Swine Influenza in Pigs: Clinical Symptoms and Differential Diagnosis

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-06-06

Key Takeaways

• Swine influenza A virus (swIAV) is a primary viral agent of acute respiratory disease in pigs, characterized by sudden onset of fever (>40.5°C), harsh coughing, serous to mucopurulent nasal discharge, anorexia, and lethargy.
• The virus targets respiratory epithelium, leading to epithelial necrosis and impaired mucociliary clearance, predisposing pigs to secondary bacterial infections which significantly increase morbidity and mortality.
• Differential diagnosis is critical due to overlapping clinical signs with other porcine respiratory disease complex (PRDC) pathogens, necessitating laboratory confirmation via molecular methods like RT-PCR targeting the M-gene.
• Multiplex RT-PCR assays are essential for simultaneously detecting swIAV alongside other common viral agents such as PRRSV, PCV2, and PRV1, as well as bacterial pathogens.
• Sample types for diagnosis include nasal swabs, tracheobronchial swabs, oral fluids, and lung tissue, with oral fluids demonstrating acceptable diagnostic sensitivity for RT-PCR detection.
• While serology (HI assays, ELISA) can confirm exposure to swIAV, it cannot differentiate between recent and past infections, making molecular diagnostics the gold standard for acute disease identification.

Swine influenza A virus (swIAV) is a primary viral agent of acute respiratory disease in pigs worldwide, contributing significantly to the porcine respiratory disease complex (PRDC) [<a href="#ref-1">1</a>]. The clinical presentation of swIAV infection ranges from subclinical to severe, often indistinguishable from other respiratory pathogens [<a href="#ref-2">2</a>, <a href="#ref-3">3</a>]. Accurate differential diagnosis is critical for implementing control measures and reducing economic losses. This article reviews the clinical symptomatology, pathophysiological mechanisms, and diagnostic approaches for swIAV, with an emphasis on distinguishing it from other viral and bacterial causes of porcine respiratory disease.

Etiology and Virological Background

SwIAV belongs to the family Orthomyxoviridae, genus Alphainfluenzavirus, and is classified by hemagglutinin (HA) and neuraminidase (NA) surface glycoproteins [<a href="#ref-4">4</a>, <a href="#ref-5">5</a>]. Endemic subtypes in swine include H1N1, H1N2, and H3N2, with frequent reassortment events generating novel strains [<a href="#ref-4">4</a>, <a href="#ref-5">5</a>]. Reverse zoonotic transmission of human seasonal influenza viruses to pigs has been documented, further expanding genetic diversity [<a href="#ref-6">6</a>, <a href="#ref-7">7</a>]. The primary site of infection is the respiratory epithelium of the nasal mucosa, trachea, bronchi, and bronchioles [<a href="#ref-8">8</a>]. SwIAV replication in ciliated epithelial cells leads to ciliary dysfunction, epithelial necrosis, and desquamation, impairing mucociliary clearance and predisposing the lower respiratory tract to secondary bacterial invasion [<a href="#ref-9">9</a>, <a href="#ref-10">10</a>].

Clinical Symptoms in Pigs

The incubation period of swIAV is typically 1 to 3 days, followed by an acute onset of clinical signs [<a href="#ref-11">11</a>, <a href="#ref-12">12</a>]. Disease expression is influenced by host immune status, viral strain virulence, and concurrent infections [<a href="#ref-13">13</a>, <a href="#ref-14">14</a>]. Core clinical manifestations include:

• Fever: Rectal temperatures may exceed 40.5°C, often peaking within 48 hours of infection [<a href="#ref-12">12</a>].
• Respiratory signs: Sudden onset of harsh, non-productive coughing, sneezing, nasal discharge (serous to mucopurulent), and tachypnea with labored abdominal breathing [<a href="#ref-15">15</a>, <a href="#ref-16">16</a>].
• Systemic signs: Anorexia, lethargy, piloerection, and huddling behavior. Weight loss and decreased growth rates are observed in growing pigs [<a href="#ref-17">17</a>, <a href="#ref-18">18</a>].
• Ocular signs: Conjunctivitis and ocular discharge are occasionally reported [<a href="#ref-11">11</a>].

