Flea Allergy Dermatitis in Dogs

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

What is flea allergy dermatitis?

Flea allergy dermatitis (FAD) is a leading cause of allergic reactions in dogs. In an allergic reaction, the body’s immune system overreacts or is hypersensitive to a substance (antigen) that is usually harmless.

FAD is a common cause of itching in dogs. Adult fleas must bite a dog and obtain a blood meal to reproduce. Fleas typically do not remain on the dog except for the minutes to hours they feed, which is why dog owners often do not see live fleas on their dogs unless there is a severe flea infestation in their immediate environment.

When fleas feed, they inject a small amount of saliva into the skin. The antigens in the saliva cause an intensely itchy response in sensitive dogs. Dogs with FAD do not have to be infested with fleas to be itchy. A single flea bite can cause itching for several days.

Are only certain dogs allergic to fleas?

FAD can develop at any age. It is important to note that dogs with other forms of allergies, such as environmental allergies (e.g., pollens, molds, dust mites) tend to be highly sensitive to a flea bite and are therefore much more susceptible to FAD than dogs that do not have other allergic conditions.

How is flea allergy dermatitis diagnosed?

Clinical signs often indicate that your pet suffers from FAD. Itching and hair loss from the middle of the back to the tail base and down the rear legs (the flea triangle) are often associated with FAD.

Intradermal allergy tests (skin tests like those performed in humans) or specialized blood tests (IgE blood tests) can confirm flea allergy in your dog. The signs of FAD are often classic, and response to treatment occurs so quickly that formal allergy testing is only necessary in some cases.

What does treatment involve?

Since the injection of flea saliva causes the allergic response, it is important to prevent fleas from biting your dog. Strict flea control is essential. Even if you do not see fleas on your dog, it is important to continue rigorous flea treatment and maintain environmental control. Most flea infestations occur in warmer weather but can happen year-round (see handout "Flea Control in Dogs" for more information). For a dog with FAD, flea prevention (using modern oral or topical medications) is the best form of treatment. Speak to your veterinarian about the best option for preventative treatment for fleas, as there are many choices available.

What about desensitization?

Desensitization, or administering allergy injections, involves increasing doses of a diluted flea antigen over a prolonged period. Sometimes, these allergy injections may be required for the duration of the pet's life. Unfortunately, desensitization is not considered very successful in treating flea allergies and is rarely used. It is important not to confuse desensitization therapy with corticosteroid or cortisone injections, which relieve the symptoms of an acute FAD flare-up.

What about steroids or other drugs?

Corticosteroids are widely used to treat acute (sudden) episodes of particularly itchy FAD. They often bring about almost miraculous relief from the itching. However, steroid use has significant potential side effects (see handout "Steroid Treatment - Effects in Dogs" for more information).

Steroids, most commonly prednisone, can safely be used for short-term relief while flea control is implemented. Combining corticosteroids with antihistamines and/or omega-fatty acid supplements is an ideal regimen for many patients. The goal is to use the lowest dosage of corticosteroids possible, as infrequently as possible, preferably on an alternating or every-third-day basis.

"Your veterinarian will discuss the pros and cons of the various treatments for FAD with you during the examination and will recommend the safest and most effective treatment for your pet's needs."

Fortunately, steroids are often unnecessary in today’s treatment of FAD because of newer and more effective flea control products for pets. Oclacitinib (Apoquel®), or Lokivetmab (Cytopoint®), are both alternatives to steroids that also help address itching and inflammation. Antibiotics may be necessary if your dog has developed a secondary bacterial skin infection because of the scratching.

Your veterinarian will discuss the pros and cons of the various treatments for FAD with you during the examination and will recommend the safest and most effective treatment for your pet's needs. Prevention is essential, relatively simple, and inexpensive.

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