Therapy For Specific Diseases Of The External Ear Canal

By | 2013-07-19

Ectoparasites

Thorough cleaning of the external ear canal, treatment of all household pets, and whole-body therapy should be considered in the treatment regimen for ear mites. Pets with no clinical signs may be asymptomatic carriers and a reservoir for reinfestation. Otic parasiticides such as pyrethrins, rotenone, amitraz, and carbaryl must be administered every 24 hours throughout the 20-day mite life cycle because they do not kill mite eggs. Thiabendazole eliminates all mite stages, but it must be applied every 12 hours for 14 days. Ivermectin (0.3 to 0.5 mg/kg) may be applied topically once weekly for 5 weeks. Otic administration of medication does not affect mites on adjacent or distant skin locations, and systemic or other total-body parasiticide may be indicated. Alternatively, ivermectin administered subcutaneously (0.2 to 0.3 mg/kg) 2 to 3 times at 10- to 14-day intervals or orally (0.3 mg/kg) every week for four treatments eliminates otic mites and those found elsewhere on the body. Other topicals proven safe and effective for ear mite treatment include selamectin (6 mg/kg) applied to the skin between the shoulder blades and fipronil spray. Selamectin administered once in cats and two times, 30 days apart in dogs gave results similar to topical pyrethrin therapy.

Idiopathic Inflammatory or Hyperplastic Otitis in Cocker Spaniels

Treatment is aimed at decreasing the secondary ear canal changes associated with this condition. Anti-inflammatory doses of corticosteroids administered orally may be useful. Topical corticosteroid preparations in combination with antimicrobials decrease the soft tissue mass affecting the ear canal but may not be as effective as oral administration. Maintenance therapy may be required both topically and orally; however, low doses of corticosteroids should be used. Re-evaluation should include attention to the potential side effects of corticosteroid therapy. Intermittent treatment of secondary bacterial or yeast overgrowth and infection may be required. Surgery is often indicated due to the severe secondary changes within the ear canal.

Excessive Moisture (Swimmer’s Ear)

Other primary disease conditions such as allergic otitis should be ruled out in any dog with erythematous ceruminous otitis. Dogs with frequent exposure to water, however, may require ear cleaning and drying agents to diminish the humidity of the ear canal. Many cleaning and drying agents also posses antimicrobial effects. Products that combine a drying agent and corticosteroid decrease the ear canal humidity and inflammation associated with allergic otitis complicated by swimming. Care should be taken to control primary disease (i.e. allergic otitis), however, and intermittendy manage the predisposing factor (i.e. excessive moisture) as necessary. The dog’s ears should be cleaned and dried the day of water exposure and for 2 to 5 days after. For continued frequent exposure, maintenance cleaning may be required every other day to twice weekly.

Chronic Bacterial Otitis

Resistant bacteria play an important role in the development of chronic otitis externa. Any dog not responding to initial therapy should be re-evaluated for primary and perpetuating conditions such as allergic disease, foreign body, neoplasia, otitis media, and secondary anatomic changes of the ear canal. Primary disease processes identified in one study included hypothyroidism, atopy, food allergy, and immune-mediated disease. Infection with Pseudomonas species frequently occurs with repeated treatment of otitis extema, and acquired resistance is common. Culture and susceptibility testing is imperative to guide therapy. Oral antimicrobials combined with topical therapy are used in severe cases with secondary changes of the ear canal. Identification of otitis media is vital to remove the middle ear as a source of otitis extema. Otitis media requires long-term treatment.

Ear cleaning prior to the application of topical medication may increase the efficacy of the agent by decreasing exudate in the ear canal that inactivates antimicrobial drugs such as polymyxin. In cases that fail to respond to first-line drug treatments such as polymyxin or gentamicin, other topical antimicrobial agents should be tried. Ophthalmic tobramycin and injectable amikacin have been described for use as topical antimicrobials in ear disease. The integrity of the tympanic membrane should be known prior to use; the clinician should avoid these medications if the tympanic membrane cannot be proven intact. Enrofloxacin or ticarcillin injectable preparations diluted in saline or water may be applied topically for resistant Pseudomonas. Parenteral ticarcillin was used in cases with a ruptured tympanic membrane until healing was observed, at which time topical therapy was instituted; clinical response occurred in 11 of 12 cases. Enrofloxacin and silver sulfadiazine combination is also available in an otic preparation (Baytril Otic, Bayer Shawne Mission, KS).

Other topical therapy may assist in eliminating resistant Pseudomonas from the ear canal.

