Allergies in Dogs

By Malcolm Weir, DVM, MSc, MPH; Catherine Barnette, DVM; Ernest Ward, DVM

What is an allergy?

An allergy is a state of over-reactivity or hypersensitivity of the immune system to a particular substance called an allergen. Most allergens are proteins from plants, insects, animals, or foods.

Exposure to the allergen, usually on multiple occasions spanning months to years, sensitizes the immune system, and subsequent exposure to the same or related allergen causes an over-reaction. Normally, the immune system protects the dog against infection and disease, but with allergies, the immune response can be harmful to the body. Allergies may be considered an unnecessary normal immune response to a benign foreign substance.

The immune reactions involved in allergies are complex. Most reactions involve allergen protein molecules combined with antibodies in the blood that attach to a cell called a mast cell. Mast cells are found in many tissues throughout the body. When the antigen and antibody react with mast cells, the mast cells release potent chemicals, such as histamines, that cause local inflammation, such as redness, swelling, and itching. This inflammation causes various signs associated with an allergic reaction.

What are the symptoms of allergies in dogs?

In dogs, the most common symptom associated with allergies is itchy skin, either localized (in one area) or generalized (all over the body). In other cases, the allergic symptoms affect the digestive system, resulting in vomiting and diarrhea.

How common are allergies in dogs?

Unfortunately, allergies are quite common in dogs of all breeds and backgrounds. Most allergies appear after six months of age, with most affected dogs over one or two years.

Are allergies inherited?

Some allergies are thought to be inherited. An inherited allergy is atopic dermatitis (atopy) or allergies to substances in the environment, such as pollens (see "What is Atopic Dermatitis (Atopy) and how is it treated?" below).

What are the common allergy-causing substances (allergens)?

A very large number of substances can act as allergens. Most are insect, plant, or animal proteins, but small chemical molecules can also cause allergies. Common allergens include pollens, mold spores, dust mites, shed skin cells (like pet allergies in humans), insect proteins such as flea saliva, and some medications.

What are the different types of allergies?

There are several ways of classifying allergies. Some examples of classifications include:

  • The allergen: flea allergy, food allergy
  • The route the allergen takes into the body: inhalant allergy, skin contact allergy, or food allergy
  • The time it takes for the immune reaction: immediate-type hypersensitivity, also called anaphylaxis or shock, and delayed-type hypersensitivity
  • The clinical signs: the areas of the body affected by the allergen
  • Inherited forms: atopy or seasonal allergies

What is flea or insect bite allergy, and how is it treated?

Insect bite allergy is the exaggerated inflammatory response to the bite or sting of an insect. Arachnids, such as spiders and ticks, and insects, including fleas, blackflies, deerflies, horseflies, mosquitoes, ants, bees, hornets, and wasps, can cause an allergic reaction in sensitive dogs.

"Most dogs experience minor local irritation from flea bites."

Flea saliva is the most common insect allergen in dogs, causing flea allergy dermatitis (FAD). Most dogs experience minor local irritation from flea bites. The FAD dog will react to a single bite with severe local itching. A dog with FAD will bite and scratch himself and may remove large amounts of hair, especially in the tail-base region. A secondary bacterial infection may develop in the areas of broken skin.

Because one flea can be a problem for a dog with FAD, strict flea control is essential. This is difficult considering the life cycle of fleas, but modern monthly flea preventives and home treatment options allow you to provide a flea-free environment for your dog (see the handout “Flea Control in Dogs” for more information). Your veterinarian can advise on protecting your dog and other pets from fleas. When strict flea control is not possible, or in cases of severe itching, your veterinarian may prescribe antihistamines or corticosteroids (steroids) to block the acute allergic reaction and give immediate relief. If a secondary bacterial infection is present, an appropriate antibiotic will be prescribed. (See the handout “Flea Allergy Dermatitis in Dogs” for more information.)

What is atopic dermatitis (atopy), and how is it treated?

The term atopic dermatitis in the dog is often used as a synonym for atopy. The main allergens are tree pollens (cedar, ash, oak, etc.), grass pollens, weed pollens (ragweed), molds, mildew, and house dust mites. Many of these allergies occur seasonally, such as ragweed, cedar, and grass pollens. However, molds, mildew, and house dust mites occur year-round. In dogs, atopic dermatitis manifests with itchy skin (pruritus). Your dog may rub his face, lick his feet, and scratch the axillae (underarrms).

