Atopic dermatitis is an allergic skin disease caused by environmental allergens: pollens, house dust mites, mould spores. In cats it is diagnosed only after fleas and food have been excluded, because it has no test of its own.

How it appears

Itching is the whole disease. It shows in four patterns, and a cat may have more than one:

Scratching at the head, face and ears, often causing sores in front of the ears
Symmetrical hair loss on the belly, inner thighs and flanks from over-grooming
Small crusty bumps along the back, felt more easily than seen
Raised plaques on the belly or thighs, an ulcer on the upper lip, or a swollen lower lip

Signs often start between one and three years of age, may be seasonal at first, and tend to become year round as the cat gets older.

Why there is no simple test

Blood and skin tests exist, but they identify what a cat has been exposed to rather than what is causing its disease. A cat can test positive to pollens it reacts to not at all. They are used to select allergens for desensitisation once the diagnosis is made, not to make the diagnosis.

The diagnosis is made by working through the causes in order:

Fleas, treated rigorously for at least eight weeks, every animal in the household, whether or not fleas are seen.
Food, tested with a strict elimination diet of a novel or hydrolysed protein for eight to twelve weeks, with nothing else given at all.
Parasites and infection, ruled out with skin scrapes and cytology.

A cat still itching after all three has atopic dermatitis.

Why owners find this frustrating

Because it is slow, and because the first two steps feel like they are not treating anything. They are: the majority of cats referred to us as atopic turn out to have flea allergy that was never properly treated, or a food trigger.

Doing the steps properly once is faster than repeating them for years.

Management

Atopy is managed, not cured.

Anti-itch medication, chosen according to the individual cat and reviewed regularly
Treating the secondary bacterial and yeast infections that follow scratching
Continuing strict flea control permanently, because a flea bite on an atopic cat causes a disproportionate flare
Reducing house dust mite exposure: washing bedding hot, vacuuming, reducing soft furnishings where practical
Essential fatty acid supplementation, which helps the skin barrier modestly

When to come in

If your cat is scratching persistently, has bald patches, sores around the face or ears, or has started over-grooming. Bring a full list of everything it eats, including treats, and the date flea treatment was last given. Those two facts determine the first eight weeks of the plan.