Like people, cats can be allergic to various substances, including plant particles and other substances in the air or in food. These substances are called allergens. Allergens are substances that, when inhaled or absorbed through the skin, respiratory tract, or gastrointestinal tract, stimulate histamine production, which results in inflammation.

Airborne Allergies (Atopy)

Airborne allergens can adversely affect the skin. Feline atopy (see Immune Disorders of Cats: Skin Allergies (Atopy)) is a condition characterized by severe itching. Affected cats have an abnormal sensitivity to inhaled or contacted environmental allergens. Excessive scratching and licking produce sores and other skin conditions including hair loss, scaling, crusts, and inflammation. The age of onset varies, but is often less than 5 years. Feline atopy may be seasonal or nonseasonal. Your veterinarian will want to eliminate other possible causes of the itching before diagnosing feline atopy. (See also Airborne Allergies, see Skin Disorders of Dogs: Airborne Allergies (Atopy).)

Food Allergies

Food allergies (see Immune Disorders of Cats: Food Allergies) are known to occur in cats. Signs of food allergy are similar to airborne allergies except there is little variation in the intensity of itching from one season to another. The age of onset is variable. The distribution and intensity of itching varies between cats; however, itching that is directed at the head and face is fairly common.

There is no reliable diagnostic test other than feeding a limited foodstuff (a hypoallergenic or elimination diet) and seeing if the itching resolves. A veterinarian should be consulted to develop a specific test plan for your cat. The ideal food elimination diet should be balanced and nutritionally complete and not contain any ingredients that have been fed previously to the cat. Owners often do not understand that if any previously fed ingredient is present in the elimination diet, the animal may be allergic to the new food and the diet trial will be a failure. The key point in any food elimination diet trial is that only new food ingredients can be fed. This includes treats and anything else the cat eats besides its regular food.

Food elimination diets can be difficult in cats because many cats are reluctant to change diets. Cats should not be starved or forced into eating a new diet, because prolonged poor appetite can lead to serious liver damage. The trial diet should be fed for up to 3 months. If obvious or complete resolution in signs occurs during the elimination diet trial, food allergy can be suspected. Response time to the elimination diets varies from 1 to 9 weeks.

To confirm that a food allergy exists and improvement was not just coincidental, the cat must be given the previously fed food ingredients and a relapse of signs must occur. The return of signs may occur in as little as 15 minutes but usually takes place within 10 days. Once a food allergy is confirmed, the elimination diet should be continued until signs disappear, which usually takes less than 14 days. At this point, previously fed individual ingredients should be added to the elimination diet for a period of up to 14 days. If signs reappear, the individual ingredient is considered a cause in the food allergy.

The foods cats are most often allergic to include fish, beef, and milk products. Avoidance of the offending allergens will control the signs associated with the food allergy.

Last full review/revision July 2011 by Karen A. Moriello, DVM, DACVD; Thomas R. Klei, PhD; David Stiller, MS, PhD; Stephen D. White, DVM, DACVD; Michael W. Dryden, DVM, PhD; Carol S. Foil, DVM, MS, DACVD; Paul Gibbs, BVSc, PhD, FRCVS; John E. Lloyd, BS, PhD; Bernard Mignon, DVM, PhD, DEVPC; Wayne Rosenkrantz, DVM, DACVD; Patricia A. Talcott, MS, DVM, PhD, DABVT; Alice Villalobos, DVM, DPNAP; Patricia D. White, DVM, MS, DACVD