The present invention has as its object to provide a process for determining the degree of pathogenicity of infecting bacterial strains in the case of canine periodontal disease. The present inventors found that Porphyromonas gulae is a major pathogen of canine periodontal disease, and demonstrated that the fimA genes encoding FimA (fimbrillin) protein which constitute fimbriae of this bacterium are classified into three major groups, and that each of the groups is correlated with the pathogenicity of this bacterium as a pathogenic bacterial of periodontal disease. Thus the inventors completed the present invention.
