Swine Actinobacillus Pleuropneumonia: Diagnosis and Treatment

Actinobacillus pleuropneumoniae (APP) causes contagious porcine pleuropneumonia, a respiratory disease responsible for economic losses in swine herds worldwide. This article provides veterinarians and swine health professionals with a syndrome-level investigation framework covering clinical presentation, postmortem lesions, diagnostic methods including culture and PCR, antimicrobial therapy considerations, and vaccination strategies. The content draws on peer-reviewed literature and official veterinary references to support evidence-led decision making in herd health management.

At a Glance

Etiology and Epidemiology

Actinobacillus pleuropneumoniae is a Gram-negative, facultatively anaerobic coccobacillus belonging to the Pasteurellaceae family. The organism was previously classified as Haemophilus pleuropneumoniae, and historical literature may use this nomenclature [10][11][14]. APP is the primary causative agent of contagious porcine pleuropneumonia, a disease recognized globally in swine-producing regions [4].

The bacterium produces several virulence factors, including Apx exotoxins (ApxI, ApxII, ApxIII, ApxIVA), lipopolysaccharides, and capsule polysaccharides. These factors contribute to the characteristic pulmonary necrosis and hemorrhagic lesions observed in affected pigs. The apxIVA gene is species-specific and serves as a reliable target for molecular diagnostic methods [5].

APP is classified into two biotypes based on nicotinamide adenine dinucleotide (NAD) dependence. Biotype 1 requires NAD for growth, while biotype 2 is NAD-independent. Biotype 2 has been documented in growing and finishing pigs, expanding the epidemiological understanding of the disease [8]. Within biotype 1, at least 19 serovars have been identified based on capsular polysaccharide antigens. Serovar distribution varies geographically, and knowledge of local serovar prevalence is important for vaccine selection and epidemiological monitoring [7].

Transmission occurs primarily through direct contact between infected and susceptible pigs via respiratory secretions. Aerosol transmission over short distances is possible, particularly in confined housing systems. Carrier animals that harbor the organism in tonsillar crypts or nasal cavities serve as reservoirs for herd-to-herd spread. Subclinical infections are common, especially in commingled feeder swine, where stress and mixing facilitate pathogen transmission without immediate clinical signs [15].

Clinical Signs and Disease Presentation

The clinical presentation of APP infection ranges from peracute death to chronic, subclinical carriage. The severity depends on host immunity, serovar virulence, concurrent infections, and environmental stressors.

Peracute and Acute Forms

Peracute cases present with sudden death in apparently healthy pigs, often without premonitory signs. Affected animals may be found dead with cyanotic extremities and bloody froth from the mouth or nostrils. Pigs observed before death show severe respiratory distress characterized by open-mouth breathing, abdominal breathing, and extended head and neck posture. Rectal temperatures typically exceed 41.0 degrees Celsius. Anorexia, lethargy, and reluctance to move are common. Vomiting and diarrhea may occur in some cases.

Acute cases develop over 12 to 24 hours with progressive dyspnea, coughing, and fever. Pigs may adopt a dog-sitting posture to facilitate breathing. Cyanosis of the ears, snout, and ventral abdomen indicates severe hypoxemia. Mortality in untreated acute outbreaks can reach 20 to 30 percent in affected groups.

Subacute and Chronic Forms

Subacute cases present with milder respiratory signs, reduced feed intake, and uneven growth within the group. Chronic infections are characterized by intermittent coughing, poor growth performance, and reduced feed conversion efficiency. Chronically infected pigs may develop pulmonary abscesses and pleural adhesions visible at slaughter. These animals serve as subclinical carriers that perpetuate herd infection.

Subclinical infection is particularly relevant in commingled feeder swine operations. Pigs from multiple sources are mixed during the nursery or grower phase, allowing APP to spread without immediate clinical signs. Stressors such as transportation, temperature fluctuations, and poor ventilation can trigger acute disease outbreaks in these populations [15].

Postmortem Lesions

Postmortem examination provides critical diagnostic information in suspected APP cases. Lesions are typically confined to the respiratory tract, with the most severe changes in the lungs and pleura.

Gross Pathology

The hallmark lesion of acute APP infection is a fibrinous, hemorrhagic, necrotizing pleuropneumonia. Affected lung lobes are firm, dark red to black, and enlarged. Cut surfaces reveal a marbled appearance with alternating areas of hemorrhage, necrosis, and fibrin deposition. The lesions are often sharply demarcated from adjacent normal lung tissue and most frequently involve the diaphragmatic and cardiac lobes.

