Abstract: 【Objective】 In this research, an experimental animal infection model of canine influenza virus (CIV, H3N2 subtype) was established to better understand the pathogenesis of canine influenza and to lay the foundation for vaccine efficacy evaluation. 【Methods】 26 beagles aged 6-13 months with negative CIV Haemagglutination Inhibition (HI) antibody (HI < 1:10) were selected. Three were challenged by 1mL nasal spray of PBS. And twenty-three challenged by 1mL nasal spray of H3N2 CIV (A/canine/China/Huabei-170607/2017(H3N2), HB strain for short) with 5 groups (10、10³、10⁵、10⁶、10⁷ 50% EID₅₀) as 3, 5, 5, 5 and 5 beagles each group respectively_. Clinical symptoms, lung lesions, histopathological changes of lung, calculating the proportion of consolidation mass, HI antibody titer and virus shedding were examined 14 days after virus challenge together with three control beagles. 【Results】 3 beagles inoculated with a dose of 10 EID₅₀ H3N2 CIV did not show any clinical symptoms , gross lesions in lungs and histopathological changes. The consolidation rate was 0%. Virus shedding was not detected. The geometric mean titer (GMT) of HI antibodies was 1:15.9. However, 4/5 of beagles inoculated with a dose of 10³ EID₅₀ H3N2 CIV showed clinical symptoms and virus shedding such as puffing, runny nose and cough. 2/5 of beagles showed light lung consolidation, whose rate was 1.4%. The GMT of HI antibodies was 1:320. All beagles (5/5) infected with a dose of 10⁵ EID₅₀ H3N2 CIV showed clinical symptoms at day 5 after challenge such as runny nose and cough, virus shedding, lung consolidation and widened alveolar septum. The consolidation rate was 4.2%. The GMT of HI antibodies was 1:2940.7. 5/5 of beagles infected with a dose of 10⁶ EID₅₀ all showed severe clinical symptoms at day 4 after challenge such as cough and elevated body temperature, virus shedding, obvious pathological features in the lungs with GMT of HI antibodies were 1:2228.7. The clinical symptoms appeared earlier 1 day than that in 10⁵ EID₅₀ dose infection group. And the degree of lung consolidation increased. The lung consolidation ratio was 17.9%. 5/5 of beagles infected with doses of 10⁷ EID₅₀ all showed severe clinical symptoms at day 3 after challenge which appeared earlier 1 day than that in 10⁶ EID₅₀ dose infection group, virus shedding and widened alveolar septum in the lungs with GMT of HI antibodies were 1:2228.7. The consolidation rate of lung was 29.0%. Control beagles did not show any clinical symptoms, gross lesions and histopathological changes in lungs. And virus shedding was not detected. GMT of HI antibodies was ＜1:10. 【Conclusions】 The dose of 10⁶ EID⁵⁰ is the minimum virus inoculation dose that can cause obvious pathogenesis in beagles. An experimental animal infection model of CIV subtype H3N2 in beagles was established.

Key words: beagles, canine influenza virus, H3N2 subtype, lung, infection model