In suckling piglets, maternal antibodies provide partial protection but do not prevent infection entirely, often resulting in milder or subclinical disease [<a href="#ref-12">12</a>, <a href="#ref-14">14</a>]. In finishing pigs and adults, clinical signs can be more pronounced, especially when co-infections with other PRDC pathogens occur [<a href="#ref-1">1</a>, <a href="#ref-19">19</a>]. Morbidity is typically high (50 to 90%), but mortality is low (<5%) unless secondary bacterial pneumonia ensues [<a href="#ref-9">9</a>, <a href="#ref-10">10</a>].

Lesions observed at necropsy include cranioventral bronchopneumonia, hyperemic and edematous lungs, tracheitis, and accumulation of serosanguineous fluid in airways [<a href="#ref-2">2</a>]. Histopathologic examination reveals necrotizing bronchitis and bronchiolitis, with loss of ciliated epithelium, peribronchiolar lymphocytic cuffing, and alveolar edema [<a href="#ref-2">2</a>, <a href="#ref-8">8</a>].

Host Factors and Immunological Response

The innate immune response to swIAV involves type I interferon production, recruitment of neutrophils and macrophages, and secretion of pro-inflammatory cytokines [<a href="#ref-13">13</a>, <a href="#ref-18">18</a>]. The adaptive response induces virus-specific IgA in the upper respiratory tract and serum IgG, which can be measured by hemagglutination inhibition (HI) assays and commercial ELISA kits [<a href="#ref-12">12</a>, <a href="#ref-20">20</a>]. Vaccination with inactivated or recombinant vaccines can reduce clinical severity but may not prevent infection or shedding entirely [<a href="#ref-21">21</a>, <a href="#ref-22">22</a>]. Vaccine-associated enhanced respiratory disease has been described under certain conditions [<a href="#ref-18">18</a>].

Differential Diagnosis

Because the acute respiratory signs of swIAV overlap with those of numerous other porcine pathogens, a structured differential diagnosis is essential. The table below summarizes key differential diagnoses based on clinical and pathological features.

Co-infections between swIAV and bacterial pathogens such as Actinobacillus pleuropneumoniae and Mycoplasma hyopneumoniae worsen clinical signs and complicate diagnosis [<a href="#ref-9">9</a>, <a href="#ref-10">10</a>]. Viral-bacterial synergy is mediated by swIAV-induced damage to the respiratory epithelium, which facilitates bacterial adherence and invasion [<a href="#ref-10">10</a>, <a href="#ref-27">27</a>].

Diagnostic Approach

Laboratory confirmation is required for definitive diagnosis. Sample types include nasal swabs (individual or pooled), tracheobronchial swabs, oral fluids, and lung tissue [<a href="#ref-11">11</a>, <a href="#ref-15">15</a>, <a href="#ref-28">28</a>]. Oral fluids have shown acceptable diagnostic sensitivity compared to nasal swabs for RT-PCR detection of swIAV [<a href="#ref-11">11</a>]. For herd-level monitoring, aggregate samples such as rope oral fluids or dust swabs can be used [<a href="#ref-15">15</a>, <a href="#ref-17">17</a>].

Molecular detection: Real-time RT-PCR targeting the matrix (M) gene is the gold standard for swIAV detection [<a href="#ref-23">23</a>, <a href="#ref-26">26</a>, <a href="#ref-29">29</a>]. Multiplex RT-PCR assays can simultaneously detect swIAV, PRRSV, PCV2, and other respiratory pathogens [<a href="#ref-23">23</a>, <a href="#ref-29">29</a>, <a href="#ref-30">30</a>]. Digital PCR (dPCR) offers absolute quantification and may be less affected by inhibitors [<a href="#ref-31">31</a>]. Duplex real-time fluorescent quantitative PCR assays have been developed for simultaneous detection of swIAV and Actinobacillus pleuropneumoniae [<a href="#ref-26">26</a>]. Quadruplex one-step RT-qPCR assays allow detection of multiple respiratory viruses in a single reaction [<a href="#ref-30">30</a>].

