Flea Allergy Dermatitis

The skin is a “target” for ectoparasites.  Fleas and ticks have developed ways to get past the thin epidermal layer (especially that of the flanks and abdomen).  They can easily pierce through skin with their blood-sucking apparatus to access the rich network of blood vessels in the dermis.  Systemic exposure to allergens is pretty much inevitable as the fleas take a blood meal.

If allergy (hypersensitivity) develops, the tissue damage associated with inflammation may become too great for the skin to repair, and opportunistic infections with common bacteria (principally Staphylococcus pseud-intermedius and yeast (Malassezia pachydermatis) can gain a foothold.  Chronic itching and scratching causes affected areas to become hairless (alopecic, lichenified, and hyperpigmented).

Flea allergy dermatitis begins with flea saliva, which contains numerous antigenic materials.  Dogs present with signs of intense scratching, chewing, licking, biting, “hotspots” and other indicators of severe itching (pruritus).

Sixty-one percent of flea-allergic dogs develop clinical signs between 1 and 3 years of age.  Most dogs that are allergic to the bite of a flea have very few fleas on them at any time because their excessive grooming activity removes the fleas.

Patients usually have papules, crusts, salivary stains, excoriations, and erythema in a wedge-shaped pattern over the lumbosacral region, caudal thighs, proximal tail, ventral abdomen, and around the umbilicus.  FAD can be diagnosed based on age of onset, distribution of the pruritus, type of lesions and the observation of fleas and/or flea feces.

Many dogs affected by flea dermatitis will have recurrent tapeworm (Diplydium caninum) infestations from ingestion of the fleas.  The diagnosis of FAD may be confirmed with an intradermal skin test with flea antigen.

Therapy for the allergic reaction is based on the severity and history of the symptoms: it may include topical treatments, medicated shampoos, steroids, antihistamines, antibiotics, and fatty acid supplements such as skin oil replacement.  The effectiveness of allergy shots, or hyposensitization, for treating flea bite hypersensitivity remains controversial.  While symptomatic relief can be provided, the only real “treatment” for a dog with this condition is to keep him flea-free if possible.