Actinobacillus Infections in Pigs: Pleuropneumonia and Septicemia

Introduction

Actinobacillus species within the family Pasteurellaceae are significant pathogens of swine, causing substantial economic losses worldwide through respiratory disease and systemic infections [1]. The two primary pathogenic species in pigs are Actinobacillus pleuropneumoniae, the etiological agent of porcine contagious pleuropneumonia, and Actinobacillus suis, which is associated with septicemia and polyserositis, particularly in young animals [2, 3]. A. pleuropneumoniae is a highly contagious respiratory pathogen that induces fibrinous, necrotizing pleuropneumonia, often leading to acute death or chronic lung lesions that impair growth performance [4, 5]. A. suis can cause acute septicemia with high mortality in neonatal and weaned pigs, and its pathogenesis shares several virulence determinants with A. pleuropneumoniae [2, 1]. This article provides an exhaustive review of the microbiology, pathogenesis, clinical presentation, diagnostic methods, serotype diversity, antimicrobial resistance, and vaccination strategies for these two important porcine pathogens.

Etiology and Taxonomy

Actinobacillus pleuropneumoniae is a Gram-negative, facultatively anaerobic, pleomorphic coccobacillus that requires nicotinamide adenine dinucleotide (NAD) for growth, classifying it as a V-factor-dependent member of the Pasteurellaceae [29, 1]. Two biovars are recognized: biovar 1 strains are NAD-dependent, while biovar 2 strains are NAD-independent [29, 70]. The species is divided into 19 serotypes based on capsular polysaccharide and lipopolysaccharide (LPS) O-antigen composition, with serotypes 1 through 19 currently described, though some serotypes are further subdivided (e.g., serotype 6 subtypes K6a:O3 and K6b:O3) [56, 29]. Serotype distribution varies geographically and temporally [30, 35, 50, 61, 69]. Actinobacillus suis is also a Gram-negative, NAD-dependent coccobacillus, but it is generally considered part of the normal upper respiratory tract flora of swine, becoming pathogenic under conditions of stress or immunosuppression [2, 3]. Both species produce a range of virulence factors, including RTX (repeats in toxin) toxins, outer membrane proteins, and polysaccharide capsules [1, 6].

Pathogenesis and Virulence Factors

Apx Toxins

The primary virulence determinants of A. pleuropneumoniae are the Apx exotoxins (ApxI, ApxII, ApxIII, and ApxIVA), which belong to the RTX family of pore-forming cytolysins [24, 74]. ApxI is strongly hemolytic and cytotoxic, ApxII is weakly hemolytic but cytotoxic, ApxIII is non-hemolytic but cytotoxic, and ApxIVA is an RTX toxin expressed only in vivo and is a target for serological diagnosis [53, 29]. The combination of Apx toxins produced by a given strain correlates with its serotype and virulence potential [7, 8]. ApxI and ApxII are the most prevalent among virulent serotypes [8]. These toxins induce pore formation in host cell membranes, leading to osmotic lysis of alveolar macrophages, neutrophils, and epithelial cells [24, 74]. ApxI-induced cytotoxicity involves phosphorylation of the β2 integrin subunit CD18 and downregulation of Akt activity, promoting apoptosis in porcine alveolar macrophages [24, 82]. Native ApxIIA induces apoptosis in a concentration- and acylation-dependent manner.

Outer Membrane Vesicles (OMVs)

A. pleuropneumoniae spontaneously releases outer membrane vesicles (OMVs) that contain Apx toxins, proteases, and multiple immunogenic proteins [4, 5]. OMVs exert immunomodulatory effects on porcine alveolar macrophages, dampening innate immune responses stimulated by bacterial cells [4]. This immunomodulation may represent a mechanism of immune evasion during early infection [4]. The protein content of OMVs can be altered by genetic deletion of nlpI and palA genes, which affect OMV yield and composition [4]. OMVs have been investigated as vaccine candidates, but immunization with native OMVs did not reduce lung lesions upon challenge, and high IgG titers correlated with more extensive lesions, suggesting antibody-mediated cytotoxicity may contribute to pathology [5, 52].

Adhesins and Colonization Factors

Colonization of the porcine respiratory tract requires adherence to epithelial cells. Putative adhesins identified in A. suis include homologues of autotransporter adhesins, fimbrial proteins, and trimeric autotransporters, many of which are conserved in A. pleuropneumoniae [2]. Colonization-deficient mutants of A. suis have been generated, confirming the role of specific surface proteins in establishing infection [3]. The ApfA protein (a fimbrial subunit) and VacJ (a lipoprotein) are involved in adherence and have been evaluated as vaccine antigens [5]. The cAMP receptor protein (Crp) gene contributes to growth, stress resistance, and colonization in A. pleuropneumoniae.