Pleural surfaces are covered with a thick layer of yellow to tan fibrin. Fibrinous adhesions between the visceral and parietal pleura develop rapidly, often within 24 to 48 hours of clinical onset. The pleural cavity may contain serosanguinous fluid mixed with fibrin clots.

In chronic cases, pulmonary abscesses and sequestra are common. These encapsulated necrotic foci contain caseous material and are surrounded by fibrous connective tissue. Pleural adhesions become organized and fibrotic, requiring manual separation during slaughter inspection.

Histopathology

Microscopic examination reveals coagulative necrosis of pulmonary parenchyma with infiltration of neutrophils and macrophages. Fibrin thrombi are present in blood vessels and lymphatics, contributing to ischemic necrosis. Gram-negative coccobacilli may be visible within necrotic tissue and alveolar macrophages. Chronic lesions show fibrous encapsulation of necrotic foci and granulomatous inflammation.

Diagnostic Methods

Accurate diagnosis of APP infection requires integration of clinical history, postmortem findings, and laboratory confirmation. Multiple diagnostic modalities are available, each with specific advantages and limitations.

Sample Collection and Submission

Appropriate sample selection and handling directly affect diagnostic accuracy. For bacteriological culture, collect lung tissue from the interface between affected and normal parenchyma. Swabs of pleural fluid, bronchial exudate, or tonsillar crypts are also suitable. Samples should be placed in transport medium and refrigerated during shipment. For PCR testing, lung tissue or tonsillar swabs in sterile containers without transport medium are acceptable.

For serotyping and antimicrobial susceptibility testing, pure isolates are required. Submit fresh lung tissue or swabs to a diagnostic laboratory capable of APP isolation and characterization.

Bacterial Culture

APP is a fastidious organism that requires enriched media for primary isolation. Chocolate agar supplemented with NAD (for biotype 1) or blood agar with a Staphylococcus nurse streak provides the necessary growth factors. Cultures are incubated at 37 degrees Celsius in 5 to 10 percent carbon dioxide for 24 to 48 hours.

Colonies appear as small, grayish, translucent, and mucoid. A characteristic satelliting pattern around the nurse streak is observed for NAD-dependent strains. Gram staining reveals Gram-negative coccobacilli. Biochemical tests, including urease production, catalase activity, and sugar fermentation patterns, confirm the identification.

Culture allows for subsequent serotyping and antimicrobial susceptibility testing, which are essential for treatment decisions and epidemiological monitoring. However, culture sensitivity is reduced in samples from chronically infected pigs or animals that have received antimicrobial therapy prior to sampling.

Polymerase Chain Reaction (PCR)

PCR targeting the apxIVA gene provides rapid and specific detection of APP directly from clinical samples. The apxIVA gene is present in all serovars and biotypes of APP, making it a reliable diagnostic target. PCR can detect the organism in samples with low bacterial loads or in animals that have received antimicrobial treatment, offering advantages over culture in these scenarios [5].

Real-time PCR assays provide quantitative results that may correlate with infection severity. Conventional PCR followed by gel electrophoresis is also used in diagnostic laboratories. PCR does not provide information on serovar or antimicrobial susceptibility, so culture remains necessary for these purposes.

Polymerase spiral reaction (PSR) is a newer molecular technique that targets the apxIVA gene for rapid diagnosis. PSR operates under isothermal conditions and requires less specialized equipment than conventional PCR, potentially increasing diagnostic accessibility in field settings [5].

Serotyping

Serotyping identifies the capsular polysaccharide serovar of APP isolates. This information is critical for vaccine selection and epidemiological surveillance. Serotyping is performed using slide agglutination, coagglutination, or indirect hemagglutination tests with serovar-specific antisera. Molecular serotyping methods using multiplex PCR are increasingly used in diagnostic laboratories [7].

Knowledge of circulating serovars in a region or production system guides vaccine formulation and helps predict cross-protection potential. Serovar distribution varies geographically and can change over time, necessitating periodic surveillance [9].

Serology

Serological testing detects antibodies against APP in serum or oral fluids. Enzyme-linked immunosorbent assays (ELISAs) targeting Apx toxins or lipopolysaccharides are commercially available. Serology is useful for herd-level screening, monitoring vaccination responses, and confirming exposure history.

Serology cannot distinguish between vaccinated and naturally infected animals unless DIVA (differentiating infected from vaccinated animals) vaccines are used. Antibody levels decline over time, and negative serology does not rule out infection in individual animals.

Antimicrobial Therapy

Antimicrobial treatment is the primary intervention for acute APP outbreaks. Treatment success depends on early administration, appropriate drug selection, adequate dosage, and sufficient duration.

