Flea Allergy Dermatitis (FAD) in Dogs

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6 min read

Key takeaways

Flea allergy dermatitis (FAD) in dogs is an inflammatory response to the saliva of an adult flea as it feeds on its host. -A dog can develop flea allergy dermatitis after exposure to only a small number of fleas; failure to find evidence of fleas does not rule out the condition

  • Not all dogs are allergic to flea saliva, so a nonallergic dog may carry thousands of adult fleas and not have FAD
  • When a flea bites an allergic dog, the dog responds by biting, licking, rubbing, and scratching
  • The resultant rash is characterized by hair loss, redness, and scabby sores
  • The rash can occur anywhere on the dog’s body, but is characteristically worst on the dog’s lower back and near the base of the tail
  • Damaged skin is vulnerable to secondary bacterial and/or fungal skin infections
  • Other unrelated conditions can cause similar symptoms, so diagnostic tests are crucial to an accurate diagnosis
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A closer look: Flea Allergy Dermatitis (FAD) in Dogs

Flea allergy dermatitis is not an emergency, but severely itchy dogs often show signs of distress and are unable to sleep well or get comfortable. As such, getting to the veterinarian for treatment at the earliest opportunity is best.

Dogs usually respond rapidly to treatment and stay comfortable as long as the owner remains compliant with ongoing treatment. The long-term goal is to use flea control consistently to avoid the need for separate allergy treatment or antibiotics/antifungals for secondary infections.

Always consult a veterinarian before choosing external parasite control. Many available products are toxic to pets, especially cats.

Ideally, a dog with FAD will remain symptom-free when kept on a veterinarian-approved flea control plan year-round.

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Risk factors

Fleas are ubiquitous in the environment. All dogs and outdoor animals are at risk of exposure to fleas.

While a dog may develop an allergy at any point in its life, most allergies appear in dogs between the ages of two and five years. Dogs with atopic dermatitis are likely to also be allergic to flea saliva.

Flea allergy dermatitis symptoms vary depending on:

  • Severity of the allergy: as with humans, repeated exposure to the allergen heightens the severity of the allergic reaction.
  • Number of bites (i.e., intensity of the infestation): the more bites, the stronger the symptoms
  • Impact of the dog’s response (biting, scratching, etc.): the longer the dog scratches, bites, licks, and rubs, the greater the likelihood of serious secondary problems, like infections.

Possible causes

As with allergies in humans, the reason why certain dogs become allergic is not known.

Main symptoms

Testing and diagnosis

The basic diagnostic workup for an itchy dog includes:

  • Flea combing, to check for evidence of fleas and flea dirt (Failure to find physical evidence of fleas does not rule out flea allergy dermatitis, because a dog may groom the fleas off before they have had a chance to create a large population)
  • Skin scraping, to check for mites (but negative results do not rule mites out)
  • Fungal culture, to check for ringworm (dermatophytosis) (false negatives can occur)
  • Skin cytology, to check for signs of secondary bacterial and fungal skin infections

Treatment for flea allergy dermatitis is targeted at eliminating fleas, relieving the allergic itch, and treating any secondary infection.

One common approach for mild symptoms is a therapeutic trial of flea medication. In many cases, itchy dogs show improvement simply with monthly flea control. A variety of options for flea treatment exist, and veterinary guidance is critical for successfully navigating the available choices.

Always consult a veterinarian before choosing external parasite control. Many available products are toxic to pets, especially cats.

In some cases, a therapeutic trial of an allergy medication may be recommended. It’s worth noting that many types of symptomatic allergy treatments suppress the immune system, and this can make conditions that look similar to FAD (like mites or skin infections) worse.

While undergoing treatment to kill the fleas, dogs may find relief of symptoms with shampoos and medications. Frequent bathing may also decrease the effectiveness of some types of flea control, so check with your vet before bathing.

Steps to Recovery

Treatment for flea allergy dermatitis requires a multimodal approach:

  • Protect the pet with products that kill fleas before they bite
  • Treat the environment with products that kill the larvae and newly emerged adult fleas
  • Administer medications to reduce the itch and inflammation resulting from the allergy
  • Administer medications to treat any secondary infections

The allergy itself lasts for the lifetime of the dog. The inflammatory response and subsequent symptoms last as long flea bites occur. Once flea bites stop, the irritation, biting and scratching, and hair loss will stop as long as any secondary infections are also addressed.


Flea allergy dermatitis is not contagious. FAD cannot be prevented, but symptoms can be kept under control with continuous maintenance of flea control medication.

Is Flea Allergy Dermatitis (FAD) in Dogs common?

Flea bites are the leading cause of allergic reactions in dogs. Flea allergy dermatitis is the most common skin disease in American dogs.

Typical Treatment

  • Immunotherapeutic medications
  • Anti-inflammatory medications
  • Antibiotics and/or antifungals for secondary infections
  • Antihistamines
  • Appropriate flea treatment used year-round

Always consult a veterinarian before choosing external parasite control. Many available products are toxic to pets, especially cats.


Stephanie Lantry, DVM. - Writing for PetMD
Jennifer Kvamme, DVM - Writing for PetMD
Ryan Llera, BSc, DVM; Tammy Hunter, DVM; Ernest Ward, DVM - Writing for VCA Animal Hospitals
Anna Burke - Writing for The American Kennel Club

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