Decreasing the pH of the ear canal with 2% acetic acid is lethal to Pseudomonas; diluted vinegar in water (1:1 to 1:3) may be used to flush the ear canal. Acetic acid combined with boric acid is lethal to Pseudomonas and Staphylococcus, depending on the concentration of each agent. Increasing the concentration of acetic acid may broaden its spectrum of activity but causes irritation of the external and middle ear. Silver sulfadiazine in a 1% solution exceeds the minimum inhibitory concentration of Pseudomonas and may be instilled into the ear canal. One gram of silver sulfadiazine powder mixed in 100 mL of water may be used for topical therapy and is also effective against Proteus species, enterococci, and Staphylococcus intermedium. Dilute acetic acid (2%) and silver sulfadiazine (1%) have not caused adverse effects in cases with a ruptured tympanic membrane.I Tris EDTA may be applied after thorough ear cleaning to increase the susceptibility of Pseudomonas to antimicrobial agents. It must be mixed, pH adjusted, and autoclaved prior to use or is available in an otic preparation (TrizEDTA, DermaPet ®, Potomac, MD), which is used to clean the ears prior to instillation of topical antibiotic. Topical antiseptics such as chlorhexidine and povidone-iodine solutions may be helpful, but ototoxicity is an issue, particularly in cases in which the tympanum is ruptured or cannot be evaluated.

Re-evaluation of the pet is important for monitoring response to therapy. Evaluation of the ear canal for progressive secondary changes and cytologic examination will allow alterations in therapy as needed. Significant narrowing of the ear canal is an indication for surgical intervention. Yeast overgrowth may occur with aggressive medical management of bacterial otitis and should be identified to maintain proper medical management.

Refractory or Recurrent Yeast Infection

Malassezia infection is a common perpetuating factor with erythematous ceruminous otitis and alterations in the otic microenvironment. Primary causes of the otitis should be identified and treated. Cytologic examination, not culture, should be relied upon for the diagnosis of yeast infection. If a case becomes refractory to therapy, reassessment of the primary condition and perpetuating factors should be done. Miconazole, clotrimazole, cuprimyxin, nystatin, and amphotericin B have all been described for treating Malassezia otitis. Climbazole had better in vitro activity against isolates of Malassezia pachydermatis in one study. Yeast were more susceptible to azole antifungals than polyene antifungals; however, oral ketoconazole, itraconazole, or fluconazole have been recommended for refractory cases. Long-term therapy may require topical antibacterial and antifungal combinations.

Ear cleaning may aid in the elimination of yeast organisms by removing cerumen, debris, or exudate and altering the microenvironment of the ear canal. Cleaning with antimicrobial agents such as chlorhexidine, povidone-iodine, and acetic acid may be beneficial; but as always the integrity of the tympanum should be established prior to use. Ear cleaning solutions may also have some efficacy against yeast organisms both in vitro and in clinical cases of otitis.

Neoplasia

Chronic otitis externa may be the result of otic neoplasia, or otitis may be a predisposing factor in the development of neoplasia. Cocker spaniels are over-represented for benign and malignant neoplasia and otitis extema. Tumors of the skin and adenexal structures of the ear predominate. Benign tumors in dogs include sebaceous gland adenoma, basal cell tumor, polyp, ceruminous gland adenoma, and papilloma. Cats are more frequendy diagnosed with malignant neoplasms, but benign conditions include inflammatory polyps, ceruminous gland adenomas, ceruminous gland cysts, and basal cell tumors. Malignant neoplasms in both species include ceruminous gland adenocarcinoma, undifferentiated carcinoma, and squamous cell carcinoma. Ceruminous gland adenocarcinomas are the most frequendy diagnosed tumors of the ear canal in dogs and cats; however, one report stated that squamous cell carcinoma occurs with equal incidence in the cat.

The biologic behavior of otic tumors cannot be judged by their gross appearance; however, benign masses are usually nodular and pedunculated. Ulceration can be secondary to otitis associated with mass lesions, but malignant masses ulcerate more frequendy than benign masses. The tympanic bulla is involved in up to 25% of aural neoplasms, and neurologic signs occur in 10% of dogs and 25% of cats with otic neoplasia. The biologic behavior of malignant neoplasms tends to be local invasion with a low metastatic rate (e.g. 10% in dogs) to draining lymph nodes or lung.

Surgery is the mainstay treatment of otic neoplasia. Conservative excision may be possible for benign lesions, depending on the location of the tumor. Malignancies should be removed by total ear canal ablation and lateral bulla osteotomy. Incomplete excision results in recurrence of the mass and secondary otitis externa. Malignant neoplasia is associated with a median survival time (MST) of more than 58 months in dogs and 11.7 months in cats. Extensive tumor involvement and lack of aggressive management are associated with a poor prognosis in dogs. In cats a poor prognosis is associated with neurologic signs, squamous cell carcinoma or undifferentiated carcinoma, vascular or lymphatic invasion, and lack of aggressive therapy. Ceruminous gland adenocarcinoma has a median disease free interval of more than 36 months and 42 months in dogs and cats, respectively. The MST associated with squamous cell carcinoma and undifferentiated carcinoma in cats is 4 to 6 months.