Drug Selection Considerations

Florfenicol has demonstrated efficacy against experimental APP infection in pigs [6]. Other antimicrobial classes used for APP treatment include tetracyclines, beta-lactams (penicillins, cephalosporins), macrolides, and fluoroquinolones. The choice of antimicrobial should be guided by culture and susceptibility testing whenever possible.

Antimicrobial resistance in APP is an increasing concern. Resistance profiles vary by region and serovar. A study of APP isolates from clinical-case pigs in Taiwan reported serovar distribution and antimicrobial resistance patterns, highlighting the need for local surveillance to inform treatment protocols [9].

Treatment Protocols

Treatment protocols should be established by the herd veterinarian based on clinical history, diagnostic results, and regulatory requirements. Key considerations include:

• Route of administration: Injectable formulations are preferred for acutely ill pigs with reduced feed and water intake. Water-soluble medications can be used for group treatment when individual injection is impractical.
• Dosage and duration: Follow label instructions and extend treatment beyond clinical resolution to reduce relapse risk.
• Withdrawal periods: Adhere to regulatory withdrawal times for meat and offal to ensure food safety.
• Concurrent infections: Address concurrent viral or bacterial pathogens that may complicate APP infection.

Supportive Care

Supportive care improves survival rates in affected pigs. Provide clean, dry bedding and ensure adequate ventilation without drafts. Reduce stocking density to minimize stress and respiratory challenge. Ensure access to fresh water and palatable feed. Nonsteroidal anti-inflammatory drugs may reduce fever and improve appetite, but consult with a veterinarian before administration.

Treatment Failure Patterns

Treatment failure occurs when clinical signs persist or recur after antimicrobial therapy. Common causes include:

• Delayed treatment initiation: APP causes rapid pulmonary necrosis that is irreversible once established.
• Inadequate dosage or duration: Subtherapeutic dosing selects for resistant organisms.
• Antimicrobial resistance: The isolate is not susceptible to the chosen drug.
• Concurrent infections: Viral pathogens such as porcine reproductive and respiratory syndrome virus or swine influenza virus complicate recovery.
• Environmental stressors: Poor ventilation, high ammonia levels, or temperature fluctuations impair immune response.

When treatment failure occurs, collect samples for culture and susceptibility testing before administering alternative antimicrobials. Review herd management practices to identify and correct predisposing factors.

Vaccination Strategies

Vaccination is a cornerstone of APP prevention in endemic herds. Multiple vaccine types are available, and program design should account for herd-specific risk factors.

Vaccine Types

Bacterins are whole-cell inactivated vaccines that contain multiple serovars. They induce antibody responses against capsular and somatic antigens but provide limited cross-protection between serovars. Bacterins reduce clinical disease severity and mortality but do not prevent colonization or transmission [12].

Subunit vaccines contain purified Apx toxins and outer membrane proteins. These vaccines target conserved virulence factors and may provide broader cross-protection than bacterins. Subunit vaccines reduce lung lesion severity and improve growth performance in vaccinated pigs [12].

Autogenous vaccines are prepared from herd-specific isolates and are used when commercial vaccines do not match circulating serovars. Autogenous vaccines require regulatory approval and are produced by licensed biological companies.

Vaccination Program Design

Vaccination programs should be tailored to herd size, production flow, disease history, and serovar prevalence. Key considerations include:

• Sow vaccination: Vaccinating sows provides passive immunity to piglets through colostrum. Immunity stabilization in reproductive sows may improve the efficiency of vaccination programs for porcine pleuropneumonia [13].
• Piglet vaccination: Piglets are vaccinated at weaning or during the nursery phase. Two doses administered 2 to 4 weeks apart are typical. Timing should account for maternal antibody interference.
• Booster vaccination: Finisher pigs may require booster doses in high-challenge environments.

Historical studies have reported experiences with vaccination against Haemophilus pleuropneumoniae, demonstrating reduced mortality and improved growth performance in vaccinated herds [14]. Adjuvant properties of certain compounds may enhance vaccine efficacy [10].

Vaccination Limitations

Vaccination does not eliminate APP from a herd. Vaccinated pigs can still become infected and serve as carriers. Vaccine efficacy varies by serovar match, challenge level, and individual animal immune status. Breakthrough disease can occur when challenge pressure is high or when immunosuppressive co-infections are present.

Cross-reacting lipopolysaccharide core antigens of Escherichia coli have been investigated for their ability to reduce mortality in herds endemically infected with Haemophilus pleuropneumoniae [11]. This approach represents an alternative immunization strategy but is not widely adopted.

Herd-Level Management and Biosecurity

Effective APP control requires integration of vaccination with management practices that reduce pathogen transmission and host susceptibility.

