A survey of the antimicrobial susceptibility of Escherichia coli isolated from Sable Island horses

The feral horses of Sable Island are a geographically isolated population located ~160 km off the east coast of Nova Scotia, Canada. Because these horses have no contact with domestic animals, minimal contact with people and have never received antimicrobials, they offer a unique opportunity to study the dissemination of antimicrobial resistance in unmanaged populations. As part of an ongoing multi-disciplinary and individual-based monitoring program, we collected feces from 508 geolocalized horses (92 % of the total population) between July and September 2014. We selectively cultured Escherichia coli on MacConkey and CHROMagar ESBL. Antimicrobial susceptibilities were determined and organisms resistant to β-lactam antimicrobials were screened for β-lactamase genes by PCR. E. coli was recovered from 146 (28.7%) individuals and the majority of isolates (97%) were susceptible to all drugs tested. Resistance to tetracycline was most common, including organisms isolated from 4 (2.7%) of the colonized horses. A single isolate resistant to ampicillin, ceftriaxone and ceftiofur was identified which possessed the CTX-M-1 gene. Our findings demonstrate that although antimicrobial resistance is not common in this remote population, clinically relevant resistance genes are present.

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