Swine Influenza in Pigs: Clinical Symptoms and Differential Diagnosis

Etiology and Virology

Swine influenza is an acute, highly contagious respiratory disease of pigs caused by type A influenza viruses of the family Orthomyxoviridae. These viruses are enveloped, negative-sense, single-stranded RNA viruses with a segmented genome comprising eight gene segments. The two major surface glycoproteins, hemagglutinin (HA) and neuraminidase (NA), determine the viral subtype and are the primary targets of the host humoral immune response. In swine populations globally, the most commonly circulating subtypes include H1N1, H3N2, and H1N2, with multiple genetic lineages (e.g., classical swine, avian-like, human-like, and reassortant strains) cocirculating in different geographic regions.

The viral hemagglutinin binds to sialic acid receptors on the apical surface of porcine respiratory epithelial cells. Swine tracheal and bronchial epithelia express both alpha-2,3- and alpha-2,6-linked sialic acid receptors, making pigs a permissive mixing vessel for avian and mammalian influenza viruses. Following receptor-mediated endocytosis, the low pH of the endosome triggers a conformational change in HA, facilitating fusion of the viral envelope with the endosomal membrane. The viral ribonucleoprotein complexes are released into the cytoplasm and subsequently imported into the nucleus, where transcription and replication occur. Neuraminidase cleaves sialic acid residues from the host cell surface, enabling the release of progeny virions.

Transmission and Epidemiology

Swine influenza virus (SIV) is transmitted primarily through direct contact between infected and susceptible pigs via respiratory secretions. Aerosol and droplet transmission over short distances (within 2 to 3 meters) is a major route in confined housing systems. Indirect transmission via contaminated fomites, including boots, clothing, feeding equipment, and transport vehicles, also contributes to herd-to-herd spread. The virus can survive on inanimate surfaces for 24 to 48 hours under cool, humid conditions, though survival time decreases with higher temperatures and desiccation.

Once introduced into a naive herd, SIV spreads rapidly, with morbidity rates often reaching 80 to 100 percent. Mortality is generally low (less than 5 percent) in uncomplicated cases but can increase substantially when secondary bacterial infections occur. The incubation period ranges from 1 to 3 days. Virus shedding in nasal secretions begins 24 hours post-infection, peaks at 2 to 3 days, and typically ceases by 7 to 10 days. Subclinically infected pigs and those with mild signs can shed virus and serve as reservoirs.

Seasonal patterns are observed in temperate regions, with outbreaks more frequent in the autumn and winter months. However, in intensive production systems with continuous farrowing and multisite operations, SIV can circulate endemically year-round. Factors such as temperature fluctuations, poor ventilation, high stocking density, and commingling of pigs from different sources increase the risk of outbreaks.

Clinical Symptoms of Swine Influenza in Pigs

The clinical presentation of swine influenza is characterized by an acute onset of respiratory signs and systemic illness. The severity of symptoms depends on viral strain, host immune status, age, and the presence of concurrent infections.

Acute Phase (Days 1 to 4)

The hallmark signs include a sudden onset of fever (rectal temperatures of 40.0 to 41.5 degrees Celsius), profound lethargy, anorexia, and a characteristic harsh, nonproductive cough. Affected pigs often huddle together, exhibit piloerection, and show signs of depression. Tachypnea and dyspnea are common, with abdominal breathing and a thumping respiratory pattern observed in severe cases. Serous to mucoid nasal discharge is frequently present, and some pigs may exhibit ocular discharge and conjunctivitis.

Sows and gilts may experience a transient drop in feed intake and milk production. In lactating sows, agalactia can lead to starvation and increased mortality in nursing piglets. Boars may show a temporary reduction in libido and semen quality.

Subacute and Convalescent Phase (Days 5 to 14)

Fever typically resolves within 3 to 5 days in uncomplicated cases. Coughing may persist for 2 to 3 weeks, particularly when secondary bacterial infections are present. Appetite gradually returns, but weight gain is reduced due to the period of anorexia. In growing pigs, this can result in significant variation in body weight within a cohort.

Complications and Severe Disease

Secondary bacterial infections are the most common complication of swine influenza. The viral damage to the respiratory epithelium impairs mucociliary clearance and disrupts the innate immune barrier, predisposing pigs to colonization by opportunistic bacteria. Common secondary pathogens include Mycoplasma hyopneumoniae, Pasteurella multocida, Actinobacillus pleuropneumoniae, Streptococcus suis, and Haemophilus parasuis. The resulting bronchopneumonia or pleuropneumonia exacerbates clinical signs, prolongs recovery, and increases mortality.

In naive herds or when highly pathogenic strains emerge, a peracute form of disease can occur with severe hemorrhagic pneumonia and death within 24 to 48 hours. Porcine reproductive and respiratory syndrome virus (PRRSV) and porcine circovirus type 2 (PCV2) co-infections further complicate the clinical picture and are associated with more severe respiratory disease.