Biosecurity Measures

• Source pigs from herds with known APP status. Test incoming stock for APP carriage before introduction.
• Quarantine new arrivals for a minimum of 30 days. Monitor for respiratory signs during quarantine.
• Implement all-in/all-out production flow by room or barn to break infection cycles.
• Clean and disinfect facilities between groups. APP is susceptible to common disinfectants including quaternary ammonium compounds and peroxygen compounds.
• Control rodent and bird populations that may mechanically transmit the organism.
• Restrict visitor access and provide farm-specific clothing and footwear.

Environmental Management

• Maintain adequate ventilation rates to remove respiratory aerosols and reduce ammonia levels.
• Avoid temperature fluctuations that stress pigs and impair respiratory defenses.
• Provide appropriate stocking density to minimize crowding and respiratory challenge.
• Ensure clean, dry bedding to reduce environmental bacterial load.

Monitoring and Surveillance

• Conduct regular slaughter checks to monitor lung lesion prevalence and severity.
• Perform serological profiling of different production stages to identify infection patterns.
• Investigate mortality events with postmortem examination and laboratory confirmation.
• Track antimicrobial use and susceptibility patterns to guide treatment protocols.

Records and Measurements

Systematic record keeping supports evidence-led decision making in APP management. Maintain the following records:

• Mortality records: Daily mortality by pen, room, and production stage. Note cause of death when determined.
• Treatment records: Pig identification, date, drug, dose, route, duration, and outcome.
• Vaccination records: Product, batch number, dose, route, date administered, and pig age.
• Diagnostic results: Culture, PCR, serology, and serotyping reports with accession numbers.
• Slaughter check data: Lung lesion scores, pleurisy prevalence, and abscess rates.
• Antimicrobial susceptibility profiles: MIC values and interpretive categories for tested drugs.

Review records monthly to identify trends and evaluate intervention effectiveness. Escalate to a veterinary diagnostic laboratory or swine health specialist when mortality exceeds baseline, treatment failure rates increase, or new serovars are detected.

Common Failure Patterns

Recognizing common failure patterns helps veterinarians troubleshoot APP control programs.

Welfare and Safety Context

APP infection causes significant pain and distress in affected pigs. Acute respiratory distress, fever, and anorexia represent severe welfare compromise. Prompt diagnosis and treatment are essential to minimize suffering.

Veterinarians should consider the following welfare and safety aspects:

• Euthanasia criteria: Pigs with severe dyspnea, cyanosis, or inability to rise should be euthanized humanely without delay.
• Pain management: Nonsteroidal anti-inflammatory drugs may provide analgesia and reduce fever.
• Handling stress: Minimize handling of acutely ill pigs to avoid exacerbating respiratory distress.
• Worker safety: APP is not considered a zoonotic pathogen, but standard hygiene practices should be followed when handling tissues and body fluids.
• Antimicrobial stewardship: Use antimicrobials judiciously to preserve efficacy and reduce resistance development. Follow veterinary oversight requirements for prescription drugs.

Professional Escalation Criteria

Veterinarians should seek specialist consultation or escalate to a diagnostic laboratory under the following circumstances:

• Mortality exceeds 5 percent in an affected group despite treatment.
• Treatment failure occurs in multiple pigs with confirmed APP infection.
• New serovars are detected that are not covered by the current vaccine.
• Antimicrobial resistance patterns suggest limited treatment options.
• Concurrent disease outbreaks complicate diagnosis and management.
• Herd-level control measures fail to reduce disease incidence over two production cycles.

Contact a veterinary diagnostic laboratory for guidance on sample collection, shipping, and test selection. Consult a swine health specialist for complex herd-level control program design.

Practical Decision Framework for APP Outbreak Investigation and Herd-Level Control

Managing Actinobacillus pleuropneumoniae requires a structured approach that moves beyond individual case treatment to systematic herd-level investigation. This section provides a practical decision framework for veterinarians responding to suspected APP outbreaks, including a stepwise investigation protocol, record system design, troubleshooting methodology, and comparison of diagnostic approaches for different herd scenarios. The framework integrates clinical observation, diagnostic sampling, treatment decision points, and long-term monitoring to support evidence-led herd health management.

Stepwise Outbreak Investigation Protocol

When a swine herd experiences an acute respiratory disease outbreak with suspected APP involvement, follow this structured investigation protocol to ensure consistent data collection and informed decision making.

Phase 1: Immediate Response (First 24 Hours)

The initial response focuses on confirming the diagnosis, implementing treatment, and preventing further spread. Begin by examining affected pigs to document clinical signs including rectal temperature, respiratory rate and character, posture, and mucous membrane color. Photograph representative cases for reference and discussion with diagnostic laboratory personnel.

