Infectious Diseases

A Guide to Empiric Therapy for Equine Foot Infections

By Paul Basilio

Most equine foot infections are simple abscesses that a veterinarian or farrier can treat without expecting major complications. More serious infections, however, can have career-ending or life-threatening complications.

Many clinicians choose to start empiric antibiotic therapy while results of culture and sensitivity tests are pending, but which ones have the best chance of working?

“With foot infections, we typically choose broad-spectrum empiric therapy because there’s commonly a mix of gram-positive and gram-negative bacteria,” according to Benjamin Flakoll, DVM, an ambulatory and podiatry associate at Rood & Riddle Equine Hospital in Saratoga Springs, N.Y. “However, there are no studies regarding typical bacterial isolates or susceptibility profiles for equine foot infections.”

Dr. Flakoll and colleagues set out to create a quantitative list of the most commonly isolated bacteria in equine foot infections and their susceptibility profiles to help clinicians guide the rational selection of antimicrobials. He discussed the results during a presentation at the 70th AAEP Annual Convention in Orlando, Fla.

The Study
The investigators analyzed cultures of equine foot infections submitted to the Rood & Riddle laboratories in Saratoga Springs and Lexington, Ky., from 2010 through 2020. Most of the samples (78%) were from Thoroughbreds, with ages ranging from 10 days to 26 years (mean, 6.9 years). A total of 1,179 organisms from 595 reports were included in the study.

The infections were predominantly polymicrobial (≥2 isolated organisms, 71%) and gram-negative (70%). Escherichia coli was present in 54.8% of foot infections, making it the most frequently isolated pathogen.

Analyses of culture and susceptibility data for organisms present in more than 5% of foot infections showed 6 gram-negative organisms (E. coli, Proteus vulgaris, Proteus mirabilis, Enterobacter cloacae, Klebsiella pneumoniae and Pantoea agglomerans), as well as 2 gram-positive organisms (alpha- and beta-hemolytic Streptococcus).

The antibiotics were then categorized by effectiveness (see Table):

• High: <10% of isolates exhibited resistance
• Moderate: 10% to 30% of isolates exhibited resistance
• Low: 30% to 50% of isolates exhibited resistance
• Ineffective: >50% of isolates exhibited resistance

“In general, the antibiotics that appear to be highly effective against gram-negative organisms and that are used commonly in equine practice include amikacin.”

For the gram-positive bacteria, no antibiotics were highly effective against alpha-hemolytic Streptococcus, but enrofloxacin, doxycycline, gentamicin and chloramphenicol were moderately effective. Most antibiotics, except for amikacin and sulfonamides, were highly effective against beta-hemolytic Streptococcus.

The study’s limitations included a lack of clinical data regarding individual infections and the inclusion of aerobic cultures only.

“Antimicrobial therapy should be reserved for severe cases, and it should be guided by culture and sensitivity results, when possible,” Dr. Flakoll said.

If empiric therapy is needed, then the results of this study may help guide clinicians to the best possible choices, he said. MeV