Equine veterinarians should prescribe antibiotics conservatively, and horse owners should support this approach, said researchers who studied resistance to antibiotics in 216 horses. Their study showed that a common strain of Escherichia coli can be resistant to commonly prescribed drugs such as sulfamethoxazole, trimethoprim-sulfamethoxazole (TMS), gentamicin, and tetracycline, particularly in horses that have been hospitalized. Experts worry that overprescribing these drugs could lead to widespread antimicrobial resistance that would render common treatments ineffective.

“In general, over time we believe that use of antimicrobial drugs inevitably results in resistance,” explained Paul Morley, DVM, PhD, Dipl. ACVIM, associate professor in Epidemiology and Biosecurity at Colorado State University. “As such, we promote the most conservative use that is appropriate for the given situation, and we believe that veterinarians are best able to guide these decisions relative to health of animals.”

Morley and four other researchers compared E. coli antibiotic resistance in 68 horses in equine hospitals that had been treated with antibiotics for at least three days with 63 hospitalized horses that had not been treated with antibiotics. As a control, they studied 85 horses that had not been hospitalized or treated with antibiotics. The results showed that for E. coli:

• Resistance to antibiotics is higher in horses that have been hospitalized than those that have not, regardless of whether or not they received antibiotics in the hospital.
  
• All hospitalized horses were likely to show resistance to multiple antibiotics.
  
• Although resistance to antibiotics is lower in healthy, non-hospitalized horses, it still occurs.

The study acknowledges that resistance to some antibiotics occurs because the drugs are widely prescribed. However, resistance to other antibiotics was observed even when the drug had not been used often, possibly due to an E. coli mutation DNA exchange. The article also speculated that resistant bacteria could be transferred among the hospital population through cleaning, brushing, feeding, and treatment