Allergies in Cats

By Malcolm Weir, DVM, MSc, MPH; Tammy Hunter, DVM; Ernest Ward, DVM

How do allergies affect cats?

One of the most common medical conditions affecting cats is allergies. An allergy occurs when the cat's immune system overreacts or is hypersensitive to foreign substances called allergens. Allergens are foreign proteins that the body's immune system tries to remove. Examples of allergens common in humans are pollens, dust, molds, and pet hair. Hypersensitivity in cats can manifest in one of three ways:

  1. The most common manifestation is itching skin, either localized to one area or a generalized reaction all over the cat's body.
  2. Another manifestation involves the respiratory system and may result in coughing, sneezing, and wheezing. Sometimes, there may be an associated nasal or ocular (eye) discharge.
  3. The third manifestation involves the digestive system and can result in vomiting, flatulence, and/or diarrhea.

Does that mean that there are several types of allergies?

Yes. There are four common types of allergies in the cat: insect (fleas), food allergy, atopic dermatitis (house dust, pollen, and molds), and contact. They share common physical expressions and signs in cats, and each has unique features.

What is a flea allergy, and how is it treated?

A flea allergy is the most common allergy in cats. Despite common belief, the average cat experiences only minor skin irritation in response to fleabites. On the other hand, a cat with flea allergies has a severe reaction to even a single fleabite. This reaction is an allergic response to proteins or antigens in the flea's saliva. When a flea bites a cat to consume a blood meal, it injects saliva into the skin. Just one fleabite may cause such intense itching that the cat may roughly scratch or chew himself, causing hair loss. There will often be open sores or scabs on the skin, resulting in a secondary bacterial skin infection (pyoderma). The area most commonly involved is over the rump or base of the tail. In addition, the cat may have numerous small scabs around the head and neck. These scabs are often referred to as miliary dermatitis, a term coined because the scabs look like millet seeds. (See the handout "Miliary Dermatitis in Cats" for more information.)

Since the flea saliva causes the reaction, the most important treatment for flea allergy is to prevent fleabites. Most flea infestations occur in warmer weather but can occur year-round. Strict flea control is the foundation of successful treatment. There are many highly effective flea control products to treat cats and control fleas in the environment. Modern monthly flea preventives have made it easier and less expensive than ever to prevent fleas from affecting your cat. (See the handout "Flea Control in Cats" for more information).

"Strict flea control is the foundation of successful treatment."

Corticosteroids can also block the allergic reaction and give immediate relief to a cat suffering from the intense itching of flea allergy dermatitis. This is often a necessary part of treatment, especially during the initial stages. If a secondary bacterial skin infection occurs from flea allergy dermatitis, appropriate antibiotics must be used, generally for two to four weeks.

See the handout "Flea Allergy Dermatitis in Cats" for more detailed information about flea allergies in cats.

What is a food allergy, and how is it treated?

Food allergies in cats are caused by an immune reaction to a food or food additive. The allergy most frequently develops in response to the protein component of the food (e.g., beef, pork, chicken, or turkey). Vegetable proteins, such as those found in corn and wheat, and food additives and preservatives may cause food allergies in some cases. A food allergy may produce any of the clinical signs previously mentioned, including itching, digestive disorders, and respiratory distress.

Food allergy testing is recommended if clinical signs have been present for several months, if the cat has a poor response to steroids, or if a very young cat itches without other apparent causes.

Testing is conducted by feeding an elimination or hypoallergenic diet. This means a diet in which the ingredients have not previously been fed to the cat (e.g., duck, rabbit, venison). Because it takes at least eight weeks for all other food products to be removed from the body, the cat must eat the special diet exclusively for a minimum of eight to twelve weeks.

Unless the diet is fed exclusively, the test is meaningless. This means absolutely no treats, other foods, people foods, or even flavored medications during this trial. This cannot be overemphasized. Even accidentally providing a tiny amount of the offending protein can invalidate the test.

If your cat’s symptoms improve after the food trial, a presumptive diagnosis of a food allergy is made. Exclusively feeding a hypoallergenic diet lifelong is highly successful in treating food allergic skin disease in many cats.

