Antibiotic resistance in bacteria associated with equine respiratory disease in the United Kingdom

05 Sep 2020

Fonseca, J. D., Mavrides, D. E., Morgan, A. L., Na, J. G., Graham, P. A., McHugh, T. D.

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Respiratory diseases account for the highest number of clinical problems in horses compared with other body systems. While microbiological culture and sensitivity testing is essential for certain cases, knowledge of the most likely bacterial agents and their susceptibilities is necessary to inform empirical antibiotic choices.


A retrospective study of microbiological and cytological results from upper and lower respiratory samples (n=615) processed in a commercial laboratory between 2002 and 2012 was carried out. A further study of lower respiratory samples from horses with clinical signs of lower respiratory disease from May to June 2012 was undertaken.


Both studies revealed Streptococcus equi subspecies zooepidemicus, Pseudomonas aeruginosa, Pasteurella species, Escherichia coli and Bordetella bronchiseptica as the most frequently isolated species. S equi subspecies zooepidemicus and subspecies equi were susceptible to ceftiofur (100 per cent) and erythromycin (99 per cent). Resistance to penicillin (12.5 per cent of S equi subspecies equi from upper respiratory tract samples) and tetracycline (62.7 per cent) was also detected. Gram-negative isolates showed resistance to gentamicin, trimethoprim-sulfamethoxazole and tetracycline but susceptibility to enrofloxacin (except Pseudomonas species, where 46.2 per cent were resistant). Multiple drug resistance was detected in 1 per cent of isolates.


Resistance to first-choice antibiotics in common equine respiratory tract bacteria was noted and warrants continued monitoring of their susceptibility profiles. This can provide information to clinicians about the best empirical antimicrobial choices against certain pathogenic bacteria and help guide antibiotic stewardship efforts to converse their efficacy.