Actinobacillus pleuropneumoniae (APP) in Pigs: Fibrinohemorrhagic Pleuropneumonia and Diagnosis

Etiology and Taxonomic Classification

Actinobacillus pleuropneumoniae is a Gram-negative, facultatively anaerobic, nonmotile coccobacillus belonging to the family Pasteurellaceae [1]. The organism is the primary etiologic agent of porcine pleuropneumonia, a highly contagious and often fatal respiratory disease characterized by fibrinohemorrhagic and necrotizing lung lesions [1, 2]. The species is divided into two biovars based on nicotinamide adenine dinucleotide (NAD) dependence. Biovar 1 strains are NAD-dependent, whereas biovar 2 strains are NAD-independent [1]. To date, 19 serovars have been described based on capsular polysaccharide and lipopolysaccharide (LPS) antigenic diversity, with serovar 15 being a more recently recognized type [3]. The serovar classification is critical for epidemiological tracking and vaccine formulation, as cross-protection between serovars is limited [1, 3].

Virulence Factors and Pathogenesis

The pathogenesis of A. pleuropneumoniae infection is multifactorial, involving a suite of virulence determinants that facilitate colonization, immune evasion, and tissue destruction. The primary virulence factors are the Apx exotoxins (ApxI, ApxII, ApxIII, and ApxIV), which belong to the repeats-in-toxin (RTX) family of pore-forming cytolysins [4]. ApxI is strongly hemolytic and cytotoxic, ApxII is weakly hemolytic but cytotoxic, and ApxIII is nonhemolytic but cytotoxic [4]. ApxIV is produced only in vivo and is a species-specific marker used for serological diagnosis [4]. These toxins induce the release of proinflammatory cytokines, including interleukin-1 beta (IL-1β), interleukin-8 (IL-8), and tumor necrosis factor-alpha (TNF-α), from porcine alveolar macrophages, thereby amplifying the inflammatory cascade and contributing to the characteristic fibrinohemorrhagic necrosis [4].

Adhesion to the porcine respiratory epithelium is a prerequisite for colonization. A. pleuropneumoniae adheres to host cells via multiple adhesins, including fimbriae, outer membrane proteins, and LPS [2, 5]. The bacterium binds specifically to phosphatidylethanolamine, a phospholipid exposed on the surface of porcine respiratory tract cells [2]. Monoclonal antibodies directed against the LPS core and O-antigen have been shown to inhibit this adherence, confirming the role of LPS as a critical adhesin [5]. An atypical strain of serovar 1, which possesses a truncated LPS outer core and lacks O-antigen, has been isolated, suggesting that the O-antigen is not absolutely required for virulence but may modulate host immune recognition [1].

Clinical Presentation and Pathology

The incubation period for porcine pleuropneumonia ranges from a few hours to several days, depending on the infective dose, serovar virulence, and host immune status [3]. Peracute infections result in sudden death with minimal premonitory signs. Acute cases present with severe dyspnea, open-mouth breathing, cyanosis, pyrexia (40.5 to 41.5 degrees Celsius), and a characteristic frothy, blood-tinged nasal discharge [3]. Chronically infected pigs may exhibit a persistent cough, reduced growth rates, and intermittent pyrexia, serving as subclinical carriers that perpetuate herd transmission [3].

Gross pathological findings are confined to the respiratory tract. The hallmark lesion is a fibrinohemorrhagic and necrotizing pleuropneumonia, typically affecting the dorsocaudal regions of the diaphragmatic lung lobes [3]. Affected lung tissue is firm, dark red to black, and covered with a thick layer of fibrin. Cut surfaces reveal a marbled appearance due to interlobular edema and hemorrhage. Histologically, lesions are characterized by coagulative necrosis, massive infiltration of neutrophils, and the presence of fibrin thrombi within pulmonary vessels [3]. In experimentally infected pigs with serovar 15, asteroid bodies (Splendore-Hoeppli phenomenon) have been observed within pulmonary lesions, representing antigen-antibody complexes surrounded by eosinophilic material [3].

Host Range and Transmission

A. pleuropneumoniae is a host-specific pathogen of swine. No other domestic or wild animal species are considered natural hosts, and the bacterium is not zoonotic [1]. Transmission occurs primarily via direct contact between infected and susceptible pigs through respiratory droplets and aerosols [1]. Fomites, contaminated equipment, and personnel can also contribute to mechanical spread. Carrier animals, particularly those with chronic or subclinical infections, are the primary reservoir for maintaining the pathogen within a herd [1]. Stressors such as overcrowding, poor ventilation, temperature fluctuations, and commingling of pigs from different sources precipitate clinical outbreaks [1].

Diagnostic Approaches

Accurate and timely diagnosis of A. pleuropneumoniae infection is essential for implementing control measures and reducing economic losses. A combination of clinical observation, gross pathology, histopathology, bacteriological culture, and molecular techniques is recommended.

Bacteriological Culture and Identification

Isolation of A. pleuropneumoniae from lung tissue, pleural fluid, or nasal swabs remains the gold standard for definitive diagnosis [1]. Samples should be collected from acutely affected, untreated animals. The organism grows on chocolate agar or blood agar supplemented with NAD (for biovar 1) when incubated at 37 degrees Celsius in 5 to 10 percent carbon dioxide [1]. Colonies are small, grayish, and mucoid, with a characteristic satellite growth pattern around a nurse colony (e.g., Staphylococcus aureus) that provides NAD. A positive Christie-Atkins-Munch-Petersen (CAMP) reaction with beta-hemolytic Staphylococcus aureus is a useful phenotypic marker [1]. Biochemical identification can be performed using commercial identification systems, though these may misidentify atypical strains [1].

