Antimicrobials and Antimicrobial Resistance

When a prescribed medication doesn’t work as it should, one cause could be antimicrobial (antibiotic) resistance. Is the widespread use of antibiotics selecting for stronger and stronger pathogens that can resist the drugs we use to fight them? This was discussed at the Antimicrobials and Antimicrobial Resistance Table Topic, moderated by Josie Traub-Dargatz, DVM, MS, Dipl. ACVIM, professor of equine medicine at Colorado State University's Veterinary Teaching Hospital and Biomedical Sciences Department in Fort Collins, Colo.; and Scott Weese, DVM, DVSc, Dipl. ACVIM, a microbiology specialist at the University of Guelph in Ontario, Canada. Traub has worked on the American Veterinary Medical Associations’ (AVMA) committee on antimicrobial resistance as a representative of the American Association of Equine Practitioners (AAEP).

The AAEP has developed guidelines for the judicious use of antimicrobials in an attempt to clarify for the membership the principles of judicious or prudent use of antimicrobials with a goal of keeping the drugs used to treat bacterial infection effective in resolving disease. In the guidelines, it was made clear that AAEP members get regular education on antimicrobial use and work with horse owners to implement methods to prevent disease through vaccination and biosecurity/hygiene methods.

When an antimicrobial treatment doesn’t work, it is hard for veterinarians to determine if it is an isolated case or if they are dealing with a resistant pathogen. Weese said that in greater than 90% of cases, commonly used antibiotics should be effective at eliminating infection. Weese said that he, Traub-Dargatz, and others at university hospitals usually see the worst cases since they are referred to them, often because they have not responded to conventional treatment. Therefore, those who work in referral clinics might be recognizing the occurrence of antimicrobial resistance before field practitioners.

Ideally, samples should be collected for bacterial culture prior to the start of antimicrobial treatment. Owners are sometimes reluctant to spend the money for bacterial culture. However, not performing a culture might increase the chance of treatment failure and result in more difficulty eliminating the infection and a longer (and more expensive) course of treatment. If an initial culture is not submitted, it would be reasonable to store a sample that could be submitted if the initial treatment fails. Bacterial culture, either with an initial or follow-up sample, should be performed in all cases where initial treatment fails.

An example of multi-drug resistant bacteria infecting horses are several Salmonella infections. These include Salmonella typhimurium DT 104 and Salmonella newport MDR Amp C, which are resistant to five and up to nine antimicrobials, respectively.

When antibiotics are used for treatment of respiratory disease, there are bacteria that live in the digestive tract that might be exposed to these antibiotics. While these bacteria do not cause disease, they might acquire, then transfer genes coding for resistance to bacteria that can cause disease. The relationship between different species of gut flora is complicated and is not fully understood.

More research is needed to learn how long each antibiotic should be used during a course of treatment for each particular bacterial disease, and to determine the maximum amount of time an antibiotic can be in general use for that treatment before resistance develops. Currently, practitioners are left wondering if a shorter and more intense treatment regime is more effective, or whether a less-intense, but longer treatment series is better and more able to prevent relapse. Attendees agreed there is much still to learn about the most risky uses of antimicrobials and the best ways to preserve their effectiveness.

Several practitioners expressed concern that indiscriminate use of antibiotics by horse owners was causing resistant organisms, since some antibiotics are available from sources other than a veterinarian and are used without veterinary advice. Many expressed the desire to have stronger regulations on the accessibility of drugs.

Another question was brought up on whether probiotics could help deliver a two-part attack on a pathogen with less concern for resistance; research is needed to answer this question. If probiotics are used, Weese highly recommended making sure that the product has an accurate label. A large percentage of commercial probiotics are mislabeled, he said, making it impossible to use them accurately. There are also concerns about poor quality control with many commercial probiotics. Horse owners should consult with their veterinarians about choosing an appropriate probiotic, and what dose to use.

Until further research is done, the issue of antimicrobial resistance will continue to pose a challenge. However, for more information on what is known about antimicrobials and antimicrobial resistance, Traub-Dargatz and Weese recommend the book Antimicrobial Therapy in Veterinary Medicine by J.F. Prescott, J.D. Begget, and R.D. Walker for veterinarians who want more information on this topic.

About the Author

Sarah Evers Conrad

Sarah Evers Conrad has a bachelor’s of arts in journalism and equine science from Western Kentucky University. As a lifelong horse lover and equestrian, Conrad started her career at The Horse: Your Guide to Equine Health Care magazine. She has also worked for the United States Equestrian Federation as the managing editor of Equestrian magazine and director of e-communications and served as content manager/travel writer for a Caribbean travel agency. When she isn’t freelancing, Conrad spends her free time enjoying her family, reading, practicing photography, traveling, crocheting, and being around animals in her Lexington, Kentucky, home.

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