Environmental-induced (eg, house dust, house dust mites, pollens of trees, grasses and weeds, molds) or food-induced atopic dermatitis are common allergic disorders of dogs and cats and frequently cause erythema and pruritus of the pinnae and external ear canals. The allergic condition predisposes to develop secondary bacterial or yeast otitis externa, which can extend to the pinna. In these cases, papules, crusts, and lichenification may develop in addition to the erythema. Other body sites such as the face (ie, periocular region, muzzle, chin), axilla, groin, and feet are also often affected. Diagnosis is based on a characteristic history, clinical signs, and the elimination of other pruritic skin diseases. A strict food elimination trial is important for animals with year-round clinical signs to determine whether food allergens are triggering the atopic signs. If food allergy is excluded or is only part of the allergic condition, intradermal or serologic allergy tests can be performed to support a presumptive diagnosis of environmental-induced atopic dermatitis and, more importantly, to institute allergen-specific immunotherapy. It is very important to identify and treat any secondary ear and skin bacterial or yeast infections, because they aggravate the allergic dermatitis. Treatment of food-induced atopic dermatitis consists of solely feeding the diet used during the food trial or avoiding food ingredients known to trigger the allergic reaction. Treatment of atopic dermatitis consists of using anti-inflammatory/immunomodulatory drugs, including oral short-acting glucocorticoids such as prednisone or prednisolone at 1 mg/kg/day and tapering to the lowest possible dosage administered every other day. Cyclosporine at 5 mg/kg/day can also be used in animals that cannot tolerate or do not respond well to glucocorticoid therapy. Omega-3 and/or omega-6 fatty acids can be used as adjunctive therapy. H1-receptor antihistamines typically are not very efficacious to control the pruritus or inflammation associated with atopic dermatitis, but their safety profile and relatively low cost make them worthwhile to try. Allergen-specific immunotherapy based on intradermal or serologic test results is currently the only specific therapy for atopic dermatitis and should be considered as a treatment option.
Feline mosquito hypersensitivity is an allergic reaction to mosquito bites that can cause an ulcerative and crusted dermatitis of the pinnae, nose, and less commonly the footpads, eyelids, chin, and lips of cats. Lesions progress from wheals to papules to plaques to crusted ulcers that coalesce to affect extensive areas. Pruritus is a consistent sign, and regional lymphadenopathy may occur. In severe cases, fever or other systemic signs may develop. Histologically, the lesions are characterized by severe superficial and deep perivascular to interstitial eosinophilic dermatitis, often associated with flame figures, folliculitis, and furunculosis. Differential diagnoses include pemphigus foliaceus, herpesvirus ulcerative dermatitis, other causes of eosinophilic dermatitis (food allergy, atopy, idiopathic), notoedric mange, and dermatophytosis. Treatment includes keeping the cat inside and using a pyrethrin repellent (avoid permethrin-containing products in cats) when exposure to mosquitoes is anticipated. Systemic glucocorticoids may be necessary in severe cases. (Also see Mosquitoes.)