Subclinical Mastitis Related to Streptococcus canis Infection in Dairy Cattle

In the present study, we isolated <i>S. canis</i> from milk samples in a dairy farm with suspicions of subclinical mastitis. Milk samples testing positive on California Mastitis Test (CMT) were collected from different cows for bacteriological and genomic analyses to identify the causative pathogen, and somatic cell counts (SCC) were determined. A multiplex qPCR assay was conducted to detect 15 potential pathogens, and all samples showed negative results. Conventional bacteriology procedures were performed; DNA of the bacterial strains was extracted, sequenced, and submitted to bioinformatic analysis. Three CMT positive milk samples showed SCC > 200 × 10³ cell/mL. However, these same three samples were positive for bacteria phenotypically identified as Streptococci, and the strains were confirmed as <i>S. canis</i> using MS MALDI-TOF methodology. In susceptibility testing, resistance against tetracycline was detected, revealing a potential chronic infection in one cow, while the presence of the same bacteria was observed in two other cows. Genomic DNA from four <i>S. canis</i> isolates, obtained in the first and second sampling, was sequenced. Genetic relationships revealed a unique sequence type (ST24). The gene (<i>tetM</i>) related with resistance to TE was highlighted. Although the association between <i>S. canis</i> and mastitis is not routinely detected, early diagnosis of bacterial infections and the study of the antimicrobial profile are crucial for effective therapy. Pets could act as a potential reservoir, so improving hygienic conditions is needed to prevent new infections.