General Facts

Influenza Change






The appearance and distribution of pathological lesions can vary between sub-types of H1N1.1 The macroscopic and micrcoscopic appearance of the 2009 H1N1 sub-type have been describe in pigs and appear to follow the 'classic' swine influenza viral pattern.1, 2 Lesions are localized to the respiratory tract in uncomplicated cases.3 Complications that do occur are often a result of secondary bacterial infections and/or underlying medical conditions and associated lesions will be found in affected organ systems.

Gross Pathology

Multifocal, well-demarcated, purple-red necrotic lesions are typically found within the lungs of infected individuals. They are located throughout the lung but are more prevelant in the cranioventral region.1 These lesions begin to appear 24 hours after viral replication within the host epithelium has begun.3 Pneumonic areas are consolidated while the nonpneumonic areas are pale and emphysematous.1,3 Mucopurulent exudate is excreted into the air passages. Edematous bronchial and mediastinal lymph nodes may be observed but they are not usually congested. Additionally, signs of serous or serofibrinous pleuritis may be present.3

Courtesy of Aleksandar Masic, Western College of Veterinary Medicine

Viral replication results in the disruption of the epithelium leading to obliterative exudative bronchiolitis.1,3 In the alveoli and other air passages mild to moderate peribronchiolar and perivascular infiltration of lymphocytes is observed.1 Additionally, interstitial pneumonia is sometimes noted.3

                                   Normal Swine Lung                                                              Lung of H1N1 Infected Pig

        normal              H1N1
Courtesy of Aleksandar Masic, Western College of Veterinary Medicine


  1. Sreta, D., Kedkovid, R., Tuamsang, S., Kitikoon, P., and Thanawongnuwech, R. (2009). Pathogenesis of swine influenza virus (Thai isolates) in weanling pigs: an experimental trail. Virology Journal. 6:34
  2. Lange, E., Kalthoff, D., Blohm, U., Teifke, J.P., Breithaupt, A., Maresch, C., Starick, E., Fereidouni, S., Hoffmann, B., Mettenleiter, T.C., Beer, M., and Vahlenkamp, T.W. (2009). Pathogenesis and transmission of the novel swine-origin influenza virus A/H1N1 after experimental infection of pigs. Journal of General Virology. 90. pp 2119-2123
  3. 'H1N1'. The Merck Veterinary Manual. (Accessed September 15, 2009)
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