Streptococcus canis, the underdog of the genus.

Streptococcus canis is a multi-host pathogen that causes disease of varying severity in a wide range of mammals, including humans. Dogs and cats appear to be the primary hosts and may play a role in transmitting infection to humans. The broader epidemiology of S. canis, however, is still poorly understood, as are its virulence mechanisms, antimicrobial resistance (AMR) and population structure. In this review we gather existing knowledge on S. canis, describing its epidemiology in animals and humans and present information on virulence factors, classification schemes and AMR prevalence. We describe the main ecological niches of S. canis in companion animals, discuss potential risk factors for infection in humans and propose a multi-host transmission cycle. We show that current knowledge on S. canis virulence determinants is limited and sometimes contradictory. We illustrate the different typing systems proposed to classify S. canis. We also report the range of known AMR phenotypes and the emergence of new mechanisms of resistance. Finally, we discuss the zoonotic potential of S. canis, highlighting the need for further evidence in this area. Streptococcus canis may be regarded as a neglected pathogen of one health concern. Further research is needed for its better understanding and effective control.