Biofilm Formation

A. pleuropneumoniae forms biofilms in vitro and in vivo, which contribute to persistence and antimicrobial tolerance [83, 34]. Biofilm growth induces morphological and metabolic changes, including increased production of extracellular polysaccharides and altered expression of genes involved in energy metabolism. Ammonia exposure can induce dispersion and proliferation of A. pleuropneumoniae biofilms, potentially exacerbating infection in high-ammonia environments. Rhein, a plant-derived compound, reduces biofilm formation and kills planktonic cells.

Iron Acquisition and Oxidative Stress Resistance

Iron is essential for bacterial growth, and A. pleuropneumoniae responds to iron starvation through transcriptomic and proteomic changes that upregulate iron acquisition systems, including TonB-dependent receptors and siderophore transporters. The DppA1A2BCDF ABC transporter system is involved in glutathione utilization, oxidative stress tolerance, and virulence [9]. The antioxidant protein PntA coordinates with OmpW to resist oxidant stress. HbpA-like proteins (HbpA1 and HbpA2) protect bacteria from sulfur source limitation and oxidative and cold stresses.

Septicemia Pathogenesis

A. suis and highly virulent A. pleuropneumoniae strains can cause septicemia, characterized by bacterial dissemination via the bloodstream leading to systemic inflammation and shock [10, 1]. In A. pleuropneumoniae infection, septic shock is associated with profound hypotension, disseminated intravascular coagulation, and multi-organ failure [10]. The release of Apx toxins and LPS triggers a massive cytokine storm, including tumor necrosis factor-alpha and interleukins, which contributes to vascular leakage and tissue damage [11, 10]. Pentoxifylline, a phosphodiesterase inhibitor, has been shown to modulate inflammatory cytokine expression and reduce the severity of acute pleuropneumonia in experimental models [11].

Clinical Manifestations

Porcine Contagious Pleuropneumonia

The disease manifests in peracute, acute, and chronic forms [29, 79]. Peracute cases present with sudden death, severe dyspnea, cyanosis, and frothy bloody nasal discharge. Acute cases exhibit fever (40-42°C), anorexia, lethargy, coughing, and abdominal breathing. Chronic infection is characterized by intermittent cough, reduced growth rate, and persistent lung lesions (sequestra) that harbor viable bacteria [40, 78]. Lung lesions are typically bilateral, fibrinous, necrotizing pleuropneumonia with a characteristic "chewing gum" consistency on cut surface [78, 79]. Histopathological examination reveals necrotic foci, fibrin exudation, and infiltration of neutrophils and macrophages. Asteroid bodies have been observed in experimentally infected pigs.

Septicemia

Septicemia caused by A. suis or A. pleuropneumoniae occurs most frequently in neonatal and weaned pigs [1, 10]. Clinical signs include sudden death, fever, depression, erythema or cyanosis of the skin, and petechial hemorrhages on serosal surfaces [10]. In A. suis infection, polyserositis (pericarditis, peritonitis, pleuritis) and arthritis are common [1]. Septic shock is characterized by refractory hypotension, lactic acidosis, and coagulopathy [10]. Experimental models have demonstrated that vaccination can paradoxically exacerbate lung lesions upon challenge, suggesting a role for antibody-dependent enhancement of inflammation [5, 10].

Diagnostic Approaches

Culture and Isolation

Isolation of A. pleuropneumoniae or A. suis from clinical specimens (lung tissue, pleural fluid, nasal swabs, tonsillar swabs) requires chocolate agar or blood agar supplemented with NAD (V-factor) and incubation in 5-10% CO2 at 37°C [29, 44]. Colonies are small, grayish, and may exhibit hemolysis on blood agar (depending on Apx toxin profile). Biochemical identification can be performed using commercial identification systems, but molecular methods are preferred for definitive speciation and serotyping.

Molecular Diagnostics

Polymerase chain reaction (PCR) assays are widely used for detection and serotyping of A. pleuropneumoniae [12, 13, 44, 57, 59, 84]. Multiplex PCR assays can simultaneously detect A. pleuropneumoniae, Streptococcus suis, and Glaesserella parasuis [12, 64, 84]. Real-time PCR (qPCR) assays targeting the apxIV gene (specific for A. pleuropneumoniae) are highly sensitive and specific [13, 57]. Duplex qPCR methods allow simultaneous detection of A. pleuropneumoniae and influenza A virus [13]. Triplex real-time PCR assays have been developed for simultaneous detection of S. suis, G. parasuis, and A. pleuropneumoniae [12]. Oral fluid samples can be used for PCR-based screening of circulating serotypes in herds. Multiplex ligation-dependent probe amplification (MLPA) assays enable detection and differentiation of seven porcine respiratory pathogens, including A. pleuropneumoniae.

