Mcr-1 and other new resistance genes: What is the threat to horses?

A scanning electron micrograph of a human neutrophil ingesting MRSA. Photo: National Institutes of Health, public domain via Wikimedia Commons
A scanning electron micrograph of a human neutrophil ingesting MRSA. Photo: National Institutes of Health, public domain via Wikimedia Commons

Equine medicine has not escaped the challenges posed by resistant bacteria, a specialist reports in the latest issue of Equine Disease Quarterly.

Professor Scott Weese, of the Ontario Veterinary College in Canada, said the recent identification of the mcr-1 resistance gene in bacteria from humans and animals had focused attention on the emerging epidemic of multidrug resistance.

Multidrug resistant Gram-negative bacteria, particularly those of the Enterobacteriaceae family, for example E. coli, Klebsiella and Enterobacter, were not new, as serial waves of resistance mediated by a wide range of genes have been encountered, he said.

Yet, emergence of mcr-1 was of concern because it conferred resistance to colistin, an antimicrobial drug that can be the only option for some highly drug-resistant infections.

Weese, a specialist in veterinary internal medicine with an interest in bacterial infections in animals and humans, said concerns have been expressed that mcr-1 had ushered in the era of pan-resistant infections – infections where there were no antimicrobial options.

“Equine medicine has not escaped the challenges posed by resistant bacteria, including multidrug resistant Enterobacteriaceae,” he said.

In recent years, there have been various reports of extended spectrum beta-lactamase (ESBL) producing Enterobacteriaceae in horse. These are resistant to penicillins and most cephalosporins, and typically also have acquired resistance to various other antimicrobials, limiting treatment options. Susceptibility to only aminoglycosides (for example, amikacin) and carbapenems (for example, meropenem) was a common pattern.

ESBL-producing bacteria have been identified in horses in multiple countries, both in healthy colonized horses and horses with clinical infections.

While carbapenem use is very limited in horses, carbapenemase-producing Enterobacteriaceae (CPE) and Acinetobacter, which are resistant to penicillins, cephalosporins, and carbapenems, and usually a range of other antimicrobials, have been found in limited numbers in horses.

“With the common co-occurrence of resistance to other drug classes, infections by bacteria such as these approach the ‘pan resistance,’ from a practical standpoint, considering limitations in antimicrobials that can be used in horses.

“Of additional note is the fact that ESBLs found in horses are often the same types that are found in people (for example, CTX-M-15), highlighting both the potential for zoonotic transmission and the likelihood that some equine infections are human in origin

“What does identification of mcr-1 mean to the equine industry and equine veterinarians?

“The likelihood of encountering a horse infected with a bacterium possessing mcr-1 is exceedingly low.

“However, if bacteria harboring genes such as these increase in humans, food animals, and the environment, equine infections are probably inevitable.

“Yet, while attracting less attention, the endemic level of resistance to cephalosporins, fluoroquinolones, and aminoglycosides is likely of greater relevance in horses because of their increasing incidence and the limited treatment options.

“Regardless, awareness of emerging trends in humans is important. As new issues emerge in humans, it is possible that the same problems will emerge in horses given the close contact between humans and horses, the potential for interspecies transmission (in both directions), and the potential for common source infection – for example, from food animals or the environment.”

Weese said veterinarians were increasingly encountering horses infected with multidrug-resistant pathogens, and the pathogens of concern continued to change.

“Twenty years ago, MRSA and ESBLs in horses were of little interest. Now, they are far from rare. It is almost certain that the next 20 years (if not the next five years) will be accompanied with new antimicrobial resistant pathogens and challenges.

“There are no simple answers for battling the scourge of antimicrobial resistance. However, awareness of the issues, optimizing antimicrobial use, and focusing on infection control measures to reduce the need for antimicrobials are important basic and practical matters that every equine veterinarian and horse caretaker can undertake.”

Equine Disease Quarterly is funded by underwriters at Lloyd’s, London.

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