In recent years, the medical world has been abuzz about concerns over antimicrobial resistance (AMR) to available antimicrobial agents. This phenomenon is relevant to treatment of infectious disease in all species, including humans. Often, equine practitioners are in the position of treating a patient empirically, without the benefit of bacterial culture and sensitivity due to time-sensitive and potentially life-threatening or performance-threatening maladies. The more that antibiotics are used, the more opportunity for microorganisms to develop resistance to those drugs.
A study in South Africa tested 1,505 bacterial isolates from 907 samples from 2007-2014 [Chipangura, J.K.; Chetty, T.; Kgoete, M.; Abernethy, D.; Naidoo, V. Prevalence of antimicrobial resistance from bacterial culture and susceptibility records from horse samples in South Africa. Preventive Veterinary Medicine, Vol 156, Issue 1, June 2017].
While a large diversity of tissues was sampled, the greatest percentage of samples was taken from uterine swabs, respiratory swabs and aspirates, and wounds. The authors pointed out that most bacterial isolates were obtained from horses that were non-responsive to empirical treatment rather than as random samplings.
The results point to a trend for increasing antimicrobial resistance. While the results of the study are specific to equine cases managed at the University of Pretoria, South Africa, their finding that more than 50% of the isolates demonstrated multiple drug resistance (MDR, resistant to three or more antimicrobial agents) is troubling.
Resistance to specific drugs:
- Amikacin — E. coli, Staphylococcus
- Amoxicillin/Ampicillin — Corynebacteria, Lactobacillus, Salmonella
- Chloramphenicol — Enterococcus, E. coli, Pseudomonas, Streptococcus, Staphyloccus, Salmonella
- Enrofloxacin — E. coli, Staphyloccus, Salmonella, Pseudomonas
- Gentamycin — Staphyloccus, Salmonella
Resistance by E. coli poses a threat in that these organisms are able to confer drug-resistant plasmids to pass resistance genes to other bacteria.
Another relevant point to be considered by equine practitioners is stated by the authors: “The finding of high resistance and MDR across all bacterial isolates could be due to noncompliance as has been concluded by Hughes et al. (2013) that veterinarians prescribing antimicrobials for horses commonly use inappropriate doses and rarely utilize guidelines for antimicrobials designed to minimize resistance.”
When and where possible, practitioners should use bacterial culture and sensitivity diagnostic procedures to obtain evidence-based information to plan an effective therapeutic strategy.