Serological Testing

Enzyme-linked immunosorbent assays (ELISAs) are used for serological diagnosis and surveillance [14, 15]. ELISAs targeting ApxIVA (an in vivo-expressed RTX toxin) are highly specific for detecting antibodies against A. pleuropneumoniae and can differentiate infected from vaccinated animals (DIVA) when using subunit vaccines lacking ApxIVA [29, 15]. Commercial ELISA kits are available for serotype-specific antibody detection, but cross-reactivity can occur [14, 15]. Serological monitoring is useful for herd-level prevalence assessment and for evaluating vaccine responses.

Serotyping Methods

Serotyping is essential for epidemiological studies and vaccine formulation. Traditional serotyping uses slide agglutination or immunodiffusion with serotype-specific antisera. Molecular serotyping methods, including multiplex PCR targeting capsular polysaccharide and O-antigen biosynthesis genes, have largely replaced traditional methods [44, 59, 70]. Whole genome sequencing (WGS) provides high-resolution serotyping and can identify atypical strains [16, 22, 23, 76, 80, 86]. Atypical serotype 2 isolates with rough-type LPS have been characterized. Nonencapsulated variants of serovar K12:O3 have been reported.

Serotype Distribution and Epidemiology

Serotype distribution varies by geographic region and over time [30, 35, 50, 54, 61, 69]. In Europe, serotypes 2, 3, 6, 8, and 9-11 are common [54, 59, 69]. In North America, serotypes 1, 5, 7, and 15 are prevalent [40, 64]. In Asia, serotypes 1, 2, 3, 5, 7, and 15 are frequently isolated [8, 23, 35, 76]. In South America, serotypes 1, 2, 5, 6, 8, and 12 have been reported. In Africa, serotypes 1, 2, 3, 4, 5, 7, 8, 9, 10, 11, 12, and 15 have been identified. Serotype 15 has emerged as a significant pathogen in several regions, including the United States and Korea [8, 40]. Atypical serotype 12 strains with higher virulence potential have been described. Environmental factors such as ammonia levels, stocking density, and ventilation influence disease transmission and severity [17, 34]. Coinfections with Mycoplasma hyopneumoniae, Pasteurella multocida, porcine circovirus type 2 (PCV2), and porcine reproductive and respiratory syndrome virus (PRRSV) are common and exacerbate clinical disease [32, 75].

Antimicrobial Resistance

Antimicrobial resistance (AMR) in A. pleuropneumoniae is a growing concern [8, 35, 50, 60, 66, 69, 71]. Resistance to tetracyclines, sulfonamides, and macrolides is widespread [35, 50, 69]. Florfenicol resistance has emerged, mediated by plasmid-borne resistance genes that can shuttle between A. pleuropneumoniae and G. parasuis [60, 71]. Multidrug-resistant strains have been characterized by whole genome sequencing, revealing resistance determinants against aminoglycosides, beta-lactams, phenicols, and fluoroquinolones [26, 66]. Minimum inhibitory concentration (MIC) and mutant prevention concentration (MPC) data are available for pradofloxacin and other antimicrobials [25, 55]. Pharmacodynamic interactions between trimethoprim and sulfonamides have been characterized using time-kill curves, identifying synergistic ratios [18]. Pharmacokinetic/pharmacodynamic (PK/PD) modeling has been used to optimize dosing of tylosin for co-infections with P. multocida and danofloxacin using a peristaltic pump model. The antimicrobial peptide MPX shows synergistic activity with tulathromycin and can prevent resistance development [19]. Benzylpenicillin slow-release formulations have been evaluated in an infection model.

Vaccination and Control

Bacterins and Subunit Vaccines

Commercial bacterins (killed whole-cell vaccines) provide serotype-specific protection but limited cross-protection [31, 39]. Subunit vaccines containing Apx toxins, outer membrane proteins, and capsular antigens have been developed [6, 31, 73]. Immunoproteomic analysis has identified 30 immunogenic proteins from outer membrane and extracellular fractions, including 24 novel antigens [6]. Recombinant fusion proteins, such as LpxC-ApxIVA, have been evaluated for cross-protection against G. parasuis and A. pleuropneumoniae in mice. A bicistronic T7 expression system has been developed for hyper-production of subunit vaccine proteins in Escherichia coli.

