A 4-month-old Tactic dog was presented for evaluation of respiratory syndrome. A strain of Streptococcus pluranimalium was isolated and confirmed by biochemical characteristics and 16S rRNA sequence analysis. To determine the virulence of this bacterium, BALB/c mice were challenged by intraperitoneal injection with 10⁸ CFU S. pluranimalium. Lung, liver, heart, kidney, spleen and brain were harvested at intervals for analysis. Lung tissue showed the earliest bacterial detection at 2 days and all the collected tissues demonstrated greater colony counts at 96h following exposure to the bacterium. Histopathological examinations showed that lung cells became widened with thickening of the bronchiolar lumen and narrowing of alveolar septa. Exudate in macrophages significantly increased lung congestion and severe consolidation. The main lesions in the brain were characterized by edema of nerve cells and non-supportive encephalitis, indicating that S. pluranimalium is capable of inducing brain damage by crossing the blood–brain barrier (BBB). It is concluded that S. pluranimalium may be one of causative agents for canine infectious respiratory disease.

Key words: Dog, Histopathology, Pathogenicity, Respiratory syndrome, Streptococcus pluranimalium