Select three to five acutely affected pigs that have not received antimicrobial therapy for euthanasia and postmortem examination. Collect lung tissue from the interface of affected and normal parenchyma, pleural fluid swabs, and tonsillar swabs. Submit samples for bacterial culture, PCR targeting the apxIVA gene, and histopathology [5]. If antimicrobial therapy has already been administered, PCR may still detect APP DNA and provide diagnostic confirmation.

Initiate antimicrobial therapy in affected groups based on clinical history and known susceptibility patterns in the region. Florfenicol has demonstrated efficacy against experimental APP infection and is a reasonable first-choice option when susceptibility is unknown [6]. Administer injectable formulations to acutely ill pigs and consider water-soluble medications for group treatment when individual injection is impractical.

Implement immediate biosecurity measures including isolation of affected groups, dedicated equipment and footwear, and movement restrictions to prevent spread to naive populations.

Phase 2: Diagnostic Confirmation (24 to 72 Hours)

Within 72 hours of sample submission, preliminary culture results and PCR results should be available. Interpret results in the context of clinical and postmortem findings. A positive PCR result for the apxIVA gene confirms APP presence in the sample [5]. Culture provides the isolate needed for serotyping and antimicrobial susceptibility testing.

Record the following information for each diagnostic submission:

• Farm identification and production stage
• Date of sample collection and submission
• Number and type of samples submitted
• Clinical signs observed at time of sampling
• Antimicrobial therapy administered before sampling
• Laboratory accession number
• Culture result (positive or negative, growth characteristics)
• PCR result (cycle threshold value if quantitative)
• Serovar identification
• Antimicrobial susceptibility profile (MIC values and interpretive categories)

Compare serovar results with vaccine serovar coverage. If the identified serovar is not included in the current vaccine, consult with the herd veterinarian and vaccine manufacturer to discuss alternative vaccination strategies.

Phase 3: Herd-Level Assessment (One to Two Weeks)

After the acute outbreak is controlled, conduct a comprehensive herd-level assessment to identify predisposing factors and develop a long-term control plan. Review the following areas:

• Production flow: Is the system all-in/all-out or continuous flow? Are pigs commingled from multiple sources?
• Environmental conditions: Measure ventilation rates, ammonia levels, temperature fluctuations, and stocking density in affected barns.
• Biosecurity practices: Evaluate pig source health status, quarantine protocols, visitor policies, and cleaning and disinfection procedures.
• Vaccination history: Review vaccine product, serovar coverage, dose, route, timing, and administration technique.
• Concurrent infections: Test for porcine reproductive and respiratory syndrome virus, swine influenza virus, and Mycoplasma hyopneumoniae, as these pathogens can exacerbate APP disease severity.

Document findings in a standardized format for comparison across production cycles and herds.

Record System for APP Management

A systematic record system supports evidence-led decision making and enables trend analysis over time. Design records that capture the following data elements for each production group.

Mortality and Treatment Records

Create a daily mortality log that records:

• Date and time of death
• Pen or room identification
• Pig identification (ear tag, tattoo, or group identifier)
• Body weight or age
• Clinical signs observed before death
• Postmortem findings (lung lesion score, pleurisy severity, other lesions)
• Samples collected and laboratory accession numbers
• Cause of death (confirmed or suspected)

Maintain a treatment log that records:

• Date and time of treatment
• Pig identification
• Clinical signs prompting treatment
• Drug name, dose, route, and duration
• Withdrawal period and estimated slaughter date
• Treatment outcome (recovered, died, euthanized, relapsed)
• Culture and susceptibility results if available

Review treatment records monthly to calculate treatment rate (number of pigs treated per 100 pigs at risk), case fatality rate (number of deaths among treated pigs divided by number of treated pigs), and relapse rate (number of pigs requiring retreatment within 14 days divided by number of treated pigs).

Vaccination Records

Record the following for each vaccination event:

• Date of administration
• Product name, manufacturer, batch number, and expiration date
• Serovar coverage
• Dose volume and route (intramuscular, subcutaneous)
• Pig age and weight
• Number of pigs vaccinated
• Adverse reactions observed
• Person administering the vaccine

Calculate vaccination coverage (number of pigs vaccinated divided by number of pigs eligible) and timeliness (percentage of pigs vaccinated within the target age window).

Slaughter Check Records

Conduct slaughter checks at least quarterly to monitor lung lesion prevalence and severity. Record:

• Date and abattoir location
• Number of pigs examined
• Lung lesion score using a standardized system (0 to 100 percent affected)
• Pleurisy prevalence and severity (none, mild, moderate, severe)
• Abscess prevalence
• Pericarditis prevalence
• Farm or group identification

Compare slaughter check results across production cycles to evaluate the impact of management changes and vaccination program adjustments.

