Swine Influenza: Pathobiology, Pulmonary Lesions, Diagnosis and Its Significance in Porcine Respiratory Disease

Swine influenza is an acute, highly contagious respiratory disease of pigs caused predominantly by influenza A viruses. The disease is important from a veterinary pathology perspective because viral replication in the respiratory epithelium produces epithelial injury, inflammation and characteristic pulmonary lesions, while disruption of mucociliary defence may facilitate secondary bacterial infection. Clinically, affected pigs may show sudden fever, depression, anorexia, coughing, sneezing, nasal discharge and respiratory distress. At necropsy, the lungs commonly show multifocal to coalescing, dark-red to purple, firm and depressed areas of consolidation, particularly in the cranial and middle lobes. Microscopically, acute bronchointerstitial pneumonia, bronchiolar epithelial degeneration and necrosis, loss of ciliated epithelium, inflammatory exudation and alveolar changes may be observed. Because similar lesions occur with other respiratory pathogens, diagnosis should integrate clinical history, gross pathology, and histopathology and laboratory confirmation, particularly real-time reverse transcription polymerase chain reaction (rRT-PCR). Control depends on biosecurity, appropriate herd management, adequate ventilation, vaccination where indicated, surveillance and timely recognition of secondary infections. The One Health importance of swine influenza also reflects the genetic diversity and reassortment potential of influenza A viruses. This article highlights the disease from a pathology-oriented perspective, emphasizing how lesion development helps explain clinical disease, complications and diagnostic decision-making.