# Subclinical Mastitis Related to Streptococcus canis Infection in Dairy Cattle

## TL;DR

The study identifies Streptococcus canis as a cause of subclinical mastitis in dairy cows and highlights antibiotic resistance and potential transmission from pets.

## Contribution

This study reports S. canis as a causative agent of subclinical mastitis and identifies tetracycline resistance and a unique bacterial strain.

## Key findings

- Streptococcus canis was isolated from milk samples of cows with subclinical mastitis.
- The bacteria showed resistance to tetracycline and shared a unique sequence type (ST24).
- Pharyngeal swabs from dogs and poor hygiene were linked to potential infection sources.

## Abstract

Mastitis is described as an inflammatory process affecting the mammary gland and is a major problem in dairy cattle. Infectious causes are among the most common in the development of inflammatory processes. Pathogens most commonly involved include bacteria. Among these, Streptococcus canis could represent a contagious pathogen causing subclinical mastitis. This study reports a case of subclinical mastitis in a dairy farm where the hygienic conditions were moderate and free domestic carnivores were present. The positive animals were treated, and another sampling was performed for the positive animals and for the rest of the lactating ones. Swabs samples from the pharyngeal mucosa of the dogs were performed in order to identify the possible source of infection. The strong selective pressure exerted by the improper use of antibiotics has allowed for the development of bacterial strains resistant to the common drugs used. Antibiotic resistance represents an increasingly present problem in the world. This aspect, together with the possibility of some bacteria to cause disease in animals and humans, pushes for greater awareness of the risks associated with the development of resistant strains in livestock farming.

In the present study, we isolated S. canis 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 S. canis 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 S. canis isolates, obtained in the first and second sampling, was sequenced. Genetic relationships revealed a unique sequence type (ST24). The gene (tetM) related with resistance to TE was highlighted. Although the association between S. canis 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.