Avoiding Antimicrobial Resistance in Hospitalized Horses

Avoiding Antimicrobial Resistance in Hospitalized Horses

Antimicrobial-resistant infections were most common in horses coming out of surgery and that have had previous treatments with antimicrobials.

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Fighting bacteria is becoming more challenging as germs develop increasing resistance to antimicrobial drugs. That’s particularly true for hospitalized horses with weakened immune systems, Swiss researchers have learned. Their recent study shows that antimicrobial-resistant infections are most common in horses coming out of surgery and that have had previous treatments with antimicrobials.

“Many of these products just don’t work anymore,” said Julia Nora van Spijk, Dr Med Vet, of the University of Zurich Veterinary School’s Equine Department. She presented her work at the 2017 Swiss Equine Research Day, held earlier this year in Avenches.

“Fortunately, we have a ‘reserve stock’ of newer variants of antibiotics which are effective,” she said. “Due to the locations of the infections, which often did not require antimicrobial treatment or our newer-generation antimicrobials, 80% of our study horses survived their resistant infections.”

In their study, van Spijk and her fellow researchers reviewed records from 108 horses that had been hospitalized in their clinic and had a total of 110 infections. Nearly half of the cases involved horses recovering from surgery, primarily colic surgery, she said. Nonsurgical wounds and musculoskeletal infections represented 15% each of the total infections in the study. Most of the horses suffering from resistant strains of bacteria had already had previous antibiotic treatments.

The resistant bacteria in this study included Staphylococcus strains, S. aureus, Enterococcus spp, Escherichia coli, and other Methicillin-resistant S. aureus (MRSA) agents—“the same as we see in humans,” van Spijk said.

Classic antibiotic treatments, such as penicillin, cephalosporins, tetracycline, and ampicillin, were almost ineffective in fighting most infections, van Spijk said. Gentamicin showed relative efficacy against Coagulase-negative staphylococci (CNS) infections, whereas ampicillin worked on about two-thirds of the Enterococcus spp cases but not on other bacterial strains. Enrofloxin and marbofloxin showed moderate efficacy in treating S. aureus but high levels of resistance in CNS and Enterococcus cases. These are considered “second-line antimicrobials” or “reserve antimicrobials” and should only be given if routine antimicrobials do not show efficacy, van Spijk stressed.

Antimicrobials that have been around for a long time but are not often administered to horses (due to side effects and lack of dosage recommendations) showed low levels of resistance, she said. These included chloramphenicol, rifampicin, and nitrofurantoin.

The “star” antimicrobial in this study was vancomycin, which showed 0% resistance in all S. aureus, CNS, and Enterococcus cases, said van Spijk. However, she cautioned that vancomycin is an important antimicrobial in human medicine and is therefore considered a drug of last resort. “It should be used very sparingly, only in cases where the bacterial isolate causing the infection shows resistance against all other antimicrobials,” she said.

“Multiresistant bacterial agents are common among horses, and that’s especially true, as we have shown, of horses with weakened immune systems due to surgery or illness,” said van Spijk. “And furthermore, and/or consequently, multiresistant germs appear to be especially frequent in hospital settings.”

The two main risk factors for developing resistant bacterial infections in this study were a weakened immune system and previous antibiotic use, she said.

“Especially in a hospital setting, we need to place a high priority on good hygiene and biosecurity,” van Spijk said. “Hand hygiene is extremely important, in particular. We should wash and disinfect hands or change gloves between patients and clean and disinfect equipment thoroughly between each use.”

As antimicrobial use is associated with the development of antimicrobial resistance, veterinarians should avoid administering antimicrobials unnecessarily, she said.

About the Author

Christa Lesté-Lasserre, MA

Christa Lesté-Lasserre is a freelance writer based in France. A native of Dallas, Texas, Lesté-Lasserre grew up riding Quarter Horses, Appaloosas, and Shetland Ponies. She holds a master’s degree in English, specializing in creative writing, from the University of Mississippi in Oxford and earned a bachelor's in journalism and creative writing with a minor in sciences from Baylor University in Waco, Texas. She currently keeps her two Trakehners at home near Paris. Follow Lesté-Lasserre on Twitter @christalestelas.

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