Why is my Pet Itching? Understanding Allergies.

Posted by Jennifer on December 23rd, 2010

Understanding Allergic Dermatitis in Dogs and Cats – Written by Dr. Jill Cline, Ph.D.

All pets occasionally scratch and bite at their skin. However, excessive and persistent scratching, licking, biting or hair loss may be signs of allergic skin disease. Just like people, dogs and cats can develop allergic reactions to a variety of agents. Understanding the types of allergies that occur in dogs and cats can help owners to seek prompt veterinary care and relief for their pet.

What is Allergic Skin Disease?

Allergies are one of the most common causes of skin problems in dogs and cats. Allergic skin disease (also called allergic dermatitis) occurs when a pet’s immune system over-reacts to a particular substance (antigen), resulting in a prolonged and chronic inflammatory response. The most common antigens that are responsible for allergic dermatitis in dogs and cats are inhaled allergens such as pollens, mites and molds (atopic dermatitis), contact allergens, fleas (flea allergy dermatitis), and food proteins. The signs of allergic skin disease are initiated by the intense and prolonged pruritus (itchiness) that the dog or cat experiences upon exposure to the offending antigen (Box 1, below).

Diagnosis can be difficult because of the many possible causes of dermatitis. Veterinarians use the pet’s medical history, the pattern and frequency of the pruritus and the elimination of other potential causes during diagnosis. In cases of suspected atopic dermatitis, intra-dermal skin tests can be helpful in identifying the exact antigens, and in cases of suspected food allergy, diet elimination trials can be used to discover the dietary component that is responsible (see below).


Although pets may be allergic to a wide variety of substances, the general signs of allergic dermatitis are common to all types. These include:

• Intense pruritus (itching); the areas of the body that are affected may be influenced by the type of allergy
• Redness and sores (lesions) in areas of affected skin; these are caused primarily by the self-trauma of constant scratching, licking and biting
• Secondary bacterial skin infections can add to pruritus and cause pustules and scabs to develop
• Reddish-brown saliva staining occurs as a result of persistent licking and chewing; most commonly seen around the groin, armpits and between the toes.
• Over time, the skin in affected areas becomes thickened and hyper-pigmented (black mottling in color instead of the normal pink).

Atopic (Inhalant) Dermatitis

Atopic dermatitis is the most common type of allergic dermatitis seen in dogs and cats. It caused by an allergic reaction to inhaled particles such as pollens, grasses, dust mites and molds and usually develops within the first two years of life. Owners often report that their pet self-grooms obsessively, often on the paws and abdomen, and may persistently rub the face and ears along carpets and furniture. Otitis externa (inflammation of the outer ear) is also common in dogs with atopic dermatitis. The ears are red and inflamed and may feel hot to the touch. The pattern of discomfort often (but not always) follows the seasonal pattern of pollens or molds that are common to the region. When atopy becomes severe, a pet may be affected year-round, with little or no period of relief.

Intra-dermal testing can be used to help identify specific antigens. Owners often learn that their pet is allergic to more than one substance. Once these are identified, a hypo-sensitization program can be used to reduce a pet’s reaction to the substance. This involves periodic injections of small amounts of the allergen(s) into the pet’s skin for up to 12 months. Responses to this treatment vary but many dogs and cats can attain sustained periods of relief and improvement through desensitization. Other treatments include the use of anti-inflammatory agents, careful attention to where the pet is walked or spends time to reduce exposure to allergens, and the use of medicated baths and topical agents to reduce pruritus.

Contact Allergens

Contact allergies are less common than atopic dermatitis, but easier to control and manage when they do occur. This type of skin disease occurs when the pet has an allergic response to direct contact with an offending substance. The most common contact allergens are particular soaps or shampoos, insecticides, certain types of carpet fiber, poison ivy or oak, and some pollens and grasses. In cats, contact allergies can develop in reaction to components found in some brands of cat litter. Because pets with contact dermatitis usually develop signs immediately after they are exposed to the antigen, it is often possible to quickly identify the causative agent. The most effective treatment is to eliminate the antigen from the pet’s environment and treat signs medically with anti-inflammatory agents and topical treatments.

