Flea Allergy Dermatitis


Flea allergy dermatitis (FAD) is a skin condition of dogs and cats that's caused by an allergic reaction to flea saliva. An allergic reaction to certain proteins in the flea's saliva occurs after a flea bites and feeds on a pet's skin. For some afflicted pets, even a single flea bite can cause a severe, prolonged reaction.

FAD is considered a highly uncomfortable condition. The resulting inflammation and itching can lead to such dramatic scratching, licking, biting, and chewing that the skin is often red, scabbed, crusty, and otherwise inflamed. Because secondary bacterial or fungal infections often occur secondary to the inflammation, untreated FAD

Though more than two thousand species of fleas exist, the one that most commonly infests dogs and cats is the common cat flea -- otherwise known as Ctenocephalides felis.

Symptoms and Identification

Itching, scratching, licking, biting and chewing of the skin is typically the first sign of this condition but other evidence of FAD includes:

  • Skin redness

  • Hair loss

  • Scabbing and crusting of the skin

  • Hot spots (oozing or crusted sores)

  • Darkening or thickening of skin (after a prolonged period of infestation)

  • Unpleasant odor (usually the result of secondary infection)

  • Fleas visualized

  • Blackish debris (flea feces) on the skin at the base of the hair

This last sign is the result of flea feces. When collected and evaluated off the animal, this blackish "flea dirt" will dissolve into a rusty, bloody paste when wetted.

Signs of FAD can be more severe during warm and humid weather, when fleas are more active. However, if a pet's home environment is infested with fleas or if the pet lives in a place that is warm year-round, FAD can be a year-round problem seemingly unaffected by the weather.

Dogs and cats typically display different signs of FAD:

In dogs, red, oozing lesions called hot spots may develop in areas where the scratching is most intense--typically on the rump, tail, and hind legs. Affected dogs typically exhibit thinning of the hair along the tail base in a classic, "Christmas tree" pattern.

Affected cats may lick off large swaths of hair in a bilaterally symmetrical pattern. Many, however, will develop tiny scabs (collectively called miliary dermatitis) that may cover most of their body but affects their necks and rumps preferentially. Still other cats may lose the fur over their noses and develop crusts and ulcerations there.

The diagnosis of FAD is typically made after finding fleas (or evidence thereof). However, because a single bite can cause a reaction and many FAD patients show no signs of ever having ever been bitten, definitive diagnosis may rest either on allergy testing (blood- or skin-based) or, more commonly, after treating with a highly effective flea product.

Affected Breeds

No breed predisposition has been specifically identified for FAD in either dogs or cats. Allergic skin disease in general, however, has been found to be more prevalent among certain breeds of dogs.


Treatment involves three approaches: flea treatment, allergy treatment, and treatment of the skin infections that invariably result from a pet's response to the allergy.Flea treatment: Many safe drugs and products have been designed to effectively kill fleas. Many of these may also be used to prevent fleas in the future.

Allergy treatment: Antihistamines and corticosteroids are effective in breaking the itch-scratch cycle initiated by the body's allergic response.

Treatment of skin infections: Oral antibiotics are sometimes necessary but in many cases topical therapies like shampoos can be highly effective.

Veterinary Cost

The cost of diagnosis and treatment of FAD is typically relegated to the cost of flea treatment/prevention and the medications (oral, topical, or injectable) required to eliminate the allergic response and treat the resulting infections.

Flea treatment/prevention drugs and products can be had for anywhere between $5 and $30 a month. The most effective preventatives tend towards the middle to higher end of this estimate.


Flea preventatives are strongly recommended for all dogs and cats that live in geographic locales that support flea populations. Continuous, year-round prevention may be necessary depending on the temperature and humidity of the region.


Dryden M, Brown H, Buck S, et al. Effective long-term protection against flea infestations. Veterinary Medicine 1998; June: 16-18.

Dryden MW, Denenberg TM, Bunch S. Control of fleas on naturally infested dogs and cats and in private residences with topical spot applications of fipronil or imidacloprid. Vet Parasitol 2000; 93:69-75.

Dryden M, Perez H, Ulitchny D. Control of flea populations on naturally infested dogs and cats and in private residences with either topical Imidacloprid spot application or the combination of oral lufenuron and pyrethrin spray. J Am Vet Med Assoc 1999;215:36-39.

Dryden MW, Rust MK. The biology, ecology, and management of the cat flea. Annu Rev Entomol 1997;42:451-473.

Portis MJ, Dryden MW, Broce AB. et al. Seasonal variation of wildlife distribution and ectoparasite load in the urban environment, in Proceedings. 7th Int Symp on Ectoparasites of Pets, League City, TX, 2003;35.