Dog: Flea Allergy Dermatitis

General information

Other common/scientific names: flea bite dermatitis, flea bite hypersensitivity

An allergy is an abnormal reaction of the body to an allergen which is a substance that triggers the allergic reaction. An allergic reaction is a hypersensitivity response occurring when the body’s immune system overreacts to the substance. Unlike people, dogs rarely have respiratory allergies but instead exhibit allergic symptoms as skin conditions.


Dogs develop flea allergy dermatitis as a result of an allergic reaction to the flea’s saliva when the flea bites the dog. In this case, the allergen is the flea’s saliva. The saliva from just one flea is enough to cause some dogs to develop this condition. Flea bite dermatitis is one of the most common skin conditions that affects dogs. In warm climates or in homes, fleas may survive year-round.

The dog flea is Ctenocephalides canis. However, the cat flea, Ctenocephalides felis, is the common flea found on the dogs in North America.

Abb. GG1XH3KY: Flea in dog hair.
This is a photograph of a flea found in dog hair. When a flea bites a dog, the saliva can lead to severe allergy symptoms.


Dogs acquire fleas by contact with other dogs, animals or humans which have fleas.

Cardinal symptom



Flea allergy dermatitis can develop in dogs of any breed, sex and age. These dogs exhibit signs of intense pruritis (itching) most commonly on or near the base of the tail but can involve any part of the dog’s body. Other signs include hair loss and red, inflamed skin from self-trauma due to scratching. Dogs can develop a secondary infection due to bacteria (Pyoderma) or yeast (Malassezia) resulting in raised papules, pustules and scabbing which further worsens the pruritis.

Abb. GG1XM45Z
Abb. GG1XM45Z: Flea Bite Dermatitis.
This photograph illustrates hair loss pattern on the back area and base of tail which is common with flea bite dermatitis.


Diagnosis of flea bite dermatitis is based on examination of the skin, presence of fleas or flea dirt (flea excrement or dried blood) and response to treatment. However, the lack of fleas or flea dirt should not rule out flea allergy. Many itchy dogs are effective at licking and grooming, so it may be difficult to find live fleas on your dog.

Allergy testing in the form of intradermal skin testing or serum allergen testing can be performed. With the intradermal skin testing, different allergens, i.e. flea saliva, are injected underneath the skin. These areas are then observed for hypersensitivity to the allergen. With serum allergen testing, a blood sample is submitted to a laboratory and specific immunoglobulins are measured. The results from allergy testing can aid in the diagnosis not only of flea allergy but also atopic dermatitis.


Treatment of flea allergy dermatitis requires year-round flea control of your dog and the dog’s environment. The oral and topical (spot-on) antiparasitic products are most commonly used to treat and prevent flea infestations. In the case of flea allergy, it is best to use a product which kills the flea before it bites the dog. This prevents the dog from contacting the allergy triggering saliva.

Secondary bacterial and yeast infections must be also be treated. Anti-inflammatory medications can be used to help alleviate the itching.


Flea allergy dermatitis has a good prognosis if stringent flea control is implemented. Concurrent allergies will require lifelong allergy management. As with any allergy, a cure is not possible but these dogs can lead a healthy, itch free life with proper treatment.


Many dogs with flea allergies are also allergic to other substances such as certain types of dog food, pollens, molds, dust mites, etc. These dogs are much more sensitive to even a small increase in the amount of allergen. The allergic threshold is the amount or level of allergens at which a dog will show clinical signs of allergies. The lower the allergic threshold, the more sensitive the dog. It is extremely difficult to eliminate all the allergens in a dog’s environment. However, in order to provide relief for your dog, you do not need to eliminate them all. Rather, you only need to prevent the allergens from exceeding threshold. Because fleas are preventable, you can keep an allergic dog symptom free by preventing flea infestation.

Abb. GGTEWWVK: A diagram illustrating the allergic threshold principle.

Update version: 4/24/2014, © Copyright by
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