Capnocytophaga canimorsus is a Gram-negative commensal of dog's mouth causing severe human infections. A strain isolated from a human fatal infection was recently shown to have a sialidase, to inhibit the bactericidal activity of macrophages and to block the release of nitric oxide by LPS-stimulated macrophages. The present study aimed at determining the prevalence of C. canimorsus in dogs and the occurrence of these hypothetical virulence factors. C. canimorsus could be retrieved from the saliva of 61 dogs out of 106 sampled. Like in clinical isolates, all dog strains had a sialidase and 60% blocked the killing of phagocytosed Escherichia coli by macrophages. In contrast, only 6.5% of dog strains blocked the release of nitric oxide by LPS-challenged macrophages, suggesting that this property might contribute to virulence. The comparative analysis of 69 16S rDNA sequences revealed the existence of C. canimorsus strains that could be misdiagnosed.