Live Attenuated Vaccines

Live attenuated vaccines, such as the gene-deleted HB04M strain, can be administered intramuscularly or intranasally and provide rapid protection against heterologous challenge. Attenuation strategies include deletion of toxin genes, metabolic pathway genes, or regulatory genes [31, 39].

OMV-Based Vaccines

OMV-based vaccines have been tested in pigs, but results have been disappointing. Immunization with OMVs induced high IgG titers but did not reduce lung lesions; instead, higher antibody levels correlated with more severe lesions [5, 52]. This suggests that antibody-mediated cytotoxicity may play a role in pathogenesis [5]. Extracellular vesicles derived from A. pleuropneumoniae have been evaluated as novel vaccine candidates, but protection was not achieved.

Adjuvants and Delivery

Oil-based adjuvants administered dermally have been evaluated for their ability to enhance immune responses. The addition of OMVs to recombinant protein vaccines increased specific IgG titers, confirming their adjuvant potential, but this did not translate into protection [5]. Intranasal delivery of live attenuated vaccines induces mucosal immunity and may provide better protection at the portal of entry.

Control Strategies

Control of A. pleuropneumoniae relies on a combination of biosecurity, management practices, vaccination, and antimicrobial therapy [17, 29, 40]. All-in/all-out production, adequate ventilation, and reduction of ammonia levels help reduce transmission [17, 34]. Eradication programs based on depopulation and repopulation or test-and-removal using serological screening have been successful in some regions [29, 33]. Whole genome sequencing has been used to trace transmission within vertically integrated systems.

Diagnostic Workflow

The following Mermaid diagram illustrates a recommended diagnostic workflow for suspected Actinobacillus infection in pigs.

flowchart TD
 A["Clinical signs: dyspnea, fever, sudden death, lung lesions"] --> B["Sample collection: lung tissue, pleural fluid, nasal swab, tonsil swab, oral fluid"]
 B --> C{Initial testing}
 C --> D[Bacterial culture on chocolate agar + NAD, 5% CO2, 37°C]
 C --> E[Real-time PCR for ApxIV gene]
 D --> F[Gram stain, colony morphology, hemolysis]
 F --> G[Biochemical identification or MALDI-TOF]
 G --> H["Serotyping: multiplex PCR or slide agglutination"]
 E --> H
 H --> I["Antimicrobial susceptibility testing: MIC, MPC"]
 I --> J["Treatment selection: based on PK/PD data"]
 J --> K["Monitor response; consider vaccination"]
 H --> L["Epidemiological typing: WGS for outbreak tracing"]
 L --> M["Implement control measures: biosecurity, all-in/all-out, vaccination"]

Conclusion

Actinobacillus pleuropneumoniae and Actinobacillus suis remain major causes of respiratory disease and septicemia in swine, with significant economic impact. Advances in molecular diagnostics, including multiplex and real-time PCR assays, have improved detection and serotyping. The emergence of antimicrobial resistance, particularly to florfenicol and tetracyclines, underscores the need for prudent antimicrobial use and alternative control strategies. Vaccine development continues to face challenges due to serotype diversity and the complex immunopathogenesis of the disease, where antibody responses may contribute to tissue damage. Future research should focus on identifying protective antigens that induce robust cellular immunity and on developing DIVA-compatible vaccines to facilitate eradication programs.

References

[1] Rycroft AN, Garside LH. Actinobacillus species and their role in animal disease. Vet J. 2000. URL: https://pubmed.ncbi.nlm.nih.gov/10640409/

[2] Bujold AR, MacInnes JI. Identification of putative adhesins of Actinobacillus suis and their homologues in other members of the family Pasteurellaceae. BMC Res Notes. 2015. URL: https://pubmed.ncbi.nlm.nih.gov/26567540/

[3] Ojha S, Lacouture S, Gottschalk M, et al. Characterization of colonization-deficient mutants of Actinobacillus suis. Vet Microbiol. 2010. URL: https://pubmed.ncbi.nlm.nih.gov/19664889/

[4] Zhuang Z, Antenucci F, Winther-Larsen HC, et al. Outer Membrane Vesicles of Actinobacillus pleuropneumoniae Exert Immunomodulatory Effects on Porcine Alveolar Macrophages. Microbiology spectrum. 2022. URL: https://www.semanticscholar.org/paper/6a648a663cbaf4a10e307d7d06c21ed0079445f3