Pathophysiology

The primary target cells for SIV replication are ciliated and nonciliated epithelial cells lining the nasal mucosa, trachea, bronchi, and bronchioles. Viral replication induces epithelial cell necrosis, desquamation, and loss of ciliary function. The resulting denudation of the respiratory epithelium exposes underlying basement membranes and triggers an intense inflammatory response. Neutrophils, macrophages, and lymphocytes infiltrate the submucosa and alveolar spaces. Pro-inflammatory cytokines, including interleukin-1, interleukin-6, and tumor necrosis factor-alpha, mediate the systemic signs of fever, anorexia, and lethargy.

Histopathological examination reveals necrotizing bronchitis and bronchiolitis, with epithelial cell sloughing, peribronchiolar lymphocytic cuffing, and alveolar edema. In cases complicated by bacterial infection, suppurative bronchopneumonia with neutrophil-rich exudate is observed.

Differential Diagnosis

The clinical signs of swine influenza are not pathognomonic and overlap with several other respiratory diseases of pigs. A systematic differential diagnosis is essential for accurate diagnosis and appropriate management.

Primary Differential Diagnoses

Porcine Reproductive and Respiratory Syndrome (PRRS) PRRSV causes respiratory disease in growing pigs and reproductive failure in sows. Respiratory signs in piglets and weaners include fever, dyspnea, and lethargy, closely resembling swine influenza. However, PRRS typically has a more protracted course, with viremia lasting 2 to 4 weeks, and is often accompanied by cyanosis of the ears and extremities. Reproductive signs such as late-term abortions, stillbirths, and mummified fetuses are characteristic of PRRS but absent in swine influenza.

Porcine Circovirus Type 2 (PCV2) Associated Disease PCV2 infection can present as porcine circovirus associated disease (PCVAD), which includes respiratory signs, weight loss, pallor, and lymphadenopathy. The respiratory form of PCVAD is more insidious in onset compared to the acute presentation of swine influenza. Co-infection with SIV is common and can complicate clinical differentiation.

Mycoplasma hyopneumoniae Infection (Enzootic Pneumonia) Enzootic pneumonia is characterized by a chronic, dry cough and reduced growth rates, typically in growing pigs aged 6 to 16 weeks. The onset is gradual, and fever is usually absent or mild. In contrast, swine influenza presents with acute fever and rapid spread through the herd.

Actinobacillus pleuropneumoniae Infection Acute pleuropneumonia caused by A. pleuropneumoniae presents with sudden death, severe dyspnea, cyanosis, and bloody nasal discharge. While swine influenza can also cause acute respiratory distress, the presence of hemorrhagic nasal discharge and high mortality in grower-finisher pigs is more suggestive of A. pleuropneumoniae. Thoracic auscultation and postmortem findings of fibrinous pleuritis and necrotizing hemorrhagic pneumonia are key differentiating features.

Pasteurella multocida Pneumonia Secondary pasteurellosis is a common complication of SIV infection. Primary P. multocida pneumonia is less common but can cause coughing, dyspnea, and fever. The chronic nature and association with environmental stressors help differentiate it from acute SIV.

Swine Influenza versus Other Viral Respiratory Infections Other viral pathogens such as porcine respiratory coronavirus (PRCV) and porcine cytomegalovirus can cause mild respiratory signs but are generally less severe and lack the acute febrile response typical of SIV.

Diagnostic Approach

A definitive diagnosis of swine influenza requires laboratory confirmation. The diagnostic workflow is outlined in the decision tree below.

graph TD
    A[Acute respiratory signs in pigs: fever, cough, dyspnea, lethargy], > B{Clinical history and herd-level pattern}
    B, > C[Sudden onset, high morbidity, low mortality]
    B, > D[Gradual onset, chronic cough, poor growth]
    C, > E[Collect nasal swabs or lung tissue from acute cases]
    D, > F[Consider Mycoplasma, PRRS, PCV2, bacterial pathogens]
    E, > G{Laboratory testing}
    G, > H[RT-qPCR for influenza A matrix gene]
    G, > I[Virus isolation in embryonated eggs or cell culture]
    G, > J[Antigen detection via commercial ELISA or immunofluorescence]
    H, > K[Positive: SIV confirmed]
    H, > L[Negative: consider other pathogens]
    K, > M[Subtype by HA/NA-specific RT-PCR or sequencing]
    L, > N[Test for PRRSV, PCV2, Mycoplasma, bacterial culture]
    M, > O[Report subtype and lineage for epidemiological monitoring]

Sample Collection and Laboratory Diagnosis

Nasal swabs are the preferred antemortem sample for virus detection. Swabs should be collected from acutely affected pigs within the first 48 hours of clinical signs. Deep nasal swabbing using flocked or polyester-tipped swabs with viral transport medium maximizes viral recovery. Oropharyngeal swabs and tracheal washes are alternative sample types. For postmortem diagnosis, samples of lung tissue from the cranioventral regions, trachea, and bronchial lymph nodes should be collected aseptically.

