Visit Your Vet: Why is my pet itching?

Understanding Allergic Dermatitis in Dogs and Cats

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 an owner 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 overreacts 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 intense and prolonged pruritus (itchiness) that the dog or cat experiences upon exposure to the offending antigen (see signs below).

Signs of Allergic Dermatitis in Dogs and Cats

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) in areas of the body— may be influenced by the type of allergy
  • Redness and sores (lesions) in areas of the skin—caused primarily by the self-trauma of constant scratching, licking and biting
  • Pustules and scabs—may be caused by secondary bacterial skin infections and add to pruritis
  • Reddish-brown saliva staining—occurs as a result of persistent licking and chewing, most commonly around the groin, armpits and between the toes
  • Thickening and hyper-pigmenting of the skin—may become mottled black in color instead of the normal pink
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, serum and intradermal skin tests can be helpful in identifying the exact antigens. In cases of suspected food allergy, diet elimination trials can be used to discover the dietary component that is responsible.

Atopic Dermatitis

Atopic dermatitis is an extremely common type of allergic dermatitis seen in dogs and cats. It is caused by an allergic reaction to 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.

Intradermal or serum 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 or longer, as the veterinarian recommends. Responses to this treatment vary, but many dogs and cats can attain sustained periods of relief and improvement through desensitization. Other treatments may be appropriate, depending on your pet’s needs. Your veterinarian can offer helpful advice to help keep your pet comfortable.

Contact Allergens

Contact allergies are less common than atopic dermatitis, but may be easier to 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, 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 actions are to eliminate the antigen from the pet’s environment and call your veterinarian.

Flea Allergy Dermatitis (FAD)

Flea bite allergy occurs when a dog or cat reacts to components found in the flea’s mouth 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 important, and your veterinarian can help you learn the best way to treat and manage FAD. Spot-on (topical) flea treatments that kill adult fleas and control other stages of the flea’s life cycle in the environment are very effective against both flea infestations and FAD.   Consult with your veterinarian to choose an appropriate flea control product.

Food Allergies

Of the various causes of allergic dermatitis, food allergy accounts for a small percentage of the allergies observed in dogs and cats. Food allergy occurs when a pet develops a hypersensitivity reaction to one or more proteins 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 that 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 diagnose. Food components that are most likely to cause food allergies in dogs include the proteins found in beef, chicken, milk, egg, lamb, pork, fish, corn, wheat or soy. In the cat, the most common food antigens are beef, chicken, lamb, wheat, egg, fish and milk products.

A food elimination trial is used to diagnosis food allergy and to help identify the antigen (see below). Once a diagnosis is made, treatment involves selecting and feeding a food that does not contain the offending ingredient(s). 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.

Elimination Trial for Diagnosis of Food Allergy 

During the elimination food trial, the pet is fed exclusively a diet containing ingredients he has never been exposed to. This “eliminates” all potential sources of food allergens from the previous 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:

  • Your veterinarian will recommend an appropriate elimination diet based on your pet’s detailed food history.
  • The food is fed for 8 to 12 weeks, and the pet is observed regularly for a reduction in signs of allergy.
  • 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.
  • If the pet shows a clear reduction or elimination of signs, this is considered suggestive of the food allergy diagnosis.The diagnosis is confirmed by re-feeding the original diet and observing for a return of signs.  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 reintroducing 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.

Managing Allergic Dermatitis in Dogs and Cats

Although there is no “cure” for allergic dermatitis, preventing exposure to triggering antigens and controlling signs 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 can be helpful in managing outbreaks.
  • The medical management of atopy and contact dermatitis is often necessary, especially if the pet’s exposure to antigens cannot be completely avoided.
Consult with your veterinarian to determine the right therapeutic approach for your pet’s allergies. 


  • excessive and persistent scratching, licking, biting or hair loss may be signs of allergic skin disease.
  • Understanding the types of allergies that occur in dogs and cats can help an owner to seek prompt veterinary care and relief for their pet.
  • Consult with your veterinarian to determine the right therapeutic approach for your pet’s allergies.