Aiden P. Foster, MRCVS (Bristol) and Douglas J. DeBoer, DVM, DACVD (Wisconsin)
Compendium on Continuing Education Volume 20 #8 August 1998 pp.909-918
Pseudomonas bacteria are an important cause of chronic otitis externa and media in dogs.
The topical application of a tromethamine (Tris)-ethylenediaminetetraacetic acid (EDTA) buffer solution can be used as part of a program to prevent relapse of Pseudomonas infections.
Ethylenediaminetetraacetic acid (EDTA) has a direct bactericidal action against P. aeruginosa by chelating metal ions important for the integrity of the bacterial cell wall. EDTA stimulates the release of cell wall lipopolysaccharides, proteins, and other cell contents.
Buffer solutions containing tromethamine (Iris) have been used to enhance the effects of EDTA on P. aeruginosa. In vitro studies of P. aeruginosa isolated from dogs with otitis externa have demonstrated bactericidal activity with a Tris-EDTA buffer solution. This bactericidal effect is synergized with neomycin or amikacin. The EDTA binds to metal ions, which compete with aminoglycosides for cell-wall receptors that allow them both into the bacteria. In vitro studies have shown that Tris-EDTA is less effective at inhibiting gram-positive than gram-negative bacteria; this may be because gram-positive bacteria have less phospholipid and more peptidoglycan in the cell wall compared with gram-negative bacteria. The optimal pH for the activity of aminoglycosides and Tris-EDTA buffer approximately 8.0. The combination of aminoglycosides and Tris-EDTA is effective against bacteria implicated in otitis, including Staphylococcus intermedius, Proteus mirabilis, Escherichia coil, and P. aeruglnosa. These buffer solutions are not recommended for yeast organisms (e.g. Candida albicans) and therefore are unlikely to be effective against Malassezia. An experimental model of canine otitis externa with Pseudomonas infection demonstrated the usefulness of a solution of EDTA, Tris, and lysozyme. Of 11 dogs that completed the 3-week study; 2 were still infected at the end of the study, 5 had smaller numbers of bacteria, and 4 were cured. A clinical study of the use of Tris-EDTA with antimicrobial reagents suggested possible synergistic effects in treating dogs with chronic otitis externa and bacterial ear infections unresponsive to antimicrobial agents alone.
Tris-EDTA buffer solutions are valuable as a long-term therapy for preventing the recurrence of Pseudomonas infection, either as part of a regular ear cleaning regimen or as a presoak before placing antibacterial solution into the ear canal. Such solutions are typically applied two to three times per week as needed.
After the bacterial infection has resolved, the frequency of topical ear medication can be reduced in anticipation of using a regular maintenance regimen (e.g. Tris-EDTA solution, applied twice weekly, may help to prevent regrowth of Pseudomonas). We have occasionally observed dramatic overgrowth of Malassezia yeast during aggressive antibiotic treatment of Pseudomonas otitis externa. Such overgrowth typically occurs 4 to 6 weeks into treatment, when the bacterial infection is nearly resolved. This situation is readily recognized if ear cytology is performed during recheck evaluations. The problem becomes self-limiting after antibiotics are discontinued. (Ed note.) Brief treatment with the DermaPet Ear/Skin Cleanser (2% acetic/2% boric acid) may be necessary.