Treating S pseudintermedius—Topical Therapy

Karen A. Moriello, DVM, Diplomate ACVD, January 7, 2010

Staphylococci have been associated with several suppurative conditions in dogs, including otitis externa. It is generally accepted that Staphylococcus intermedius is the major pathogenic Staphylococcus species in dogs. Recent studies, however, have shown canine staphylococcal isolates previously identified as S intermedius to be S pseudointermedius. Infections with S pseudointermedius are commonly treated with antibiotics, but acquired antimicrobial resistance often occurs in canine isolates. Acetic and boric acids have been proposed as an alternative local treatment for bacterial infections. To study the bactericidal effect of a commercially available solution of 2% acetic acid and 2% boric acid, 2 phenotypically identical S pseudointermdius isolates were incubated with several dilutions of the acetic and boric acid solution. Approximately 5 × 107 colony-forming units (CFUs) of each isolate were suspended in 5 mL of undiluted, 1:2 diluted, and 1:4 diluted (in distilled water) acetic acid and boric acid solution. Controls consisting of 5 × 107 CFUs of each isolate suspended in distilled water were also included. The suspensions were incubated at 30o C; samples were collected at 0, 30, 60, 120, 180, 240, 300, and 360 minutes; and bacterial titers (CFUs per mL) were determined. Bacterial titer did not change in the controls throughout the experiment. Bacteria in the undiluted sample were inactivated within 30 minutes. In the 1:2 and 1:4 diluted samples, bacteria were inactivated within 60 and 120 minutes, respectively. No difference was noted between the 2 bacterial isolates. The authors concluded that an aqueous solution containing at least 0.5% acetic acid and 0.5% boric acid has bactericidal activity against S pseudointermedius and may be useful for treating local infections. In vivo infections may be more complex, however, and further clinical studies are needed to confirm clinical efficacy.

COMMENTARY:

The rising recognition of methicillin-resistant Staphylococcus in the veterinary community has forced us to reconsider our approache to the treatment of bacterial skin infections. From a client’s perspective, “popping a pill” is the easiest treatment approach; however, from a medical perspective, systemic therapy for every focal skin infection may be “overkill.” Topical therapy is a reasonable alternative for the management of focal bacterial infections or bacterial overgrowth syndrome (eg, fold pyoderma).

This study was conducted on only 2 strains of S pseudointermedius, neither of which were isolated from a dog with bacterial pyoderma/folliculitis (isolates were from the uterus and umbilicus); however, the results suggest that this solution may be a useful antibacterial topical therapy for focal skin lesions. In vitro testing of a larger pool of isolates is needed to confirm the findings of this pilot study and guide veterinarians on in vivo use strategies.