Canine influenza virus (CIV) is an emerging pathogen that causes acute respiratory disease in dogs. To better understand the mechanism(s) responsible for the virulence of the virus, we conducted immunological, virological, clinical, and histopathological analyses in CIV-infected dogs. CIV replicated efficiently in the respiratory system of dogs and caused severe respiratory disease. Notably, the infection induced the marked elevation and sustained expression of chemokines that resulted in severe bronchointerstitial pneumonia with extensive neutrophil infiltration. In clinicopathological findings, CIV infection resulted in regenerative anemia, perhaps due to pulmonary hemorrhage. The observations indicate that active replication of CIV in the canine respiratory system results in intense inflammatory responses central to the pathogenesis of H3N2 CIV.
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