Two-thirds of the 1711 Staphylococcus samples taken from horses that were submitted to a major American diagnostic laboratory over a 17-year period were resistant to at least one antimicrobial drug, researchers report.
Some showed resistance to up to 12 of the 16 antimicrobials assessed in the study, which the researchers described as quite concerning.
Antimicrobial resistance has become a global scientific and public health concern in both human and veterinary medicine. It limits traditional treatment options and increases costs.
Researchers from the University of Tennessee and University of Kentucky set out to investigate the burden and patterns of antimicrobial resistance among equine Staphylococcus samples submitted to the University of Kentucky Veterinary Diagnostic Laboratory from 1993 to 2009.
The testing of the 1711 samples, which came from 26 horse breeds, looked for evidence of resistance to 16 drugs. The most common breed was Thoroughbreds (74.3%) followed by Tennessee Walking Horses (5.6%). Overall, more samples were submitted from female horses (83.7%) than male horses (16.3%)
A total of 66.3% of the samples proved resistant to at least one antimicrobial, most of which were Staphylococcus aureus (77.1%), while a quarter were found to be multi-drug resistant.
The highest level of resistance was to penicillins (52.9%).
Among drug classes, samples had the highest rate of antimicrobial resistance to at least one type of β-lactams (49.2%), followed by aminoglycosides (30.2%).
Almost half of the multi-drug resistance samples in the study had resistance profiles that included penicillin, kanamycin, sulfonamides, and trimethoprim-sulfadiazine.
Agricola Odoi and her colleagues, writing in the journal BMC Veterinary Research, said they found significant associations between the odds of antimicrobial resistance and horse breed, species of organism and year.
Of the samples with known breed information, the highest proportion of resistant samples was from Thoroughbreds (70.5%) followed by Standardbreds (68.6%) and Arabians (68.4%), while the lowest proportion was seen in mixed breeds (40.0%).
Standardbreds had the highest proportion of multi-drug resistant samples (37.1%), followed by Thoroughbreds (31.1%), and Quarter Horses (18.3%).
The lowest proportion of multi-drug resistance was in the Tennessee Walking Horse (3.4%).
Although females seemed to have a slightly higher level of antimicrobial resistance (68.1%) than males (64.0%), these differences were not statistically significant.
“However, the same does not apply to the levels of multi-drug resistance between the sexes,” they reported. “In fact, males had a markedly higher proportion of multi-drug resistance (32.9%) than females (25.4%).”
Similarly, significant associations were identified between odds of multi-drug resistance and breed and age.
Foals under a year old showed the highest levels of antimicrobial resistance (75.9%), followed by horses 2–4 years old (67.3%), and yearlings (1–2 years old) (65.6%). Horses aged 4 or older had the lowest levels of antimicrobial resistance (60.0%).
Foals again showed the highest levels of multi-drug resistance (37.6%) when compared with other age groups – the levels in horses aged 2–4 were found to be 28.9% and those aged 1–2 were 18.8%.
“While some isolates had resistance to up to 12 antimicrobials, antimicrobial resistance profiles featuring single antimicrobials such as penicillin were more common than those with multiple antimicrobials,” they reported.
Demographic factors were found to be significant predictors of antimicrobial resistance and multi-drug resistance.
“The fact that some isolates had resistance to up to 12 of the 16 antimicrobials assessed is quite concerning,” they said.
The researchers said the proportion of antimicrobial resistant samples seen in the study suggested that levels of antimicrobial resistance were high for both pathogenic and non-pathogenic Staphylococcus species.
To address the high levels of resistance seen, future studies will need to focus on antimicrobial prescription practices and education of both practitioners and animal owners on better use of antimicrobials to slow development of resistance, they said.
“Future studies will need to focus on improving our understanding of antimicrobial use in horses as this will allow for more informed antimicrobial stewardship programs.”
They said resistance surveillance in horses needed to include better record-keeping and lab submission information (such as pre-treatment history).
The study team comprised Odoi and Ronita Adams, both from the University of Tennessee; and Jackie Smith, Stephen Locke, Erica Phillips, Erdal Erol and Craig Carter, all from the University of Kentucky.
An epidemiologic study of antimicrobial resistance of Staphylococcus species isolated from equine samples submitted to a diagnostic laboratory
Ronita Adams, Jackie Smith, Stephen Locke, Erica Phillips, Erdal Erol, Craig Carter and Agricola Odoi.
BMC Veterinary Research 2018 14:42 https://doi.org/10.1186/s12917-018-1367-6