Antimicrobial susceptibility of bacterial isolates from 33 thoroughbred horses with arytenoid chondropathy (2005-2019).

Objective: To describe the prevalence and antimicrobial susceptibility of bacterial isolates cultured from surgical specimens of infected arytenoid cartilage and granulomas.

Study Design: Retrospective cohort study.

Animals: Thirty-three thoroughbred horses.

Methods: Hospital records were retrieved for all horses admitted to a referral hospital for arytenoid chondropathy surgery that had samples submitted for culture and sensitivity between 2005 and 2019. Descriptive analyses were performed.

Results: In total, 56 bacterial isolates were obtained. Gram-positive bacteria (58%), Gram-negative bacteria (54%), and anaerobes (33%) were cultured from samples. Fifty-eight percent of horses had multiple bacteria isolated. Streptococcus spp were the most common (32%), followed by Enterobacteriaceae (13%). Bacterial isolates were sensitive to ceftiofur (83%), followed by ampicillin (64%), tetracycline (48%), enrofloxacin (45%), trimethoprim-sulfamethoxazole (41%), and gentamicin (18%). Multidrug resistance (MDR) was present in 44% of bacterial isolates.

Conclusion: A wide variety of bacteria was cultured, providing evidence that secondary opportunistic infection by common respiratory bacteria is likely a factor in arytenoid chondropathy. Multidrug resistance was higher than what has been previously reported in equine respiratory samples. Trimethoprim-sulfamethoxazole had low effectiveness.

Clinical Significance: Because culture and sensitivity testing is not available in the diagnosis of mild to moderate arytenoid chondropathy, the information from this study may allow for more targeted broad-spectrum antimicrobial treatment to limit disease progression when the disease is first identified. The antimicrobial susceptibilities and MDR found in this study emphasize the importance of following current antimicrobial guidelines and highlight the requirement for surgical intervention rather than continued medical treatment in cases that do not resolve with initial antimicrobial therapy.