Serological Testing

Serological assays are used for herd-level surveillance and to identify carrier animals. The most widely used method is the enzyme-linked immunosorbent assay (ELISA) targeting ApxIV, a toxin produced exclusively in vivo [4]. This approach provides high specificity and distinguishes naturally infected pigs from those vaccinated with bacterins lacking ApxIV [4]. Complement fixation tests and indirect hemagglutination assays are also available but are less commonly employed due to lower sensitivity and specificity [1].

Molecular Diagnostics

Polymerase chain reaction (PCR) assays offer rapid, sensitive, and specific detection of A. pleuropneumoniae directly from clinical specimens. Multiplex PCR panels targeting the Apx toxin genes (apxI, apxII, apxIII, apxIV) and serovar-specific capsular polysaccharide genes enable simultaneous detection and serotyping [1, 4]. Real-time quantitative PCR (qPCR) provides quantification of bacterial load, which can be correlated with disease severity. High-resolution melting analysis and loop-mediated isothermal amplification (LAMP) are emerging techniques that offer field-deployable alternatives to conventional PCR [1].

Histopathology and Immunohistochemistry

Histopathological examination of formalin-fixed, paraffin-embedded lung tissue reveals the characteristic fibrinohemorrhagic and necrotizing lesions. Immunohistochemistry using monoclonal antibodies directed against A. pleuropneumoniae LPS or capsular antigens can confirm the presence of the bacterium within lesions, particularly in cases where culture is negative due to prior antimicrobial therapy [3, 5].

Diagnostic Workflow

The following Mermaid diagram illustrates a recommended diagnostic decision tree for suspected porcine pleuropneumonia.

flowchart TD
    A["Clinical suspicion: acute dyspnea, fever, sudden death"] --> B[Postmortem examination]
    B --> C{"Gross lesions: fibrinohemorrhagic pleuropneumonia?"}
    C -->|Yes| D[Collect lung tissue, pleural fluid, nasal swabs]
    C -->|No| E[Consider other respiratory pathogens]
    D --> F[Bacteriological culture on NAD-supplemented media]
    F --> G["Isolate identification: CAMP test, biochemical profile"]
    G --> H["Serovar determination: multiplex PCR or serotyping"]
    D --> I[DNA extraction for molecular testing]
    I --> J[Real-time PCR targeting apxIV and serovar-specific genes]
    J --> K["Positive: confirm APP infection"]
    J --> L["Negative: consider culture or histopathology"]
    D --> M[Histopathology and immunohistochemistry]
    M --> N["Characteristic lesions + positive IHC: confirm diagnosis"]
    F --> O[Antimicrobial susceptibility testing]
    O --> P[Guide treatment and herd management]

Differential Diagnosis

The clinical and pathological presentation of porcine pleuropneumonia must be differentiated from other swine respiratory diseases. Key differentials include infection with Pasteurella multocida (which typically causes a suppurative bronchopneumonia rather than a fibrinohemorrhagic pleuropneumonia), Haemophilus parasuis (Glasser's disease, which presents with polyserositis and arthritis), Streptococcus suis (meningitis and arthritis), and Mycoplasma hyopneumoniae (enzootic pneumonia with cranioventral consolidation) [1]. Coinfections with porcine reproductive and respiratory syndrome virus (PRRSV) or swine influenza virus can exacerbate the severity of A. pleuropneumoniae infection [1].

Control and Prevention

Control strategies are based on biosecurity, management practices, and vaccination. All-in/all-out production systems, strict quarantine of incoming stock, and segregation of age groups reduce the risk of introduction and spread [1]. Commercial bacterins provide partial protection against homologous serovars but do not prevent colonization or the carrier state [1]. Autogenous vaccines formulated from herd-specific isolates may offer improved coverage. Antimicrobial therapy, including injectable ceftiofur, florfenicol, or tulathromycin, is effective in treating acute cases, but antimicrobial susceptibility testing is recommended to guide selection and mitigate resistance development [1].

References

[1] Jacques M, Labrie J, St Michael F, et al. Isolation of an atypical strain of Actinobacillus pleuropneumoniae serotype 1 with a truncated lipopolysaccharide outer core and no O-antigen. J Clin Microbiol. 2005. URL: https://pubmed.ncbi.nlm.nih.gov/16000496/

[2] Jeannotte ME, Abul-Milh M, Dubreuil JD, et al. Binding of Actinobacillus pleuropneumoniae to phosphatidylethanolamine. Infect Immun. 2003. URL: https://pubmed.ncbi.nlm.nih.gov/12874346/

[3] To H, Konnai M, Teshima K, et al. Pulmonary lesions with asteroid bodies in a pig experimentally infected with Actinobacillus pleuropneumoniae serovar 15. J Vet Med Sci. 2023. URL: https://pubmed.ncbi.nlm.nih.gov/37612056/

[4] Chen ZW, Chien MS, Chang NY, et al. Mechanisms underlying Actinobacillus pleuropneumoniae exotoxin ApxI induced expression of IL-1β, IL-8 and TNF-α in porcine alveolar macrophages. Vet Res. 2011. URL: https://pubmed.ncbi.nlm.nih.gov/21314908/

[5] Paradis SE, Dubreuil JD, Gottschalk M, et al. Inhibition of adherence of Actinobacillus pleuropneumoniae to porcine respiratory tract cells by monoclonal antibodies directed against LPS and partial characterization of the LPS receptors. Curr Microbiol. 1999. URL: https://pubmed.ncbi.nlm.nih.gov/10525835/

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.