Background: Strangles is a highly infectious disease, endemic throughout the world. There is a paucity of reported data on antimicrobial susceptibility in clinical isolates.
Objectives: To describe the antimicrobial susceptibility results of Streptococcus equi subsp. equi (S. equi) isolated from horses at a referral center. Animals: 45 horses that were diagnosed with S. equi infection in a referral hospital between 2015 and 2023 that had positive cultures and antimicrobial susceptibility testing.
Methods: Retrospective, observational study. Data were collected from the medical records: signalment, clinical signs, sampling site, and MIC to a panel of antimicrobials were recorded.
Results: 45 horses met the inclusion criteria, with 46 different isolates of S. equi. The median age was 9.78 (0.1-25) years. There were 64% males and 36% females. The most common breeds were Quarter Horse (47%) and Thoroughbred (13%). 65% of samples were obtained from the guttural pouches; 17.4% were swabs from submandibular abscesses, and 17.4% were from other sites, including other abscesses and sinus aspirate. 47% of the horses had received antimicrobial treatment before sampling. 100% of tested isolates were susceptible to ceftiofur (n=46), doxycycline (n=45), minocycline (n=33) and trimethoprim sulfamethoxazole (n=46). 97.8% (n=46) of tested isolates were susceptible to penicillin, ampicillin and rifampin. 97.6% and 97.0% of tested isolates were susceptible to azithromycin and clarithromycin, respectively. Conclusions and clinical importance: This study provides information on the antimicrobial susceptibility of S. equi isolates from clinical equine cases. S. equi isolates remain highly susceptible to ß-lactams, minocycline, doxycycline, rifampin, potentiated sulfonamides, azithromycin and clarithromycin.