Most dogs with atopic dermatitis start showing signs between one and three years of age. Affected dogs often react to several allergens and experience concurrent flea or food allergies. If the offending allergens can be identified by intradermal skin tests (skin testing) or blood tests, the dog should be protected from exposure to them as much as possible. Because most of these allergens are environmental, this is difficult, and recurrent bouts are likely. Symptoms of atopy can be controlled, but a permanent cure is not usually possible.

Treatment depends largely on the length of the specific allergy season. It may involve one or more of the following three therapies:

Anti-inflammatory therapy. Treatment with anti-inflammatory drugs, such as corticosteroids or antihistamines, will quickly block the allergic reaction in most cases. Fatty acid supplementation in the diet can sometimes improve the response to steroids and antihistamines. Newer alternatives exist to block specific chemical signals associated with itch in dogs. These drugs include daily oral medications, such as oclacitinib (Apoquel®), and long-acting injections, such as lokivetmab (Cytopoint®). Your veterinarian can help you determine whether these medications may be appropriate for your dog.

Shampoo therapy. Frequent bathing with a hypoallergenic shampoo can be soothing to itchy, inflamed skin. Bathing also rinses out allergens in and on the coat that can be absorbed through the skin. Some therapeutic shampoos also contain anti-inflammatory ingredients that may further benefit your pet.

Hyposensitization or desensitization therapy. If the offending antigens are identified by allergy testing, a specific allergy serum is made for your dog and is given in a series of injections or liquid under the tongue. With this treatment, very small amounts of the antigen are administered on a regular basis. This repeated dosing has the objective of reprogramming or desensitizing the immune system. Success rates vary with this treatment. Approximately 50% of treated dogs see significant improvement in their clinical signs, while approximately 25% more will see a decrease in the amount or frequency or of corticosteroid usage.

Nutritional therapy. Certain diets are formulated to reduce the itch caused by atopic dermatitis. With ingredients that improve skin health and reduce the inflammatory response, these diets can reduce itching in allergic pets. These diets are usually available from your veterinarian.

See the handout “Atopic Dermatitis in Dogs” for more information on this type of allergy.

What is food allergy, and how is it treated?

A food allergy or hypersensitivity can develop to almost any protein or carbohydrate component of food. It most commonly develops in response to the protein in the food; beef, chicken, lamb, eggs, dairy products, and soy are commonly associated with food allergies in dogs. A food allergy can develop at almost any age. Common clinical signs include itching, digestive disorders, and respiratory distress. A dog may have multiple types of allergies, such as food allergy and atopy, making the exact diagnosis of a dog’s itching challenging.

Food allergies typically do not respond well to corticosteroids or other medical treatments. Treatment requires identifying the offending component(s) of the diet and eliminating them. The most accurate way to test for food allergies is with an elimination diet trial using a veterinary hypoallergenic diet. Because it takes at least eight weeks for all other food products to be eliminated from the body, your dog must eat the special diet exclusively for eight to twelve weeks. If there is a positive response and improvement in your dog’s clinical signs, your veterinarian will advise you on how to proceed.

It must be emphasized that a food trial will be invalid if the diet is not fed exclusively. All table food, treats, and flavored vitamins must be discontinued during the testing period. There may be problems with certain types of chewable tablets or medications, such as heartworm preventives. Your veterinarian will discuss the specific diet and restrictions recommended for your dog. (See the handout on “Food Allergy in Dogs” for more information.)

What is contact allergy? 

A contact allergy is the least common type of allergy in dogs. It results from direct contact with allergens, such as pyrethrins, found in flea collars, pesticides, grasses, and materials, such as wool or synthetics, used in carpets or bedding. Contact allergies can develop into practically anything and at any age.

"A contact allergy is the least common type of allergy in dogs."

If your dog is allergic to any of these substances, there will be skin irritation and itching at the points of contact, usually the feet and stomach. Removing the allergen (once identified) often solves the problem, although topical or systemic treatments may be required.

Caution: The symptoms of allergies can be confused with other disorders or occur concurrently. Therefore, do not attempt to diagnose your dog without veterinary assistance. Be prepared for your pet to receive a full diagnostic evaluation to rule out other causes of itching and skin problems. If an allergy is diagnosed, your whole family must follow your veterinarian's advice very closely to successfully relieve your pet's discomfort

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