Flea Allergy Dermatitis in Cats

By Ryan Llera, BSc, DVM; Tammy Hunter, DVM; Ernest Ward, DVM

What are allergies, and how do they affect cats?

One of the most common medical conditions affecting cats is allergies. An allergy occurs when the cat's immune system overreacts or is hypersensitive to foreign substances called allergens. Allergens are foreign proteins that the body's immune system tries to remove. Examples of allergens common in humans are pollens, dust, molds, and pet hair.

Hypersensitivity in cats can manifest in one of three ways:

  1. The most common manifestation is itching of the skin, either localized to one area or a generalized reaction all over the cat's body.
  2. Another manifestation involves the respiratory system and may result in coughing, sneezing, and wheezing. Sometimes, there may be an associated nasal or ocular (eye) discharge.
  3. The third manifestation involves the digestive system and can result in vomiting, flatulence, and diarrhea.

Does that mean that there are several types of allergies?

Yes. The cat has four common types of allergies: insect (fleas), food, atopic (house dust, pollen, and molds), and contact. Each has some common physical signs in cats, and some have unique features.

What is a flea allergy?

Despite common belief, an average cat experiences only minor skin irritation in response to flea bites. Even in the presence of dozens of fleas, there will typically be minimal itching. On the other hand, a cat with flea allergies will have a severe reaction to even a single flea bite. This reaction is an allergic response to proteins or antigens in the flea's saliva.

When a flea bites a cat to consume a blood meal, some of its saliva is injected into the skin. In an allergic cat, just one bite can result in intense itching that can last for days. Cats with flea allergy dermatitis (FAD) do not have to be infested with fleas; a single flea is enough to cause a problem.

What does a reaction to flea bites do to the cat?

A cat with FAD experiences intense itching and chews, licks, or scratches the affected site(s) nonstop. This causes hair loss and can lead to open sores or scabs on the skin, allowing a secondary bacterial infection to develop.

The area most commonly involved in FAD is over the rump, just in front of the tail. Many flea-allergic cats chew or lick the hair off their legs. Itching and hair loss around the tail base, neck, and head should be considered suspicious for flea allergy dermatitis. In addition, an affected cat may have numerous small scabs around the head and neck. These scabs are often referred to as miliary dermatitis, a term coined because the scabs look like millet seeds (see handout "Miliary Dermatitis in Cats" for more information on this skin condition).

How is flea allergy dermatitis diagnosed?

Clinical signs often give the first indication that your cat may suffer from FAD. Cats are such fastidious groomers that it is frequently impossible to find any evidence of fleas or flea dirt on the coat, especially if only one or two fleas are causing the problem. Intradermal allergy tests (skin tests like those performed in humans) or specialized blood tests (IgE tests) can confirm a flea allergy in your cat.

What is the treatment for flea allergy dermatitis?

Since the flea saliva causes the reaction, the most important treatment for flea allergy is to prevent flea bites. Most flea infestations occur in the warmer weather but can occur year-round. Strict flea control is the foundation of successful treatment. There are many highly effective flea control products, both for treating the cat and for controlling fleas in the environment (for more details, see the handout "Flea Control in Cats"). Modern monthly flea preventives have made it easier and less expensive than ever to prevent fleas from affecting your cat. Speak to your veterinarian about the best prevention for your cat.

What about allergy shots?

Some cats can be desensitized to the adverse effects of allergens through a series of special injections. However, flea allergy desensitization does not tend to be used for FAD in cats, as the success rate is highly variable.

What about steroids or other drugs?

Corticosteroids can be used to block the allergic reaction and give immediate relief to a cat suffering from the intense itching of FAD. This is often necessary for treating the condition, especially during the initial stages. Some cats respond best to long-acting injections of methylprednisolone (Depo-Medrol®) and others to oral medication, most often prednisolone.

While it is true that cats are more resistant to the adverse side effects of steroids than humans and dogs, significant side effects of steroids can occur if not used correctly. For this reason, the goal is to administer the smallest amount of the steroid needed to keep the cat comfortable. For some patients, combining corticosteroids with antihistamines and/or omega-fatty acid supplements will provide the ideal form of relief. Antibiotics may also be necessary if your cat develops a secondary bacterial skin infection (pyoderma) because of the scratching. Occasionally topical treatments with shampoos and sprays may be considered.

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