Allergic dermatitis

Itching is the leading sign. Atopic dogs often start with itching before lesions appear, then develop redness, self-induced hair loss, excoriation, darkening and thickening of the skin, typically on the front feet and ear pinnae while sparing the ear margins and dorsal lumbar area. Secondary staphylococcal and Malassezia infections are common, as is otitis externa. Flea allergy concentrates on the lumbosacral area, tail base and caudal thighs. Food-induced disease may add vomiting, diarrhoea or soft stools. Cats show head, neck and ear itching, self-induced alopecia, miliary dermatitis or eosinophilic plaques, granulomas and lip ulcers.

Allergic dermatitis covers the hypersensitivity skin diseases of dogs and cats: atopic dermatitis, flea (and other insect) bite hypersensitivity, and cutaneous adverse food reactions. Canine atopic dermatitis is a genetically predisposed, inflammatory, itchy disease usually linked to IgE against environmental allergens. Genetic and environmental factors produce a defective epidermal barrier, with disorganised and reduced stratum corneum lipids including ceramides, raised transepidermal water loss, skin dysbiosis and immune dysregulation. Allergens and microbes then penetrate the skin and drive a type 2 inflammatory response, which further damages the barrier in a self-perpetuating cycle. Flea bite hypersensitivity is an immediate (type I) and/or delayed (type IV) reaction to flea saliva. Food reactions can be immune-mediated, including IgE-mediated food allergy, or non-immune intolerance. In cats a role for IgE is unproven.

Canine atopic dermatitis is one of the most common chronic skin diseases of dogs, estimated to affect about 20-30% of the canine population, often beginning early in life, with typical onset before three years of age; hypersensitivities appear to be increasing. Flea allergy dermatitis has historically been the most frequently diagnosed hypersensitivity in cats and is commonest where fleas are endemic; fleas can be a year-round problem in subtropical and tropical climates and seasonal in temperate ones. Many atopic dogs also have flea allergy, and allergic cats may have more than one hypersensitivity. Mosquito bite hypersensitivity affects cats that go outdoors where mosquitoes are endemic.

No sign is pathognomonic, so look-alikes must be excluded. Ectoparasites (fleas, sarcoptic mange, demodicosis, cheyletiellosis, lice, ear mites, trombiculiasis) are ruled out by flea combing, skin scrapings, hair plucks and a flea control trial. Staphylococcal pyoderma and Malassezia dermatitis are identified by cytology and may be secondary. Food reactions look identical to atopic dermatitis and are excluded only by strict elimination diet trials. Cutaneous lymphoma is also listed; in cats, infectious, genetic and neoplastic causes should also be considered.

Skin (epidermis, stratum corneum), ear pinnae and ear canals, paws, face, lumbosacral region, conjunctiva; gastrointestinal tract in food reactions

Consistent adulticidal flea control for all household pets plus environmental treatment; keep mosquito-sensitive cats indoors; allergen-specific immunotherapy after testing. Skin-barrier prevention in dogs is unproven.

Canine atopic dermatitis is incurable, waxes and wanes with flares, and needs lifelong management with a heavy quality-of-life burden. Flea and mosquito bite hypersensitivity are controlled by strict avoidance plus anti-itch treatment in flares.