Rapid point-of-care options: Recombinase polymerase amplification (RPA) combined with CRISPR/Cas12a systems provides visual field detection without thermal cyclers [<a href="#ref-32">32</a>]. RT-LAMP paired with lateral flow strips offers another rapid alternative [<a href="#ref-33">33</a>]. Microfluidic devices incorporating photonic integrated circuits enable label-free detection [<a href="#ref-28">28</a>].

Serology: Hemagglutination inhibition (HI) assays and commercial ELISA kits detect anti-swIAV antibodies [<a href="#ref-12">12</a>, <a href="#ref-20">20</a>]. Serology can confirm exposure but cannot distinguish recent from past infection [<a href="#ref-20">20</a>].

Metagenomic and next-generation sequencing: Nanopore metagenomic sequencing on tracheobronchial swabs can identify viral and bacterial co-infections with high resolution [<a href="#ref-27">27</a>]. This approach is valuable for characterizing circulating strains and detecting novel reassortants [<a href="#ref-5">5</a>, <a href="#ref-34">34</a>].

Host gene expression assays: A 3-transcript host expression assay has been described to differentiate between viral and bacterial infections in pigs, potentially guiding antimicrobial use [<a href="#ref-35">35</a>].

Point-of-care and integrated diagnostics: Aggregate sample monitoring combined with artificial intelligence-based respiratory scoring can track disease patterns in endemically infected herds [<a href="#ref-17">17</a>].

Differential Diagnosis Decision Tree

The following Mermaid diagram outlines a structured decision pathway for swIAV differentiation from other PRDC pathogens.

flowchart TD
 A["Acute onset respiratory signs in pigs<br>cough, fever, anorexia"] --> B{"'Collect samples<br>(nasal swab / oral fluid / lung')"}
 B --> C["RT-PCR for swIAV M-gene"]
 C --> D{"Positive?"}
 D -->|"Yes"| E["swIAV confirmed<br>Consider subtyping by HA/NA RT-PCR"]
 D -->|"No"| F["Simultaneous multiplex RT-PCR<br>PRRSV, PCV2, PRV1, SOV, PHEV"]
 F --> G{"Any positive?"}
 G -->|"Yes"| H["Identify primary viral pathogen<br>Consider co-infection"]
 G -->|"No"| I["Bacterial workup:<br>Culture, PCR for App, Mhyo, S. suis, etc."]
 H --> J["Assess severity and co-infections<br>metagenomic sequencing if indicated"]
 I --> K{"Bacterial pathogen<br>isolated?"}
 K -->|"Yes"| L["Treat accordingly + antimicrobial sensitivity"]
 K -->|"No"| M["'Consider non-infectious causes<br>(e.g., environmental, toxic')"]
 E --> N["Evaluate herd immunity:<br>vaccination history, maternal antibodies"]
 N --> O["Implement control: biosecurity, vaccination"]

The decision tree emphasizes early molecular detection and systematic exclusion of other PRDC agents. The use of multiplex assays allows simultaneous screening of the most common viral and bacterial pathogens [<a href="#ref-23">23</a>, <a href="#ref-29">29</a>, <a href="#ref-30">30</a>].

Conclusion

Swine influenza A virus remains a major cause of acute respiratory disease in pigs, with clinical signs that overlap extensively with other infections. Accurate differential diagnosis relies on a combination of clinical observation, molecular diagnostics, and consideration of co-infections. Multiplex RT-PCR and emerging point-of-care technologies enable rapid detection and differentiation, facilitating timely intervention. Understanding the local viral and bacterial ecology, as well as host immune status, is essential for effective herd health management.

References

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