Glossary
Flea allergy dermatitis
An itchy skin disease caused by an allergic reaction to flea saliva, where even a few bites trigger scabs and hair loss.
Also called: FAD, flea bite allergy, flea allergic dermatitis, miliary dermatitis
Flea allergy dermatitis, or FAD, is the most common allergic skin problem in cats. The trigger is a protein in flea saliva. A cat with the allergy can react intensely to just one or two bites, so owners often find no fleas on the animal and wonder what is wrong.
The signs are itching and overgrooming, with small crusty bumps often along the back, around the base of the tail, on the neck and the belly. Vets call this pattern miliary dermatitis. Some cats lick patches bald, scratch until the skin is raw or develop raised sores on the lips or thighs. Constant scratching can lead to infection, so a cat with open sores or hair loss needs a vet.
Other conditions look alike, including food allergy, ringworm, mites and environmental allergies, so a vet examines the cat before settling on a cause. Care combines treating the skin and getting every flea out of the household. This takes consistent work across all pets and the home, using products your vet recommends. Cats with this allergy need year-round flea control, even if you rarely see an insect. Ask your vet how to choose a safe product.
Last reviewed November 7, 2026