Molecular Detection Real-time reverse transcription polymerase chain reaction (RT-qPCR) targeting the influenza A matrix gene is the gold standard diagnostic method. This assay is highly sensitive and specific and can detect viral RNA in samples with low viral loads. Subtype-specific RT-qPCR assays targeting HA and NA genes are used for subtyping. Full genome sequencing using high-throughput sequencing platforms provides detailed genetic and phylogenetic information for surveillance and outbreak investigations.

Virus Isolation Virus isolation in embryonated chicken eggs or Madin-Darby canine kidney (MDCK) cells is used for confirmatory diagnosis and for obtaining viral isolates for antigenic characterization. Isolation is less sensitive than RT-qPCR and requires 3 to 7 days.

Serology Serological testing using hemagglutination inhibition (HI) assays or commercial enzyme-linked immunosorbent assays (ELISAs) can detect antibodies against SIV. Paired serum samples collected 2 to 3 weeks apart are required to demonstrate seroconversion. Serology is useful for herd-level surveillance and vaccine response monitoring but is not suitable for acute diagnosis due to the lag between infection and antibody production.

Antigen Detection Direct immunofluorescence or immunoperoxidase staining of frozen or fixed lung tissue sections can detect viral antigens. These methods are less sensitive than RT-qPCR but provide rapid results when molecular testing is unavailable.

Prevention and Control

Biosecurity

Strict biosecurity measures are the cornerstone of SIV prevention. All-in/all-out production systems reduce the risk of pathogen carryover between groups. Quarantine of incoming stock for at least 2 to 3 weeks, with clinical monitoring and testing, prevents introduction of infected animals. Personnel should follow hygiene protocols including changing boots and coveralls between barns and using footbaths with appropriate disinfectants. Organic acids, quaternary ammonium compounds, and oxidizing agents are effective against SIV on surfaces.

Vaccination

Commercial inactivated vaccines are available for SIV and typically contain H1N1, H3N2, and H1N2 antigens. Vaccination of sows provides passive immunity to piglets via colostral antibodies. However, the efficacy of inactivated vaccines is limited by antigenic drift and the emergence of new genetic lineages. Autogenous vaccines prepared from locally circulating strains may offer better protection in some herds. Vaccination strategies should be tailored to the specific subtypes and lineages present in the region.

Management Practices

Optimizing environmental conditions reduces the severity of disease. Adequate ventilation, temperature control, and reduction of ammonia levels minimize respiratory tract irritation. Reducing stocking density and minimizing mixing of pigs from different age groups and sources lowers transmission risk. Early detection and isolation of sick pigs, combined with supportive care including access to water and palatable feed, can reduce mortality.

Antimicrobial Use

Antimicrobial therapy is not effective against the virus itself but is indicated when secondary bacterial infections are suspected or confirmed. Culture and susceptibility testing should guide antimicrobial selection to minimize resistance development. Group water medication with appropriate antimicrobials may be used during outbreaks to control bacterial complications.

Conclusion

Swine influenza is a major cause of acute respiratory disease in pigs worldwide. The clinical symptoms of swine influenza in pigs, including sudden fever, coughing, dyspnea, and lethargy, are characteristic but not specific. Accurate diagnosis requires laboratory confirmation, primarily through RT-qPCR, and differentiation from other respiratory pathogens such as PRRSV, PCV2, Mycoplasma hyopneumoniae, and Actinobacillus pleuropneumoniae. Effective control relies on robust biosecurity, vaccination, and management practices that reduce viral transmission and secondary bacterial infections. Ongoing surveillance of circulating subtypes and genetic lineages is essential for vaccine updates and outbreak preparedness.

References

1. Brown, I.H. (2000). The epidemiology and evolution of influenza viruses in pigs. Veterinary Microbiology, 74(1-2), 29-46.
2. Olsen, C.W., Brown, I.H., Easterday, B.C., & Van Reeth, K. (2006). Swine influenza. In B.E. Straw, J.J. Zimmerman, S. D'Allaire, & D.J. Taylor (Eds.), Diseases of Swine (9th ed., pp. 469-482). Blackwell Publishing.
3. Van Reeth, K., Brown, I.H., & Olsen, C.W. (2012). Influenza virus. In J.J. Zimmerman, L.A. Karriker, A. Ramirez, K.J. Schwartz, & G.W. Stevenson (Eds.), Diseases of Swine (10th ed., pp. 557-571). Wiley-Blackwell.
4. Vincent, A.L., Ma, W., Lager, K.M., Janke, B.H., & Richt, J.A. (2008). Swine influenza viruses: a North American perspective. Advances in Virus Research, 72, 127-154.
5. World Organisation for Animal Health (WOAH). (2021). Manual of Diagnostic Tests and Vaccines for Terrestrial Animals. Chapter 3.9.7: Influenza A virus of swine.