Interpreting Antimicrobial Sensitivity: More Than Sensitivity/Resistance 

Combine common sense with culture and sensitivity when assessing antimicrobial reports in equine patients.
Veterinarian preparing antibiotic for horse
Veterinarians must consider what antibiotic is best suited for a specific patient’s needs. | Getty Images

At the beginning of her presentation during a Burst Session at the 2023 AAEP Convention in San Diego, Emily Berryhill, DVM, Dipl. ACVIM, assistant clinical professor of equine internal medicine at the University of California, Davis, shared a typical antimicrobial sensitivity/resistance report associated with a long list of antibiotics for an isolated microbe. She then asked that we focus on a few specific features of the report: 

  1. Ask yourself, does the bacteria make sense and does the isolate make sense for anatomic location and disease process?  

“For example, if you do a transtracheal wash and get things you’ve never heard of, you need to consider whether the report is actually providing useful information or if the sample contaminated,” said Berryhill. “But, if you grew bacteria like Escherichia coli and Streptococcus zooepidemicus, they’re probably pathogens implicated in the disease.” 

If faced with an unexpected bacterium, we should be asking, “Was the collection process maximized to get a representative sample?”  

“For example, is taking a nasal swab in a case of bronchopneumonia appropriate? No,” she said.  

  1. Look at the tested antimicrobials. 

“The list varies depending on the laboratory and may include antibiotics used in human or small animal medicine that aren’t relevant to equine practice,” said Berryhill. “Just because an antibiotic is listed doesn’t mean it’s appropriate.”  

  1. Look at the MIC (mean inhibitory concentration). 

Recall that these levels are based on in vitro resistance and susceptibility. 

“A main take-home here is that we’re not always focused on the lowest MIC,” she emphasized. “We need to also consider the type of bacteria and the origin of infection.”  

For example, if you’re dealing with a case of osteomyelitis, then consider which antibiotic will best penetrate bone.  

Veterinarians must also consider the patient. What antibiotic would be best suited for that specific horse’s needs? Enrofloxacin, for example, would not be appropriate for a foal.  

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