Actinobacillus suis: Respiratory and Septicemic Disease in Pigs – Diagnosis and Control

Etiology and Taxonomy

Actinobacillus suis is a Gram-negative, facultatively anaerobic, pleomorphic coccobacillus belonging to the family Pasteurellaceae. The organism is phylogenetically related to Actinobacillus pleuropneumoniae and Haemophilus parasuis (now Glaesserella parasuis) [1]. A. suis requires nicotinamide adenine dinucleotide (NAD; V factor) for growth but does not require hemin (X factor), distinguishing it from certain other Pasteurellaceae. Colonies on chocolate or supplemented blood agar are small, smooth, and grayish, with a characteristic sweetish odor.

The bacterium possesses a range of virulence factors, including capsular polysaccharide, lipopolysaccharide (LPS), outer membrane proteins, and RTX (repeats in toxin) toxins. Genomic analyses have revealed that A. suis isolates harbor variable numbers of toxin gene copies (apxI, apxII) which correlate with virulence potential [2]. Serotyping is not routinely performed, but most clinical isolates belong to serovars 1 and 2, with serovar 1 associated more frequently with septicemia.

Epidemiology

A. suis is a commensal inhabitant of the upper respiratory tract and tonsils of clinically healthy pigs, including both domestic swine and feral populations [3, 4]. The primary reservoir is the carrier pig, and transmission occurs horizontally via direct contact, aerosolized respiratory secretions, and contaminated fomites. The tonsil serves as a major colonization site; A. suis adheres to extracellular matrix components of the tonsillar epithelium, facilitating persistent colonization [5].

Disease is most commonly observed in nursery and early grow-finish pigs, typically between 3 and 12 weeks of age, when maternal antibody wanes and stressors such as weaning, mixing, transport, and concurrent viral infections predispose to clinical disease [1]. Outbreaks can occur in herds naïve to the pathogen or in situations where virulent strains are introduced. The prevalence of A. suis in respiratory disease complex cases varies; in diagnostic surveillance from a US veterinary laboratory (2017-2022), A. suis was detected in approximately 5.2% of submissions [1], while cross-sectional studies in China reported detection in 7.3% of porcine respiratory disease complex (PRDC) cases [6, 7]. Infection dynamics are influenced by co-infections with primary viral pathogens such as porcine reproductive and respiratory syndrome virus (PRRSV) and swine influenza virus [8, 9].

Clinical Signs and Pathogenesis

Two major clinical presentations are recognized: acute septicemia and subacute/chronic respiratory disease. In peracute septicemia, pigs present with fever (40.5-42.0°C), lethargy, anorexia, cyanosis of the extremities, and sudden death. Cutaneous lesions, including erythema, petechiae, and ecchymoses, are sometimes noted, resembling those seen in erysipelas and other Gram-negative septicemias [10].

The respiratory form is characterized by cough, dyspnea, tachypnea, and nasal discharge. Affected pigs may adopt a dog-sitting posture to facilitate breathing. Morbidity can reach 20-30% in affected groups, with mortality varying from 2% to 50% in untreated septicemic outbreaks.

Pathogenetically, A. suis invades the tonsillar crypts and multiplies in the upper respiratory tract. Under immunosuppressive conditions or in the presence of the porcine respiratory disease complex (PRDC), the organism breaches mucosal barriers and enters the bloodstream, causing septicemia with disseminated intravascular coagulation and multi-organ failure. The RTX toxins (ApxI, ApxII) induce pore formation in host cell membranes, leading to necrosis of alveolar macrophages and pulmonary endothelium, resulting in pulmonary edema, fibrinous pleuritis, and hemorrhagic necrosis [2]. Lipid profiling of A. suis during co-infection with PRRSV shows altered host lipid metabolism, potentially enhancing bacterial survival [11].

