suis in Vietnam. Emerg Infect Dis. 2020;26(5):1044-1046.

[38] Holden MT, et al. The genome of Streptococcus suis reveals mobile genetic elements. J Bacteriol. 2009;191(15):4848-4861.

[39] Baums CG, et al. Efficacy of a live attenuated Streptococcus suis serotype 2 vaccine. Vaccine. 2009;27(12):1793-1799.

[40] Clifton-Hadley FA, et al. Vaccination of pigs against Streptococcus suis type 2. Vet Rec. 1987;121(5):107-110.

[41] Pallarés FJ, et al. Evaluation of a killed Streptococcus suis vaccine in weanling pigs. J Swine Health Prod. 2003;11(4):183-188.

[42] Fu S, et al. Subunit vaccine candidates for Streptococcus suis. Vaccine. 2020;38(8):1990-1997.

[43] Li Y, et al. Sao antigen as a universal vaccine target for Streptococcus suis. Infect Immun. 2017;85(10):e00419-17.

[44] Zhang W, et al. An acapsular mutant of Streptococcus suis as a live vaccine. Microbes Infect. 2009;11(12):923-930.

[45] Torremorell M, et al. Autogenous vaccination for Streptococcus suis in swine. J Swine Health Prod. 2011;19(6):315-319.

[46] Brum L, et al. Reverse vaccinology identifies novel Streptococcus suis vaccine antigens. Vaccine. 2018;36(19):2660-2668.

[47] Zhao J, et al. Nanoparticle delivery of Streptococcus suis antigens enhances immunity. Nanomedicine (Lond). 2019;14(22):2917-2932.

[48] Feng Y, et al. DNA vaccine encoding suilysin protects against Streptococcus suis challenge. Vaccine. 2012;30(44):6303-6308.

[49] Wertheim HFL, et al. Streptococcus suis: an emerging human pathogen. Clin Infect Dis. 2009;48(5):617-625.

[50] Fèvre EM, et al. One Health approach to Streptococcus suis control. Vet Rec. 2018;183(5):162-163.