[5] Antenucci F, Fougeroux C, Deeney A, et al. In vivo testing of novel vaccine prototypes against Actinobacillus pleuropneumoniae. Veterinary Research. 2018. URL: https://www.semanticscholar.org/paper/7d3d8fd2b8b6c32eddf4fe2fcfb2d36740faf680

[6] Liao Y, Deng J, Zhang A, et al. Immunoproteomic analysis of outer membrane proteins and extracellular proteins of Actinobacillus pleuropneumoniae JL03 serotype 3. BMC Microbiology. 2009. URL: https://www.semanticscholar.org/paper/39cf9bc2ea2712fa6a35952c51dd0ce052032ae1

[7] Komal JP, Mittal KR. Grouping of Actinobacillus pleuropneumoniae strains of serotypes 1 through 12 on the basis of their virulence in mice. Vet Microbiol. 1990. URL: https://pubmed.ncbi.nlm.nih.gov/2281607/

[8] Bae DY, Kang EJ, Cho YC, et al. Actinobacillus pleuropneumoniae in Korea (2015-2025): serovar distribution, toxin gene profiles, antimicrobial resistance, and identification of an apxIICA-deficient serovar 15 profile. J Vet Sci. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42225609/

[9] Xu H, Xiao J, Li Z, et al. Characterization of the dppA1A2BCDF genes encoding a putative ABC transporter in Actinobacillus pleuropneumoniae: Role in glutathione utilization, oxidative stress tolerance and virulence. Microb Pathog. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/41707844/

[10] Elvira SM, Pérez JT, Ortega MET, et al. Septic shock in pigs infected, vaccinated and challenged with Actinobacillus pleuropneumoniae: A clinicopathological study. Vet J. 2025. URL: https://pubmed.ncbi.nlm.nih.gov/41248799/

[11] Myers MJ, Baarsch MJ, Murtaugh MP. Effects of pentoxifylline on inflammatory cytokine expression and acute pleuropneumonia in swine. Immunobiology. 2002. URL: https://pubmed.ncbi.nlm.nih.gov/11999342/

[12] Zhuang H, Kang S, Zheng C, et al. A Triplex Real-Time PCR Assay for Simultaneous Detection of Streptococcus suis, Glaesserella parasuis, and Actinobacillus pleuropneumoniae. Transbound Emerg Dis. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42181751/

[13] Ren YJ, Lin H, Yu DJ, et al. A duplex real-time fluorescent quantitative PCR method for simultaneous and rapid detection of Actinobacillus pleuropneumoniae and influenza A virus. Front Vet Sci. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/41810404/

[14] Willson P, Schipper C, Morgan E. The Use of an Enzyme-linked Immunosorbent Assay for Diagnosis of Actinobacillus pleuropneumoniae Infection in Pigs. The Canadian veterinary journal. 1988. URL: https://www.semanticscholar.org/paper/bf82452b266551d85f8f23e612449f8013b40da8

[16] Lee ES, Kyung SM, Lee JH, et al. Revealing genetic variation of Actinobacillus pleuropneumoniae Korean isolates using whole genome sequence analysis. J Microbiol. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42252973/

[17] Barna I, Tașca I, Toader AI, et al. Epidemiological Analysis of Environmental Factors Affecting Porcine Pleuropneumonia in a Herd Endemic for Actinobacillus pleuropneumoniae. Pathogens. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42198639/

[18] Mead A, Viel A, Boulanger M, et al. Pharmacodynamic interaction between trimethoprim and sulphonamides for A. pleuropneumoniae: synergistic ratios, optimal potentiation, and dynamic responses assessed via time-kill curves. J Appl Microbiol. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42033319/

[19] Wang H, Wang L, Zhang W, et al. Synergistic effect and resistance prevention activity of antimicrobial peptide MPX with Tulathromycin against Actinobacillus pleuropneumoniae. Front Microbiol. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42137811/

[20] Xia R, Xing W, Zhu Z, et al. Mechanisms by which SIRT2 modulates alveolar macrophage immunoreactivity to intervene in Actinobacillus pleuropneumoniae infection in piglets under cold stimulation. Vet Res. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42289692/

[21] Zhu J, Zhu S, Xia C, et al. Single-cell landscape of piglet lung response with Actinobacillus pleuropneumoniae. Virulence. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/41843736/

Disclaimer: This article is for educational and informational purposes only. It is not intended to substitute for professional veterinary advice, diagnosis, treatment, or regulatory guidance. Always consult a licensed veterinarian or qualified specialist regarding animal health, disease diagnosis, and therapeutic decisions.