Pathology

Gross lesions in septicemic cases include generalized vascular congestion, petechial hemorrhages on serosal surfaces, epicardium, and kidneys, as well as fibrinonecrotic pneumonia. Serosanguinous pleural effusion and fibrinous polyserositis (pleuritis, pericarditis, peritonitis) are common. The lungs often exhibit cranioventral consolidation with multifocal necrotic foci. In the respiratory form, lesions are predominantly in the cranial and middle lung lobes, with purulent bronchopneumonia and interlobular edema. Histologically, there is necrotizing bronchopneumonia with intense infiltration of neutrophils, fibrin exudation, and thrombosis of pulmonary vessels. These lesions are difficult to differentiate from those caused by A. pleuropneumoniae and other Pasteurellaceae without ancillary diagnostic testing [12, 13].

Diagnosis

Accurate diagnosis of A. suis infection requires a combination of clinical, pathological, and laboratory methods.

Sample Collection

Suitable specimens include: tonsil swabs, nasal swabs, bronchoalveolar lavage fluid, lung tissue (affected and unaffected), pleural fluid, and blood from acutely febrile pigs. Swabs should be placed in transport medium (e.g., Amies with charcoal) and shipped refrigerated. Postmortem samples should include fresh lung and tonsil for culture and molecular testing.

Bacteriological Culture

A. suis grows on chocolate agar or blood agar supplemented with NAD (V factor) when incubated at 37°C in 5% CO2. Satellite growth around a Staphylococcus aureus nurse streak (which provides NAD) is a classical isolation method. Colonies appear after 24-48 hours as small, grayish, translucent, and non-hemolytic (or weakly hemolytic on bovine blood agar). Biochemical identification reveals urease-positive, oxidase-positive, indole-negative, and fermentation of glucose, sucrose, and mannitol. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) provides rapid and accurate identification [1].

Molecular Diagnostics

Polymerase chain reaction (PCR) assays targeting the apxIV gene (conserved among Actinobacillus species) or the 16S rRNA gene are used for species-specific detection. Multiplex real-time PCR panels allow simultaneous detection of A. suis alongside other PRDC pathogens such as Streptococcus suis, Glaesserella parasuis, and A. pleuropneumoniae [14, 15, 16, 17]. A triplex real-time PCR for S. suis, G. parasuis, and A. pleuropneumoniae has been validated [14]; incorporating A. suis in similar panels is recommended for diagnostic efficiency.

Targeted next-generation sequencing (tNGS) panels offer a comprehensive approach, enabling simultaneous detection and differentiation of multiple bacterial and viral respiratory pathogens, including A. suis, with high sensitivity and specificity [18]. This technology can be particularly useful for polymicrobial PRDC cases.

Serological assays, such as ELISA, are available for population-level screening to detect exposure, but are not generally used for individual diagnosis.

Mermaid Diagnostic Workflow Diagram

flowchart TD
    A[Acute respiratory disease / sudden death in pigs 3-12 wks], > B{Postmortem examination}
    B, > C[Gross lesions: fibrinous pneumonia, serosanguinous pleural effusion, petechiae]
    C, > D[Select fresh lung, tonsil, pleural fluid]
    D, > E{Primary laboratory methods}
    E, > F[Gram stain: Gram-negative coccobacilli]
    E, > G[Culture on NAD-supplemented agar (chocolate / satellite growth)]
    G, > H[Biochemical / MALDI-TOF identification]
    E, > I[Molecular detection: real-time multiplex PCR (apxIV / 16S rRNA) or tNGS panel]
    H, > J[Positive: Actinobacillus suis]
    I, > J
    J, > K{Antimicrobial susceptibility testing}
    K, > L[MIC determination by broth microdilution or disk diffusion]
    L, > M[Select appropriate antimicrobial based on susceptibility profile]

Treatment

Antimicrobial therapy should be initiated immediately based on clinical suspicion, ideally guided by antimicrobial susceptibility testing (AST) of the isolated strain. A. suis is generally susceptible to beta-lactams, cephalosporins, fluoroquinolones, trimethoprim-sulfonamides, and certain macrolides, but resistance patterns vary geographically.