Flea Allergy Dermatitis (FAD)

Flea bite allergy occurs when a dog or cat reacts to components found in the flea’s mouthparts when the flea bites. Because this is a hypersensitive reaction, the bite of just a single flea can cause intense itching and discomfort all over the body, not just in the area of the bite. In other words, the pet does not need to be infested with fleas to develop FAD. Eliminating fleas from the pet’s environment is the best way to treat and manage FAD. Spot-on (topical) flea treatments that kill adult fleas before they bite and control other stages of the flea’s life cycle in the environment are very effective against both flea infestations and FAD. Because the pet will react to just a single bite, management of FAD must always involve selecting a product that kills adult fleas before they bite, not after. Consult with your veterinarian when choosing an appropriate flea control product.

Food Allergies

Of the various causes of allergic dermatitis, food allergy is the least common, accounting for only about 5 to 10 percent of the allergies observed in dogs and cats. Food allergy occurs when a pet develops a hypersensitivity reaction to one or more ingredients (usually a type of protein) found in the diet. This type of allergy can cause both skin reactions and gastrointestinal problems such as diarrhea or vomiting. An allergy to a food component can develop at any time, even in pets who have been consuming the same food for months or years. This feature, plus the fact that presenting signs are similar to those of other types of allergies, can make food allergies difficult to diagnosis. (While the intra-dermal skin testing is excellent for diagnosing atopy, it is not effective for food allergies). Food components that are most likely to cause food allergies in dogs include the proteins found in beef, chicken, milk, corn, wheat, or soy. In the cat, the most common food antigens are fish and milk products.

Diagnosis of food allergy involves first ruling out all other possible causes of the allergic reaction. Once atopy, contact dermatitis and FAD have been eliminated as potential causes, a food elimination trial is used to confirm diagnosis and to help to identify the antigen (see Box 2). Once a diagnosis is made, treatment involves selecting and feeding a food that does not contain the offending ingredient. This can be accomplished either by feeding a commercial complete and balanced food that contains ingredients that the pet does not react to or by feeding a homemade food. If a homemade diet is used, it is essential that the recipe has been proven to be nutritionally balanced and complete. Your veterinarian can make recommendations for the selection of an appropriate diet.


During the elimination food trial a diet containing ingredients that the pet has never been exposed to previously is fed exclusively. This “eliminates” all potential sources of food allergens from the diet. The elimination diet that is selected for a diagnostic trial may be a commercially-prepared limited-antigen product or a homemade food. Here is the sequence of steps that are used:

• A typical elimination diet contains a single protein and single carbohydrate source, both of which are novel to the pet. Another option is to use a hydrolyzed protein diet, in which the protein source is hydrolyzed (cleaved) to small units that are incapable of invoking an immune response. These diets are available only through veterinarians.

• No other treats, chews, or food additives may be fed during the elimination trial period. Owners must also prevent the dog or cat from eating table scraps, garbage or another pet’s food.

• The food is fed for 8 to 12 weeks and the pet is observed regularly for a reduction in signs. (Although some home-made elimination diets are not complete and balanced, feeding for this period for diagnostic purposes is not be detrimental)

• If the pet shows a clear reduction or elimination of signs (i.e. stops itching), this is considered suggestive of food allergy. If there has been no change in symptoms but a food allergy is still strongly suspected, a second food trial using a different novel food source may be warranted.

• The diagnosis is confirmed by re-feeding the original diet and observing for a return of signs (itchiness). This is called the “challenge phase” of the trial. Dogs and cats with food allergy will typically become pruritic within four days to two weeks of being fed their original diet.

• A final optional step is to sequentially challenge the pet with individual allergens (food proteins) to attempt to identify the exact food component to which the pet is allergic. However many owners choose to simply select a food that is complete and balanced and contains novel protein and carbohydrate sources to manage their pet’s food allergy.

Managing Allergic Dermatitis in Dogs and Cats

Although there is no “cure” for allergic dermatitis, preventing exposure to triggering antigens and controlling symptoms in affected pets can often manage allergies successfully. Here are a few additional tips:

• In all cases, preventing exposure to the offending antigen (s) is most important. This is usually a fairly simple task with contact dermatitis and flea allergic dermatitis, but can be more challenging when pets are allergic to components in their environment or in food.

• The use of topical sprays and soothing shampoos and rinses are helpful in managing outbreaks. Some of these can be purchased through pet supply stores while others are available by prescription only from your veterinarian.

• The medical management of signs is often used with atopy and contact dermatitis, especially if the pet’s exposure to antigens cannot be completely prevented. A variety of medications are available, with varying levels of effectiveness, side effects and long-term risks. Consult with your veterinarian about medications that might be right for your pet’s type of allergy.

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