Allergic Dermatitis in Cats
A thorough history and physical exam are imperative to making an appropriate diagnosis. The distribution of skin lesions will often allow the clinician to limit the list of differential diagnoses. Some commonly performed tests include:
Skin scrapes- A surgical blade is used to gently scrape the skin surface to look for various types of mites or mite eggs.
Skin cytology- Smears of fluid or pus filled vesicles or inflamed skin are helpful to diagnose bacterial and yeast infections.
Complete blood count and biochemical profile- These tests are not necessary in every case of dermatitis. They evaluate the blood cells and organ function, and may be useful if an underlying systemic disease is suspected. These tests are especially useful if the patient has other signs of illness in addition to the dermatitis.
Allergy blood tests- These are special tests designed to find out which allergens the animal is responding to. Although they can be useful, the test results are not always reliable.
Intradermal allergy testing- This involves testing the skin for allergic responses by injecting small amounts of known allergens into pre-determined locations in the skin. This type of testing is generally more reliable than the blood tests, but interpretation can be difficult and false test results may occur if not performed properly. Only a trained dermatologist, or a doctor who does many intradermal allergy tests and is familiar with the test interpretation should perform these. Allergy testing of this sort is usually done in cases of suspected atopy.
Dietary trials- If a cat is suspected of having a food allergy, special dietary trials are the best way to make the diagnosis. This entails putting the cat on an elimination diet, consisting of a protein source that the animal has not previously eaten. Home-cooked diets or prescription diets can be fed. In order for the trial to be successful, all other foods must be excluded. This includes treats, catnip, flavored vitamins and table food. It may take a number of weeks on the diet before the animal shows any improvement in the dermatitis.
The specific therapy for your cat will depend on what type of allergy is diagnosed, as well as how severe the signs are. A combination approach to therapy is often more successful than any single therapy.
Avoidance of offending allergens- Whenever a specific allergen can be identified based on diagnostic testing, exposure to the allergen should be minimized as much as possible. In cases of flea allergy, a strict flea control program should be started. This includes treating the animal with products to kill adult fleas as well as insect growth regulators, which inhibit flea larvae. Careful eradication of fleas from the environment is also imperative. As discussed, food allergies require eliminating certain foods from the diet. Animals with atopy may benefit from more frequent cleaning of their home environment to limit dust and molds. Limited exposure to pollens and grasses may also be important.
Shampoos- Anti-pruritic (anti-itch) shampoos can often provide relief to very itchy patients. Additionally, animals with secondary bacterial or yeast infections may benefit from medicated shampoos with antibacterial and antifungal properties.
Topical anti-inflammatory ointments may be appropriate in cases with mild, regional skin lesions. Additionally, animals with ear inflammation and secondary infection will often require combination antibacterial/antifungal/anti-inflammatory medications for the ears.
Antihistamines- This class of drug is available in both over the counter and prescription medications. Although not usually effective as a sole therapy, many animals will benefit from antihistamine use to relieve the itch and inflammation. If one antihistamine does not work, a different one may sometimes be more effective.
Corticosteroid therapy- Corticosteroids are potent anti-inflammatory drugs that are widely used in veterinary medicine for a variety of reasons. They may be necessary in many cases of allergic dermatitis, and can be very effective in relieving the itch and inflammation associated with atopy and flea allergy. They are generally less effective for treating food allergy. Long term use of corticosteroids can have many negative side effects, so they must be used judiciously, and under strict supervision of a veterinarian.
Immunotherapy- Use of allergy vaccines, specifically designed for each individual based on allergy test results, can often be helpful in cases of atopy. They are often used in conjunction with corticosteroids and antihistamines, in an attempt to reduce drug dosages. As many as 60-80% of patients with atopy may have improved clinical signs related to immunotherapy. It may take months to see positive results.
Fatty acid supplementation- Supplementing the diet with fatty acids may reduce the degree of inflammatory response seen in the skin of allergic animals. This treatment is very unlikely to eliminate signs, but can be used together with other treatments.
Dietary management- Once a diet trial has been conducted, specific diets may be required for the life of the cat. Over time, certain ingredients may be selectively added to the diet one at a time, to test for hypersensitivity to a specific ingredient. Eventually the pet may be able to return to eating his normal diet, but this is unpredictable.
Antibiotics- Oral antibiotic therapy is often needed due to the prevalence of secondary bacterial skin infections.