Mobile colistin resistance (mcr) genes (mcr-1 to mcr-10) threaten the efficacy of colistin (COL), a polymyxin antibiotic that is used as a last-line agent for the treatment of deadly infections caused by multidrug-resistant and extensively drug-resistant bacteria in humans and animals. COL has been used for more than 60 years for the prophylactic control and treatment of infections in livestock husbandry but not in horses. Polymyxin B is used for the prophylactic control and empirical treatment of infections in horses without conducting sensitivity tests. The lack of sensitivity testing exerts selection pressure for the acquisition of the mcr gene. By horizontal transfer, mcr-1, mcr-5, and mcr-9 have disseminated among horse populations globally and are harbored by Escherichia coli, Klebsiella, Enterobacter, Citrobacter, and Salmonella species. Conjugative plasmids, insertion sequences, and transposons are the backbone of mcr genes in the isolates, which co-express genes conferring multi- to extensive-drug resistance, including genes encoding extended-spectrum β-lactamase, ampicillinase C, fosfomycin, and fluoroquinolone resistance, and virulence genes. The transmission of mcr genes to/among bacterial strains of equine origin is non-clonal. Contact with horses, horse manure, feed/drinking water, farmers, farmers' clothing/farm equipment, the consumption of contaminated horse meat and its associated products, and the trading of horses, horse meat, and their associated products are routes for the transmission of mcr-gene-bearing bacteria in, to, and from the equine industry.
Keywords: antimicrobial resistance; colistin resistance; equine; mobile colistin resistance (mcr) gene.