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The word allergy means 'altered working'. Just like humans, cats can have allergies too. Allergies are a common cause of skin disease in cats.
The purpose of the immune system is to keep infectious microorganisms, such as certain bacteria, viruses, and fungi, out of the body, and to destroy any infectious microorganisms that do invade the body.
Allergies are caused by an inappropriate response to a substance which would usually be considered harmless. In an allergic animal, the immune system overreacts to the substance and mounts an immune response against it.
Eosinophilic Granuloma Complex has been linked to allergies.
There are four causes of allergies in cats, insect, contact, inhalant and food.
Flea allergy dermatitis is the most common cause of insect allergies in cats. There are 15 known allergens in flea saliva, each one is capable of causing an allergic reaction in the cat. Just one bite from a flea is enough to trigger an allergic reaction in your cat. FAD is one of the major causes of miliary dermatitis in cats.
Symptoms of insect allergies:
Some symptoms of insect allergy include:
- Itching and scratching
- Frequent scratching and licking at the fur, especially over the base of the tail and back of the thighs
- Red, crusty bumps, especially around the head, neck, and back, often with intense itching
Diagnosis of insect allergy:
- This is typically diagnosed by seeing the characteristic lesions on the cat and ruling out other causes.
- Biopsy of the affected area.
- Treating the cat for fleas or keeping it indoor and therefore reducing his exposure to mosquitoes to see if the condition improves.
Treatment of insect allergies would include:
- Eliminating fleas from the cat and the environment.
- Treating secondary skin infections caused by excessive biting and scratching of the skin. This may involve a course of antibiotics, medicated shampoo and or a topical medication.
- Antihistamines or steroids may be prescribed by your veterinarian to control inflammation and reduce itching.
- Hyposensitisation (allergy shots/allergy desensitization). This involves injecting gradually greater amounts of flea antigen in the hope that it will re-programme the immune system so it's not hypersensitive.
This is caused by an allergic reaction to one or more ingredients in the cat's food. The most common causes of food allergies are fish, beef, eggs and wheat.
Symptoms of food allergies:
- Non seasonal itching, especially on the front half of the body, head, face, ears and neck.
- The ears may also be swollen and or infected
- Hair loss (due to excessive scratching, grooming etc)
- Vomiting and/or diarrhea
Diagnosis of food allergies:
Your veterinarian will place your cat on a food trial which usually lasts between 8-12 weeks. During this time, you must not give your cat any other foods, vitamins, minerals or chewable medications apart from the prescribed diet. If any other foods or vitamins are given during this trial it will invalidate the results. If the allergy clears up after the specified time then a food allergy is the likely cause. The diet given to your cat during the trial is most often a food the cat has never had before such as rabbit, duck or venison. The diet may be home made or a special 'prescription' diet. This is known as 'elimination food trial'. After the trial, if the cat's allergies have cleared up he will be placed back onto his regular food and if after a short period of time the allergies return then it is safe to conclude that the food was the cause of the allergy.
After the trial you may be asked to 'challenge' your cat by re-introducing one ingredient back into the diet. If after 2 weeks the re-introduced food hasn't caused a flair up then add another ingredient and so on. If the allergy returns with the re-introduction of the ingredient then this is eliminated from the diet once again.
Occasionally new food allergies develop over time, if this happens then it will be necessary to re-evaluate the new diet.
Treatment of food allergies would include;
Avoiding the food which caused the allergies is the best method of treatment. This may either be a home made diet or a commercial one. If you are feeding a home made diet it is important to ensure that your cat is receiving the correct nutrients in the diet.
This is an allergic reaction caused by breathing pollens, dust mites, moulds and other allergens in the environment. It is under diagnosed in cats 
Symptoms of inhalant allergy:
Symptoms can be seasonal or non seasonal and often begin in young cats, worsening with age.
- Miliary dermatitis (red and crusty rash around the head, neck and back, often with intense itching)
- Eosinophilic granuloma complex
- Head, ear and neck pruritus (intense itching)
- Facial crusty, scabby lesions
- Symmetrical alopecia (hair loss)
Diagnosis of inhalant allergy:
Your veterinarian will obtain a complete medical history from you and perform a physical examination. He will want to rule out other possible conditions such as mange, flea allergy, contact dermatitis, ringworm and food allergy.
- Skin scrapings and fungal cultures to rule out some of the conditions above.
- Intradermal skin test: A small amount of allergen is injected through the skin, which is then observed to determine if elicit an allergic reaction. Certain medications such as antihistamines may interfere with results.
- Blood test: This tests for antibodies to specific antigens in the blood. There are two tests used in cats, radioallergosorbent test (RAST) and enzyme-linked immunosorbent assay (ELISA). 
Treatment of inhalant allergy:
Treatment may include;
- Avoiding the cause of the allergy if possible is the ideal method to treat inhalant allergy but is often not possible because of an inability to identify the allergen or halt exposure.
- Hyposensitisation (allergy shots/allergy desensitization). This involves injecting gradually greater amounts of the allergen in question in the hope that it will re-programme the immune system so it's not hypersensitive to the antigen.
- Corticosteroids are beneficial in relieving symptoms but are not a cure.
- Essential fatty acid supplements have also been effective in some cases of inhalant allergy.
This is the least common of the four types of allergy in cats. Contact dermatitis is a result of the cat coming into contact with a substance. The cat's fur acts as a barrier, protecting it from allergy producing compounds.
There are two causes of contact dermatitis; allergic and irritant.
Irritant Contact Dermatitis:
Irritant dermatitis, the most common type of contact dermatitis, involves inflammation resulting from contact with acids, alkaline materials such as soaps and detergents, medications, solvents, or other chemicals.
Allergic Contact Dermatitis:
The second most common type of contact dermatitis is caused by exposure to a material to which the person has become hypersensitive or allergic. It arises some hours after contact with the responsible material, and settles down over some days providing the skin is no longer in contact with it. Plants, wool, medications are a common cause of allergic dermatitis in cats.
Symptoms of contact allergy:
- Non seasonal itching, especially in areas where there isn't much fur. Typical areas include the chin, ears, toes, underbelly and anus.
- Lesions of any type: redness, rash, papules (pimple-like), vesicles, and blisters
- Skin redness or inflammation
- Thickening of the skin
Diagnosis of contact allergy:
Your vet will perform a physical examination and take a history. He/she will want to rule out other causes of dermatitis (such as flea bite hypersensitivity) first.
- A skin biopsy may be performed.
- A patch test may be performed. This involves applying various substances to the skin, which is then bandaged over. This is then examined at 24 and 48 hours. A positive reaction will be seen as redness and swelling at a specific site.
- A presumptive diagnosis may be made by removing your cat from the environment for a week , if lesions begin to heal, then recur once returned to the environment.
Treatment of contact allergy:
- Identification and elimination of the substance causing contact allergy is the best solution.
- Corticosteroids may be prescribed to relieve pruritus.
- Antibiotics may be necessary to treat secondary bacterial infections.
 Cat Health Encyclopedia - Lowell Ackerman (p 14)
 The Feline Patient -Gary D. Norsworthy, Mitchell A. Crystal, Sharon K. Fooshee, Larry P. Tilley (p 122)