Bacterial infection, often secondary to viral respiratory disease, is an important cause of upper and lower respiratory diseases in horses. The cytological examination and bacterial culture of lower airway respiratory fluid samples are useful tools for the diagnosis of these infections and the selection of adequate antibiotic treatment. However, culture and sensitivity are not always performed and treatment is often initiated on an empirical basis when bacterial infection is suspected to avoid delays in treatment.
Selection of empirical therapy should be based on current knowledge of the prevalence and antibiotic susceptibility patterns of the bacteria most commonly isolated from affected horses. Protected antimicrobials, such as ceftiofur and enrofloxacin, should not be used as first line treatment due to their importance in human medicine.
A recent study has investigated antibiotic resistance of bacteria isolated using retrospective microbiological and cytological results from equine upper and lower respiratory fluid samples (n=615) processed in a commercial laboratory between 2002 and 2012. A further study of lower respiratory tract 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 the protected antimicrobial ceftiofur (100%) and to erythromycin (99%). Resistance to penicillin (12.5% of S equi subspecies equi from upper respiratory tract samples) and tetracycline (62.7%) was detected. Gram-negative isolates showed resistance to gentamicin, trimethoprim-sulfamethoxazole and tetracycline but susceptibility to the restricted use antimicrobial enrofloxacin (except Pseudomonas species, where 46.2% were resistant). Multiple drug resistance was detected in 1% of isolates.
The authors conclude that resistance to first-choice antibiotics in common equine respiratory tract bacteria was evident and warrants continued monitoring. This information can guide clinicians about the best empirical antimicrobial choices against certain pathogenic bacteria and help direct antibiotic stewardship efforts to conserve their efficacy.
References
Fonseca, JD., Mavrides, DE., Morgan, AL., Na, JG., Graham, PA., McHugh, TD. (2020) Antibiotic resistance in bacteria associated with equine respiratory disease in the United Kingdom. Veterinary Record 187, 189.