Analyzing the Results of the Trial

If the clinical signs stop during the time that the prednisone and ivermectin are given and never return, diagnose ectoparasites (most likely scabies) or ear mite hypersensitivity.

If the response is more gradual and the pruritus does not return after bathing with the acetic acid/boric acid shampoo and ketoconazole or itraconazole, cutaneous Malassezia is the likely cause. Unfortunately, Malassezia is often a secondary complication that perpetuates otic pruritus in many atopic dogs. An acetic acid/boric acid ear cleaner (Malacetic Otic, DermaPet) may need to be used on a maintenance basis once or twice a week to control otic Malassezia yeasts.

If the pruritus returns between days 12 and 21, a cortisone-responsive hypersen­sitivity is most likely and allergy testing should be done. The frozen pretreatment serum can then be submitted for in vitro allergy testing.

If itching remains controlled comfortably after the original pretreatment diet is reinstituted, the possibility that this is a fatty acid/shampoo therapy-responsive atopy is high. Suspect primary food allergy if the itching returns within 72 hours after the hypoallergenic diet is withdrawn and the pet is returned to the pretreat­ment diet.

Unresponsive allergy cases have recently been described to respond favorably to cyclosporine at 5 mg/kg. This may account for why some researchers have treated unresponsive end-stage ear cases with cyclosporine, allowing dogs to become comfortable while not reversing the progressive pathology.

Bacterial hypersensitivity is a disease entity that may be suspected if the itching returns within 30 days of stopping the antibiotic, provided that the pruritus was controlled during antibiotic therapy. If the itching disappears after a second course of antibiotic therapy, the diagnosis is confirmed.

Keratinization disorders, including hypothyroidism, skin neoplasia, and other less common primary skin diseases, will prevent the itch from fully resolving. Incomplete treatment of Malassezia dermatitis or failure to treat all in-contact animals for scabies may cause persistent pruritus. Biopsy of the skin may be helpful in many circum­stances to identify the etiology of pruritic skin diseases.

In many cases of acute otitis externa without progressive pathologic changes, resolution of pruritic skin disease also decreases otic pruritus. However, the skin of the ear is treated differently than the skin of the trunk, primarily with concentrated topical medications.