Merck Manual

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Pet Owner Version

Allergies of Cats


Stephen D. White

, DVM, DACVD, University of California, Davis;

Karen A. Moriello

, DVM, DACVD, Department of Medical Sciences, School of Veterinary Medicine, University of Wisconsin-Madison

Reviewed/Revised Aug 2018 | Modified Oct 2022

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 antibody production. When an animal comes in contact with these allergens in the future, more antibodies are produced, triggering histamine release and inflammation.

Airborne Allergies (Atopy, Atopic Dermatitis)

Airborne allergens can adversely affect the skin. Feline atopic dermatitis Skin Allergies (Atopy or Atopic Dermatitis) In a Type I reaction, the animal has been previously exposed to an antigen and produces an excess of antibodies. If this antigen appears in the blood, the reaction can be either body-wide (such... read more is a condition characterized by severe itching and redness and swelling of the skin. 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. Although the skin is usually affected, inflammation of the nasal passages (rhinitis) and asthma can also occur in 15% of animals. The age of onset varies, but is often less than 5 years. Feline atopy may be seasonal or nonseasonal. Purebred cats are more likely to be affected than domestic shorthair cats.

The diagnosis of airborne allergies is difficult because there are no tests available that can positively identify the condition. Your veterinarian will start by reviewing your cat's history, examining the body and skin, and performing skin tests. These tests may include flea combing, microscopic analysis of skin samples and scrapings, fungal cultures, and allergy skin tests (called intradermal allergy tests). Intradermal allergy testing cannot diagnose allergies but can be used to identify the offending allergens and to formulate a specific immunotherapy treatment program ("allergy shots"). Monitoring your cat's response to treatment may also help. Your veterinarian will also eliminate other possible causes of the itching before diagnosing feline atopy.

Atopic dermatitis is a lifelong disease that requires longterm management and regular veterinary examinations. Treatment involves a number of options: avoidance of the offending allergens (if possible), controlling the signs of itching, improving coat hygiene, controlling flare factors (such as fleas or secondary infections), and immunotherapy ("allergy shots"). A good management plan requires the use of several different treatments, the understanding and reasonable expectations for response from the pet owner, and frequent progress evaluations so that the plan can be adjusted as needed. Treatments used for sudden flare-ups often vary from those used for longterm management. Follow your veterinarian's instructions closely.

Immunotherapy attempts to increase a cat’s tolerance to environmental allergens. Your veterinarian will match the test results with the prominent allergens during the time of year when the cat has signs. Immunotherapy can then be administered as injections or as allergy drops. The cat must be cooperative enough to receive allergy injections or drops. If injections are used, you may have to administer some doses yourself. Your veterinarian can provide training, and most owners learn to administer the allergy injections very well, whereas others may need assistance from a capable friend or veterinary staff member. Your veterinarian will determine the frequency of the treatments and the dosage given.

Immunotherapy takes a longterm commitment. You must be willing to follow instructions accurately, be patient, and be able to communicate effectively with your veterinarian. Treatment may initially increase signs. If this occurs, contact your veterinarian immediately. Improvement may not be visible for 6 months, and a year of treatment may be required before you can tell if the immunotherapy is working. The best way to evaluate the treatment is to compare the degree of disease or discomfort between similar seasons. Anti-itch medication and antibiotics are often required during the initial phase of treatment and may be necessary intermittently throughout the year. Do not assume that immunotherapy has failed if signs appear during therapy because they may be due to another cause (such as an infection). Talk to your veterinarian if signs return, change, or do not resolve during treatment.

Allergy shots improve the condition but do not cure the disease. Many animals may still require anti-itch medications during seasonal flare-ups.

Food Allergies

Food allergies Food Allergies In a Type I reaction, the animal has been previously exposed to an antigen and produces an excess of antibodies. If this antigen appears in the blood, the reaction can be either body-wide (such... read more occur in cats about as frequently as airborne allergies do. Signs of food allergy are similar to airborne allergies except there is little variation in the intensity of itching from one season to another. Cats may also have multiple, small crusty bumps; hair loss on both sides of the body; or areas of flat, reddened, swollen skin (called eosinophilic plaques Eosinophilic Granuloma Complex in Cats This group of skin conditions affects cats, dogs, and horses. Eosinophilic granuloma complexes have varying signs but seem to be caused most commonly by an allergic hypersensitivity. In cats... read more ). The age of onset is variable, ranging from 3 months to 11 years. However, one study showed that most cats will show signs of food allergies before they are 2 years of age. The distribution and intensity of itching varies between cats; however, itching that is directed at the head and neck is fairly common.

There is no reliable diagnostic test other than feeding a limited foodstuff (elimination or hydrolyzed diet) and seeing if the itching resolves. Blood and skin tests are not reliable for diagnosis. 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. An alternative to an elimination diet is a prescription diet that contains hydrolyzed proteins. These proteins are broken down into components that your cat's body no longer recognizes. Whichever diet is chosen, cats should only eat the food and treats recommended by their veterinarian during the duration of the trial.

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, chicken, and milk products. Avoidance of the offending allergens will control the signs associated with the food allergy.

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