Troubleshooting Methodology for APP Control Failures

When APP control measures fail to achieve expected outcomes, use a systematic troubleshooting approach to identify root causes and implement corrective actions.

Step 1: Define the Problem

Quantify the problem using objective measures. Compare current performance to historical baselines and target thresholds. Examples include:

• Mortality rate in affected groups exceeds 5 percent despite treatment
• Treatment failure rate exceeds 20 percent (pigs that die or relapse after treatment)
• Slaughter pleurisy prevalence exceeds 30 percent
• Lung lesion scores increase by more than 20 percent compared to previous cycle
• New serovar detected that is not covered by current vaccine

Step 2: Collect Data

Gather data from the record system described above. Interview farm personnel to identify changes in management, personnel, pig source, or environment that may have contributed to the failure. Review the following:

• Pig source health status and vaccination history
• Transportation and commingling events
• Environmental monitoring data (temperature, humidity, ammonia, ventilation rates)
• Vaccine storage, handling, and administration practices
• Antimicrobial use records and susceptibility profiles
• Concurrent disease monitoring results

Step 3: Generate Hypotheses

Based on the data collected, generate hypotheses for the control failure. Common hypotheses include:

• Serovar mismatch between vaccine and circulating strains
• Maternal antibody interference with piglet vaccination
• Immunosuppression due to concurrent viral infection or mycotoxin exposure
• Antimicrobial resistance in the circulating APP strain
• Inadequate biosecurity allowing introduction of new strains
• Environmental stress overwhelming immune protection
• Suboptimal vaccine administration (dose, route, timing)

Step 4: Test Hypotheses

Design diagnostic investigations to test each hypothesis. For serovar mismatch, submit isolates from acute cases for serotyping and compare to vaccine serovar coverage. For maternal antibody interference, measure antibody levels in piglets at vaccination age using serology. For antimicrobial resistance, perform culture and susceptibility testing on isolates from treatment failure cases.

For environmental stress, measure ventilation rates, ammonia levels, and temperature fluctuations in affected barns. Compare these measurements to recommended thresholds. For biosecurity breaches, conduct a biosecurity audit and trace pig movements and personnel traffic.

Step 5: Implement Corrective Actions

Based on the confirmed root cause, implement targeted corrective actions. Examples include:

• Switch to a vaccine that includes the circulating serovar
• Adjust vaccination timing to avoid maternal antibody interference
• Treat concurrent infections and address immunosuppressive factors
• Change antimicrobial class based on susceptibility results
• Enhance biosecurity protocols including quarantine, testing, and disinfection
• Improve ventilation and environmental control
• Provide booster vaccinations in high-challenge groups

Step 6: Monitor and Adjust

Monitor the impact of corrective actions over at least two production cycles. Compare mortality rates, treatment rates, slaughter check results, and serological profiles before and after implementation. Adjust the control plan based on ongoing monitoring data.

Comparison of Diagnostic Approaches for Different Herd Scenarios

Different herd scenarios require different diagnostic approaches. The following comparison guides test selection based on clinical context and diagnostic objectives.

Scenario 1: Acute Outbreak in a Previously Unaffected Herd

Objective: Confirm APP as the cause of mortality and identify the serovar for vaccine selection.

Recommended approach: Submit lung tissue and pleural fluid from three to five acutely affected, untreated pigs for bacterial culture, PCR targeting the apxIVA gene, and histopathology. Culture provides the isolate for serotyping and susceptibility testing. PCR provides rapid confirmation even if culture is negative due to prior antimicrobial therapy [5]. Histopathology confirms the characteristic fibrinous, necrotizing pleuropneumonia.

Limitations: Culture sensitivity is reduced in samples from pigs that have received antimicrobial therapy. Collect samples before treatment initiation whenever possible.

Scenario 2: Chronic Cough and Poor Growth in a Vaccinated Herd

Objective: Determine whether APP is contributing to chronic respiratory disease and assess vaccine efficacy.

Recommended approach: Submit tonsillar swabs from 10 to 20 pigs representing different production stages for PCR targeting the apxIVA gene. PCR can detect carrier pigs with low bacterial loads that may be missed by culture [5]. Conduct serological profiling using ELISA to measure antibody levels against Apx toxins. Compare antibody profiles across age groups to identify infection patterns.

Limitations: PCR does not provide serovar information. Serology cannot distinguish vaccinated from infected pigs unless DIVA vaccines are used. Submit samples from pigs that have not received antimicrobial therapy for at least two weeks.

