Fosmidomycin Treatment for Canine Bacterial Otitis Externa

In the Literature

Citron LE, Stefanovski D, You Y, et al. Fosmidomycin for the treatment of canine otitis externa: a randomised, double-blinded, controlled ‘split body’ clinical trial. Vet Dermatol. 2026;37(3):427-436. doi:10.1111/vde.70049

The Research…

Fosmidomycin, an antibiotic and antimalarial drug in the phosphonic acid class, targets the nonmelovanate pathway specific to domestic animal–adapted staphylococci.¹ Because many antibiotics used in veterinary medicine overlap those used in human medicine, there is a One Health concern for transfer of resistance genes. Fosmidomycin may represent a novel therapeutic in veterinary medicine that effectively targets staphylococcal infections via inhibition of the nonmelovanate pathway while reducing acquired resistance of Staphylococcus spp in humans.

This double-blinded, controlled study evaluated fosmidomycin for treatment of naturally occurring, bilateral, bacterial otitis externa in client-owned dogs (n = 15). Enrolled dogs were diagnosed with either cocci-exclusive or mixed bacterial (cocci and rods) infections, which were further characterized via aerobic culture and susceptibility.

One ear in each dog was treated with a compounded otic solution (1.25 mg/mL of fosmidomycin in sterile water and polyethylene glycol); the contralateral ear was treated with 9 mg/mL of enrofloxacin in sterile water, allowing each dog to serve as its own active control. Twice-daily administration in each ear was continued for 28 days in addition to prednisone (0.5-1 mg/kg PO every 24 hours for 14 days, then every other day until study end). Pet owners were instructed not to clean the ears at home; ears were cleaned by investigators with sterile saline only if excessive debris prohibited otoscopic examination. Otic cytology, otic discomfort, and otitis severity (using the 0-3 Otitis Index Score scale) were evaluated on days 0, 14, and 28. Owners assessed patient quality of life and otic pruritus and completed a questionnaire-based hearing assessment at each visit.

On bacterial culture, Staphylococcus schleiferi was the most commonly isolated organism, followed by Staphylococcus pseudintermedius, both of which had predominantly methicillin-susceptible isolates as well as 1 and 5 methicillin-resistant isolates, respectively. Proteus mirabilis, Pseudomonas aeruginosa, Corynebacterium spp, and beta-hemolytic Streptococcus spp were also isolated. There were no significant differences between the 2 treatment groups; both resulted in significant improvement in otic cytology, pruritus, and otitis severity. No adverse effects were attributable to either fosmidomycin or enrofloxacin. Investigators and owners reported good to excellent treatment efficacy on day 28.

… The Takeaways

Key pearls to put into practice:

• The most commonly isolated bacteria from canine otitis externa are Staphylococcus spp (eg, coagulase-negative S schleiferi, coagulase-positive Staphylococcus coagulans, S pseudintermedius).²

• Fosmidomycin topical otic solution combined with oral prednisone is a safe and effective therapy for bacterial otitis externa in dogs, including for Staphylococcus spp, P mirabilis, and P aeruginosa. Fosmidomycin is not currently available as a commercially formulated, FDA-approved product in veterinary medicine. Although this drug could be potentially compounded into an otic solution as described in this study, state and federal regulations for compounded medications should be followed. Fosmidomycin should not be considered as a first-choice treatment option due to the potential selection for resistance in gram-negative flora (eg, Escherichia coli).³

• Exploring antibiotic options (eg, fosmidomycin) that minimize antimicrobial resistance in human staphylococci is a key component of antimicrobial stewardship.