Systemic Antimicrobials for Otitis

Lynette K. Cole, Columbus, Ohio

Topical antimicrobial therapy is the most common and important treatment for infectious otitis. The use of systemic antimicrobial therapy for infectious otitis externa and otitis media is controversial. However, there are indications for the use of systemic antimicrobial agents for the treatment of otitis.

One indication in dogs is chronic, recurrent, or end-stage otitis externa complicated by otitis media. In these cases, not only is infection present within the lumen of the canal but the epithelium of the ear canal and middle ear can be infected as well. Additionally, the ear canal epithelium can be involved with infections and ulcerations caused by gram-negative organisms such as Pseudomonas aeruginosa. Epithelial involvement also is likely when inflammatory cells are identified cytologically, because this finding indicates deeper skin involvement. Another situation where systemic antimicrobial agents may be needed is one in which the owner is incapable of treating the otitis topically in the home setting.

The selection of a systemic antimicrobial agent must be based on the results of culture and susceptibility (C/S) testing of specimens from the external ear (for otitis externa) or middle ear (for otitis media). However, therapy may be initiated based on cytologic findings while the C/S results are awaited.

Indications for systemic antifungal agents are similar to those mentioned previously for antibacterial medications. These include yeast otitis media, severe yeast otitis externa, and inability of the owner to administer topical therapy.

Systemic Treatment of Staphylococcal Otitis

The most common coccoid bacterium isolated from dogs with otitis externa or otitis media is Staphylococcus pseudintermedius. Good empiric choices while C/S results are awaited include cephalexin (22 mg/kg q12h PO) or amoxicillin trihydrate/clavulanate potassium (Clavamox; 13.75 to 22 mg/kg q12h PO).

Systemic Treatment of Pseudomonas Otitis

Probably the most challenging bacterial otic infections are those associated with P. aeruginosa (Table 111-1). Although there is insufficient evidence for the use of systemic antimicrobial agents for the treatment of Pseudomonas otitis, the cause is mainly the lack of published reports of randomized controlled studies in the veterinary literature evaluating systemic treatments in this setting. At the present time the fluoroquinolones are the only oral systemic antibiotic available for the treatment of P. aeruginosa infection. Most veterinary dermatologists recommend starting treatment with an oral fluoroquinolone while the C/S results are awaited. When the fluoroquinolones are used in dogs, a dosage at the upper end of the dosage range should be administered. Blindness caused by retinal degeneration has been reported in rare cases in cats given enrofloxacin; therefore a dosage at the low end of the range for an oral fluoroquinolone should be administered in cats. In one multicenter trial, 54 dogs with Pseudomonas otitis externa were administered marbofloxacin at a dose of 5 mg/kg PO once daily for 21 or 42 days, with an overall improvement in 38 of 54 dogs: 15 were cured and 23 partially improved, whereas 16 did not improve (Carlotti et al, 1998).

TABLE 111-1

Systemic Drugs and Suggested Dosages for the Treatment of Pseudomonas aeruginosa Otitis Externa and Otitis Media