Scenario 3: Treatment Failure in an Acute Outbreak

Objective: Identify antimicrobial resistance patterns and guide alternative treatment selection.

Recommended approach: Submit lung tissue or pleural fluid from pigs that died or failed to respond to treatment for bacterial culture and antimicrobial susceptibility testing. Request MIC values for a panel of antimicrobials including florfenicol, tetracyclines, beta-lactams, macrolides, and fluoroquinolones [6][9].

Limitations: Culture requires viable organisms. Submit samples before administering alternative antimicrobials. Susceptibility testing results may not correlate perfectly with clinical response due to pharmacokinetic and pharmacodynamic factors.

Scenario 4: Herd-Level Surveillance in an Endemically Infected Herd

Objective: Monitor APP infection dynamics and evaluate control program effectiveness.

Recommended approach: Conduct quarterly slaughter checks to monitor lung lesion prevalence and severity. Perform serological profiling of 10 to 20 pigs per production stage (nursery, grower, finisher, sow) using ELISA. Submit tonsillar swabs from cull sows for PCR to monitor carrier status.

Limitations: Slaughter checks provide retrospective information and may not reflect current infection status. Serology results require interpretation in the context of vaccination history. PCR on tonsillar swabs detects APP DNA but does not distinguish viable from nonviable organisms.

Common Failure Patterns in APP Control Programs

Recognizing common failure patterns helps veterinarians troubleshoot APP control programs efficiently.

Welfare and Safety Context for Outbreak Management

APP infection causes severe respiratory distress, fever, and pain in affected pigs. Prompt diagnosis and treatment are essential to minimize suffering. Consider the following welfare and safety aspects during outbreak investigation.

Euthanasia Criteria

Pigs with severe dyspnea, cyanosis, inability to rise, or failure to respond to treatment within 24 hours should be euthanized humanely without delay. Prolonged suffering is not acceptable. Use approved euthanasia methods consistent with the World Organisation for Animal Health standards [1][3].

Pain Management

Nonsteroidal anti-inflammatory drugs may provide analgesia and reduce fever in affected pigs. Consult with a veterinarian before administration. Record drug, dose, route, and withdrawal period.

Handling Stress

Minimize handling of acutely ill pigs to avoid exacerbating respiratory distress. Use boards or panels for movement instead of electric prods. Provide a quiet, low-stress environment for recovery.

Worker Safety

APP is not considered a zoonotic pathogen, but standard hygiene practices should be followed when handling tissues and body fluids. Wear gloves, protective clothing, and eye protection during postmortem examination. Wash hands thoroughly after handling pigs or samples.

Antimicrobial Stewardship

Use antimicrobials judiciously to preserve efficacy and reduce resistance development. Follow veterinary oversight requirements for prescription drugs. Base treatment decisions on culture and susceptibility testing whenever possible. Record antimicrobial use for stewardship monitoring.

Professional Escalation Criteria

Veterinarians should seek specialist consultation or escalate to a diagnostic laboratory under the following circumstances:

• Mortality exceeds 5 percent in an affected group despite treatment
• Treatment failure occurs in multiple pigs with confirmed APP infection
• New serovars are detected that are not covered by the current vaccine
• Antimicrobial resistance patterns suggest limited treatment options
• Concurrent disease outbreaks complicate diagnosis and management
• Herd-level control measures fail to reduce disease incidence over two production cycles

Contact a veterinary diagnostic laboratory for guidance on sample collection, shipping, and test selection. Consult a swine health specialist for complex herd-level control program design. The World Organisation for Animal Health provides international standards for disease surveillance and reporting [1][3].

Frequently Asked Questions

What are the first clinical signs of Actinobacillus pleuropneumonia in pigs?

The earliest signs include sudden fever above 41 degrees Celsius, rapid breathing, lethargy, and reduced feed intake. Peracute cases may present as sudden death without observed premonitory signs. Close observation of high-risk groups such as recently mixed or stressed pigs is essential for early detection.

How is Actinobacillus pleuropneumoniae transmitted between pigs?

Transmission occurs through direct contact with respiratory secretions from infected or carrier pigs. Aerosol spread over short distances is possible in confined housing. Carrier pigs with tonsillar colonization serve as the primary reservoir for herd-to-herd transmission. Commingling pigs from multiple sources increases transmission risk [15].

What samples should be collected for APP diagnosis?

For culture and PCR, collect fresh lung tissue from the interface of affected and normal parenchyma. Swabs of pleural fluid, bronchial exudate, or tonsillar crypts are also suitable. Submit samples in appropriate transport medium for culture or in sterile containers for PCR. Contact your diagnostic laboratory for specific submission requirements.

