Back to Care

The Allergic Dog

There are three main types of allergies in dogs – atopy (inhalant allergies), flea allergies, and food allergies. In dogs, just like in humans, the incidence of allergies is increasing.

Dogs are allergic to many of the same environmental allergens as humans. Common allergens are pollens, dander, grasses, trees, and fabrics. In food allergic patients, the allergen is usually a major protein or carbohydrate ingredient such as beef, chicken, pork, corn, wheat or soy. However, any food, preservative or dye can be an allergen. Some dogs have many allergies. It is not particularly unusual for a dog with a food or inhalant allergy to also be allergic to flea bites, especially considering that flea bite allergy is extremely common among pets. Flea bite allergy is caused by the flea’s saliva, so it only takes a few bites to cause the problem. 

Features of Allergy Types

Atopy usually starts out as a seasonal itchiness. After several years, the itchy period extends until in many cases the dog is itchy almost all year round. Atopy tends to begin between the ages of one and three. Itchiness due to atopy responds rapidly to steroids.  Atopy is associated with skin irritation around the eyes and mouth, ears, armpits, abdomen, legs, and around the anus.

Food allergy requires time to develop, so most animals have been eating the same diet for years before problems develop.

Food allergy is one of the itchiest conditions known to dogs. Many people assume itching due to food allergy requires a recent diet change of some sort. In fact, the opposite is true.  Food allergy requires time to develop, so most animals have been eating the same diet for years before problems develop. The following clues point us toward a diagnosis of food allergy ~ itchiness is not and has never been a seasonal problem. There has been partial response to steroids at best and there is a combination of facial itching, foot or limb chewing, belly itching, and recurrent ear infections. Some dogs will also have itching and irritation around the anus. 

Dogs that have flea allergies will bite at the base of their tail and scratch frequently. Many dogs have a characteristic thinning of the hair above the base of the tail. Fleas or flea dirt can be found on the dog the majority of the time; however, it can occasionally be difficult to find fleas as only a few fleas are needed to cause a problem in an allergic dog. 


Atopy is often diagnosed based on history and symptoms. In dogs that do not respond to symptomatic treatment, further testing is often needed. This may be in the form of biopsies or allergy testing. Testing is best done during the pet’s non-itchy season. The test involves injections of small amounts of allergen extracts into the skin. Reactions noted are compared to reactions produced by two controls ~ pure histamine (very inflammatory) and pure saline (very non-inflammatory). Prior to testing, many medications commonly used to treat atopy must be withheld. Allergic skin testing is generally performed only by veterinary dermatologists. As an alternative to skin testing, several blood tests have been developed to check for the presence of allergy-type antibodies in the blood. These tests can be submitted by any veterinarian and drugs generally do not need to be withheld prior to testing. Skin testing is ideal, but blood tests may be considered for animals suspected of having inhalant allergies that simply cannot go without medication, who have negative skin testing or for whom skin testing is unavailable for other reasons. 

To determine whether or not a food allergy is causing the itching, a hypoallergenic diet is fed for a set period of time (generally 10-12 weeks). If the skin improves, the original diet is fed for up to two weeks to see if itching returns. The test diet must be something that the dog could not possibly be allergic to. The traditional method is the use of a protein and carbohydrate source that the dog has never eaten before. No edible chew toys, such as rawhides or bones, should be given. Treats must be based on the same food sources as the test diet or better yet avoided altogether. (Beware of rice-cakes as wheat is commonly used as filler.) Chewable heartworm preventatives should be replaced with tablets or drop-ones. A newer option is the use of diets made from hydrolyzed proteins. This means that a conventional protein source is used but the protein is broken down into molecules too small to excite the immune system.

WARNING: Diets that include duck, venison, and other proteins commonly used in hypoallergenic diets are now readily available. Before feeding these foods, remember that these ingredients will not be useable for diet trials in pets that have eaten them as their normal diet. That may complicate the diagnosis of food allergy in the future. 


There is no cure for allergy and it is usually a life-long problem. The goal is to control allergies and improve the quality of life for dog and owner. Prednisone and other related steroids tend to be useful as the first line of defense against itchy skin. Dogs are started on a higher dose, which is tapered to every other day. An atopic dog will respond within a few days. Problems occur when the dog’s need for itch control demands excessive use of prednisone or there are an excessive number of undesirable side effects. Common side effects include excess thirst, excess appetite, urinary incontinence, lethargy, and panting. 

Antihistamines are less harmful and have fewer side effects than prednisone; however, only 10-20% of dogs will respond to any given antihistamine. Often several different antihistamines must be tried to determine which is most effective. Diphenhydramine (Benadryl), clemastine, hydroxyzine, and chlorpheniramine are four of the most commonly tried. Even if there is not adequate response to an antihistamine alone, combining the antihistamine with a steroid will generally lower the required dose of the steroid. Fatty acid supplementation alone is helpful in 10-25% of itchy dogs and will often boost the effectiveness of antihistamines. 

Atopica (cyclosporine) is an immune system modulating drug originally developed for use in organ transplant patients but also useful in other immune-mediated diseases. Since allergy is an immune-mediated condition, cyclosporine has been found to be an alternative to steroids. Atopica tends to be quite effective with minimal side effects; however, it can be quite expensive. 

Topical sprays and shampoos are helpful in some dogs but rarely adequate on their own.  When bathing, it is important to remember that cool water is considered much more soothing than warm water. It is also important to remember that ten minutes of skin contact is the minimum requirement for any medicated shampoo to allow for the therapeutic benefit to be realized. 

Allergy injections (hyposensitization) may be considered in pets just like in people; however, hyposensitization should not be expected to end all scratching. Allergy shots require approximately 6 to 12 months to begin working ~ 25% of atopic dogs will not respond, 25% of dogs will require prednisone at least some of the time, and you will need to give the injections.  


Severe itching has a major impact on a dog’s quality of life. It is important not to develop the mindset that steroids should be avoided at all cost. This is not fair to the itching pet.  Steroids are valuable tools in the treatment of itchy pets. The goal is not to avoid steroid use but to avoid long-term dependence on steroids if possible. However, there are a few dogs that will still require long-term steroid use in order to achieve any reasonable comfort. These dogs need to be monitored with checkups, routine blood work and, other testing as needed.