Antimicrobial Resistance

Antimicrobial drugs in veterinary medicine have made a dramatic improvement in the health of our horses, but it's alarming when infections do not respond to these drugs. "It is not a new phenomenon, but there has been ever-growing concern about antimicrobial resistance," said Josie Traub-Dargatz, DVM, MS, Dipl. ACVIM, of Colorado State University, in her presentation at the AAEP Convention. (See article #4047 online.)

The frequency of treatment failure in equine patients due to antimicrobial resistance is unknown because there is no national system to monitor resistance of animal pathogens. However, there are documented cases of resistant infections in horses, and many reasons why veterinarians should be concerned about the issue.

Traub-Dargatz said veterinarians should be concerned about this issue because it is unlikely that any new antimicrobials will arrive on the market for veterinary use in the near future.

"There hasn't been a new class of antimicrobial since the '80s," she said. "The cost to bring a new drug to human practice is estimated at $300 million."

Types of antimicrobial resistance were reviewed, including intrinsic resistance (constitutive) and acquired resistance (mechanism, organism-dependent). Intrinsic resistance occurs when the organism is not affected by the antimicrobial naturally because a lack of target binding sites or structures with which the antimicrobial interacts. If organisms develop mechanisms to circumvent the effects of antimicrobials, acquired resistance occurs. "Resistance is not caused by antimicrobial use, but antimicrobials can select for resistant organisms," she explained.

She emphasized the importance of knowing the prudent antimicrobial use guidelines developed by the AAEP in 2000. These can be found at

The Centers for Disease Control has led a public education campaign using the following steps, which can be directly applied to animal owners:

  • Prevent infection;
  • Diagnose and treat infection effectively;
  • Use antimicrobials wisely, and;
  • Prevent transmission of the agent once an infected patient is identified.

"I think there's going to be increased scrutiny of antimicrobial use," Traub-Dargatz predicted. In the meantime, she stressed that veterinarians should stay appraised of the issues, use antimicrobials judiciously, and work with clients to avoid the risks of horses acquiring infections in the first place.

About the Author

Stephanie L. Church, Editor-in-Chief

Stephanie L. Church, Editor-in-Chief, received a B.A. in Journalism and Equestrian Studies from Averett College in Danville, Virginia. A Pony Club and 4-H graduate, her background is in eventing, and she is schooling her recently retired Thoroughbred racehorse, Happy, toward a career in that discipline. She also enjoys traveling, photography, cycling, and cooking in her free time.

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