Antimicrobial Resistance in Australian Companion Animals: A Growing Problem?

In the first study of its kind in Australia, researchers measured the antimicrobial susceptibility of coagulase-positive staphylococci (CoPS) isolated from infections in companion animals and identified risk factors for methicillin resistance. Study results, published in PLoS ONE, indicate the need for effective biosecurity and infection control measures in veterinary practices, the authors wrote.

The emergence of methicillin-resistant strains of CoPS, particularly methicillin-resistant Staphylococcus aureus (MRSA) and Staphylococcus pseudintermedius (MRSP), has compounded the difficulty of treating infections in humans and animals; resistance to methicillin and all other beta-lactams is conferred through acquisition of the mecA or mecC genes. MRSA is recognized as a public health issue, while both MRSA and MRSP are major veterinary health issues.

Although increasing incidences of MRSA and MRSP infections in Australia have been reported, the frequency of methicillin resistance in the clinical infections of Australian companion animals remained unknown prior to the current study.

Study Design

Researchers collected 888 CoPS isolates obtained from infection sites or specimens from dogs, cats, and horses. Each isolate underwent susceptibility testing to 16 antimicrobials from 12 antimicrobial classes.

Methicillin resistance for presumed MRSA and MRSP strains was phenotypically confirmed by colony appearance and resistance to cefoxitin, oxacillin, or both. Using demographic factors and clinical history data accompanying the isolates, researchers performed logistic regression analysis to identify potential risk factors for methicillin resistance.

Results

S. pseudintermedius and S. aureus were the most commonly identified CoPS, with S. pseudintermedius comprising 71% of the isolates. Methicillin resistance was observed in 12% of S. aureus isolates and 13% of S. pseudintermedius isolates; researchers considered this a moderate level of resistance.

Resistance Characteristics

S. pseudintermedius was identified in dog and cat isolates. In isolates from both species, resistance to amoxicillin—clavulanate was most common, followed by trimethoprim–sulfamethoxazole. Resistance to amikacin and rifampicin was low to undetectable. Approximately half of S. pseudintermedius isolates demonstrated full susceptibility to 8 antimicrobial classes, and 13% demonstrated multidrug resistance (MDR); the majority of MDR isolates were MRSP.

S. aureus was identified in dog, cat, and horse isolates. Resistance to amoxicillin—clavulanate was most common in isolates from all three species. Frequency of resistance to other antimicrobials was variable between the species. Nearly 70% of S. aureus isolates demonstrated full susceptibility to 8 antimicrobial classes, and 13% demonstrated MDR; all MDR isolates were MRSA. Fluoroquinolone resistance was more likely in dogs and cats than in horses, while amikacin resistance was most likely in horses. Interestingly, fluoroquinolone resistance in dogs and cats was always associated with methicillin resistance.

Risk Factors

Logistic regression analysis was performed only for S. pseudintermedius isolates from dogs. This analysis indicated that infection site and prior antimicrobial treatment strongly correlated with MRSP infection. Specifically, S. pseudintermedius in surgical site and skin and soft tissue infections was significantly more likely to be methicillin-resistant in dogs receiving prior antimicrobial treatment than in untreated dogs. This finding, the researchers wrote, “[reinforces] the need for veterinarians to place a high priority on implementing infection control procedures, biosecurity and antimicrobial stewardship.”

Conclusion and Discussion

Researchers observed greater antimicrobial resistance in MRSP isolates than MRSA isolates. Given the isolates’ infrequent resistance to amikacin, researchers recommended continuing to reserve this antimicrobial for MDR infections as a last resort.

With the frequency of antimicrobial resistance of CoPS demonstrated in this study, researchers proposed continued surveillance to “[understand] the trends in emergence and dissemination of antimicrobial resistance in companion animals.”

Dr. JoAnna Pendergrass received her Doctor of Veterinary Medicine degree from the Virginia-Maryland College of Veterinary Medicine. Following veterinary school, she completed a postdoctoral fellowship at Emory University’s Yerkes National Primate Research Center. Dr. Pendergrass is the founder and owner of JPen Communications, a medical communications company.