Can PCR replace bacterial culture for APP diagnosis?

PCR provides rapid detection of APP DNA directly from clinical samples and is useful for screening and confirmation. However, PCR does not provide information on serovar or antimicrobial susceptibility. Culture remains necessary for serotyping and susceptibility testing, which are essential for vaccine selection and treatment decisions [5][7].

What antimicrobials are effective against APP?

Florfenicol has demonstrated efficacy against experimental APP infection [6]. Other antimicrobial classes used include tetracyclines, beta-lactams, macrolides, and fluoroquinolones. Antimicrobial susceptibility varies by region and serovar, so culture and susceptibility testing should guide drug selection whenever possible [9].

Does vaccination prevent APP infection completely?

Vaccination reduces clinical disease severity and mortality but does not prevent infection or carriage. Vaccinated pigs can still become infected and transmit the organism to pen mates. Vaccine efficacy depends on serovar match, challenge level, and individual animal immune status. Vaccination should be combined with biosecurity and management measures for optimal control [12].

How long does APP survive in the environment?

APP is a fragile organism that survives poorly outside the host. It is susceptible to drying, sunlight, and common disinfectants. Survival time in organic material such as manure or bedding is limited but can be extended in cool, moist conditions. Thorough cleaning and disinfection between groups reduces environmental contamination.

What is the role of carrier pigs in herd infection?

Carrier pigs harbor APP in tonsillar crypts or nasal cavities without showing clinical signs. These animals shed the organism intermittently, particularly during stress periods such as weaning, mixing, or transport. Carrier pigs introduced into naive herds can trigger acute outbreaks. Identifying and managing carrier pigs is a key challenge in APP control [15].

Related Veterinary Guides

• Swine Respiratory Disease Observation And Diagnostics
• Veterinary Clinical Methods Procedures Surgical Interventions
• Manure Management For Pig Farms
• Production Records For Pig Farms
• Pig Lameness Monitoring And Flooring Management

References and Further Reading

• World Organisation for Animal Health
• Merck Veterinary Manual. Merck Veterinary Manual.
• Animal Health and Welfare. World Organisation for Animal Health.
• Porcine pleuropneumonia.. Journal of the American Veterinary Medical Association, 1994.
• Rapid detection of Actinobacillus pleuropneumoniae targeting the apxIVA gene for diagnosis of contagious porcine pleuropneumonia in pigs by polymerase spiral reaction.. Letters in applied microbiology, 2022.
• Efficacy of florfenicol on experimental Actinobacillus pleuropneumonia in pigs.. The Journal of veterinary medical science, 1995.
• Actinobacillus pleuropneumoniae: An Update on Epidemiology, Biovar, Serotyping, Virulence, and Laboratory Diagnosis.. Microbiology and immunology, 2025.
• Pleuropneumonia caused by Actinobacillus pleuropneumoniae biotype 2 in growing and finishing pigs.. Journal of veterinary diagnostic investigation : official publication of the American Association of Veterinary Laboratory Diagnosticians, Inc, 1992.
• Serovars and antimicrobial resistance profiles of Actinobacillus pleuropneumoniae isolates from clinical-case pigs in Taiwan.. BMC veterinary research, 2025.
• Adjuvant properties of propionibacterium avidum KP-40 in vaccination against endemic viral and bacterial infections II. Swine immunized with inactivated haemophilus pleuropneumoniae vaccine and experimentally infected with different virulent serotypes of H. pleuropneumoniae. Zentralblatt Fur Bakteriologie, 1992.
• Mortality in swine herds endemically infected with Haemophilus pleuropneumoniae: effect of immunization with cross-reacting lipopolysaccharide core antigens of Escherichia coli.. American Journal of Veterinary Research, 1986.
• Actinobacillus pleuropneumoniae vaccines: from bacterins to new insights into vaccination strategies.. Animal Health Research Reviews Conference of Research Workers in Animal Diseases, 2008.
• Effect of immunity stabilization in reproductive sows on the efficiency of vaccination programs for porcine pleuropneumonia. Revista Cientifica De La Facultad De Ciencias Veterinarias De La Universidad Del Zulia, 2002.
• Experiences with vaccination against Haemophilus pleuropneumonia of swine. Berliner Und Munchener Tierarztliche Wochenschrift, 1989.
• Effect of subclinical infection with Actinobacillus pleuropneumoniae in commingled feeder swine.. Journal of the American Veterinary Medical Association, 1993.

This article is educational and is not a substitute for veterinary diagnosis or treatment. Contact a veterinarian for advice about an individual animal.