See the handout "Food Allergies in Cats" for more information on food allergies.

What is atopic dermatitis or atopy?

Atopic dermatitis or atopy generally refers to allergic reactions to environmental allergens such as pollens, grasses, molds, mildew, and house dust mites. Many of these allergies occur seasonally, such as ragweed, cedar, and grass pollens. However, others are always with us, such as molds, mildew, and house dust mites. When humans inhale these allergens, we express the allergy as a respiratory problem. In humans, atopy is sometimes called 'hay fever'. The cat's primary reaction to atopy is severe, generalized itching.

How is atopy treated?

Treatment depends largely on the length of the cat's allergy season. It involves one of two approaches:

  • The first approach involves using corticosteroids (e.g., prednisolone) and improving the health of the hair and skin coat using a therapeutic ‘spot-on’, spray, and/or shampoo. Steroids will dramatically block the allergic reaction in most cases and rapidly improve the cat's clinical signs. Steroids may be given orally or by injection, depending on the cat's condition.
  • The response to treatment with antihistamines and essential fatty acids is variable. Some cats respond well to certain antihistamines, such as chlorpheniramine maleate (Chlor-trimeton®, Chlortripolon®), while others are ineffective. It is important to understand that it can take 7–10 days before antihistamines become effective. Therefore, they are often infective in sudden flare-ups. Likewise, essential fatty acids (fish oils) are similarly ineffective during sudden episodes because they require several weeks to take effect. Cats predisposed to atopic dermatitis should be tried on fatty acid supplements to see if they help reduce future flare-ups and clinical signs.
  • Another treatment for cats with atopy is immunosuppressive drug therapy, such as cyclosporine (Atopica®). These drugs specifically target the immune cells involved in atopic dermatitis to reduce the hypersensitivity reaction that the body is experiencing. It can take up to 30 days to appreciate the drug’s maximum benefit. Therefore, it is not used for sudden allergic flare-ups.
  • The final treatment approach to chronic atopic dermatitis is desensitization with specific therapy or allergy shots. This is not to be confused with injections of corticosteroids. Once the specific sources of allergy (allergens) are identified through allergy blood tests (most commonly IgE blood tests) or intradermal skin testing, a specific allergy serum is made using very small amounts of the allergens your cat is sensitive to. This serum is given in a series of injections or liquid under the tongue. The aim is to 'reprogram' the body's immune system response to the allergens. It is hoped that the immune system will become less reactive to the allergens as time passes. For most cats, a realistic goal is to reduce the severity of the itching rather than eliminate it. In some cats, the itching and associated clinical signs may completely resolve, while others may experience minimal improvement. Steroids may be used until the allergy shots become effective (up to 12 months) or on an intermittent basis to control severe itching.

Any cat suspected of having atopic dermatitis should also be considered for a hypoallergenic food trial. Many cats with atopic dermatitis are also allergic to an ingredient in their food, making the diagnosis and treatment more challenging. As previously mentioned, this food trial should usually last 8 to 12 weeks.

"Many cats with atopic dermatitis are also allergic to an ingredient in their food"

It is important to remember that atopic dermatitis is a lifelong condition and frequent relapses are common. There is no 'cure' for allergic skin disease, only treatments that lessen clinical signs and improve quality of life. While cats appear less likely to develop side effects associated with chronic steroid usage than dogs or humans, their prolonged use must be carefully monitored. It is essential to work closely with your veterinarian to provide the best care for your cat’s allergic condition and fully understand the risks and benefits of each treatment. (See the handout "Steroid Treatment - Effects in Cats" for more information).

What are contact allergies, and how are they treated?

Contact allergies are the least common type of allergies in cats. They result in a local reaction on the skin from contact with an allergic substance. Examples of contact allergies include reactions to shampoos, flea collars, or certain types of bedding, such as wool. If the cat is allergic to such substances, skin irritation and itching will occur at the contact points. Removal of the contact irritant solves the problem. However, identifying the allergen can be challenging in many cases.

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