Commonly Used Antimicrobials

Beta-lactams: Ceftiofur (third-generation cephalosporin) is frequently used for parenteral treatment of septicemic and respiratory infections. Pharmacokinetic/pharmacodynamic (PK/PD) modeling supports optimized dosing regimens for ceftiofur to achieve target attainment against porcine respiratory pathogens [19]. Amoxicillin administered in drinking water has been evaluated for respiratory pathogens, but time-restricted dosing may be necessary to improve exposure and efficacy; predicted PK/PD inadequacy has been noted for amoxicillin in water against A. suis [20, 21].

Fluoroquinolones: Pradofloxacin and marbofloxacin exhibit strong bactericidal activity against A. suis and other swine respiratory pathogens [22, 23]. Resistance is still low in European isolates (less than 5%) [24].

Macrolides: Gamithromycin administered intramuscularly has shown efficacy against bacterial swine respiratory disease, though data specific to A. suis are limited [25].

Other classes: Trimethoprim-sulfonamides, tetracyclines, and florfenicol are also options, but susceptibility must be confirmed. European surveillance of respiratory pathogens from pigs (2009-2012; 2018-2021) showed that A. suis maintains high susceptibility to ceftiofur, tiamulin, and florfenicol, while resistance to tetracyclines and sulfonamides is more common [26, 27, 28, 24].

The emergence of multidrug resistance (MDR) is a concern. Applying standardized definitions for MDR, extensive drug resistance (XDR), and pandrug resistance (PDR) to veterinary pathogens [29] is important for monitoring. In vitro mixed biofilm formation between A. suis and other porcine respiratory bacteria (e.g., S. suis) can reduce antimicrobial susceptibility [30], complicating therapy in polymicrobial infections.

Control and Prevention

Control of A. suis infections relies on a combination of management practices, vaccination, and prudent antimicrobial use.

Biosecurity and Management

All-in/all-out production and strict hygiene reduce pathogen load and transmission. Segregated early weaning can reduce exposure to older carrier dams [31]. Minimizing stressors such as overcrowding, poor ventilation, and temperature fluctuations is critical. Good ventilation systems help reduce ammonia levels and aerosolized bacteria.

Biosecurity measures include quarantine of incoming stock, footbaths, and dedicated farm clothing. Feral swine should be excluded from domestic pig premises, as they may carry A. suis and other bacterial pathogens [3, 4].

Vaccination

Commercial vaccines for A. suis are not widely available in all regions. Autogenous (bacterin) vaccines are sometimes used in endemically infected herds. The efficacy of bacterial vaccines for swine respiratory disease has been systematically reviewed, but evidence specific to A. suis is limited [32]. Vaccination combined with other control measures may reduce the severity of outbreaks.

Antimicrobial Stewardship

Prophylactic antimicrobial use should be minimized and replaced with metaphylaxis targeting affected groups. Controlled time-restricted dosing of amoxicillin in water has been proposed to improve exposure while reducing overall antimicrobial consumption [20]. Regular surveillance of antimicrobial resistance patterns (e.g., through national programmes) is essential to guide empirical therapy [26, 28].

Role of Coinfections

Given the importance of viral triggers (especially PRRSV), control of primary viral infections through vaccination and biosecurity indirectly reduces the incidence of secondary A. suis disease [8, 9]. The impact of feed additives on clinical symptoms and the microbiome has been studied in PRRSV/S. suis co-infections [33]; similar approaches may benefit A. suis control, although data are scarce.

In summary, A. suis is an important but sometimes overlooked component of the porcine respiratory disease complex and a cause of acute septicemia in young pigs. Accurate diagnosis using culture and molecular methods, combined with AST-guided therapy and robust preventive management, is essential to reduce losses. Continued surveillance and research into vaccine development and antimicrobial alternatives are warranted.

References

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