Flea Allergy Dermatitis (FAD) in Dogs


A flea allergy, or flea allergy dermatitis (FAD), is an inflammatory response to flea bites, caused by the saliva of an adult flea as it feeds on its host. Not all dogs are allergic to flea saliva, so a non-allergic dog may carry thousands of adult fleas and show no symptoms whatsoever. Conversely, a flea-allergic dog may break out with a severely itchy rash while no fleas are visible. Having a flea infestation is entirely different from being flea allergic. Since 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. When a flea bites an allergic dog, the dog responds by biting, licking, rubbing, and scratching. A rash characterized by hair loss, redness, and scabby sores results. The rash can occur anywhere on the dog’s body, but characteristically is worse on the dog’s lower back and rump, near the base of the tail. Once the skin is broken, secondary bacterial and/or fungal skin infections can develop. Other unrelated conditions can cause similar symptoms, so diagnostic tests like a skin scrape and fungal culture are utilized to narrow down the diagnosis. Flea allergy dermatitis is not an emergency, but the 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 the quickest path to relief. 

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 living there 

  • Administer medications to reduce the itch and inflammation resulting from the allergy

  • Administer medications to treat any secondary infections Ideally, a dog with flea allergy dermatitis will remain symptom-free when kept on a veterinarian-approved flea control plan throughout the year

Risk Factors

A flea allergy is not life threatening. The profound intensity of the itch escalates the need for veterinary care in a timely manner. 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.

Possible Causes

As with allergies in humans, why particular dogs develop allergies is unknown.

Main Symptoms

Dogs with flea allergies may 

  • Be severely itchy (severe pruritus)

  • Jump up suddenly from resting 

  • Bite, lick, rub and scratch their backs and tails

  • Be restless and not sleep well, waking up to scratch

  • Develop rashes characterized by red, thickened skin with scabby sores

Detailed Characterization

Flea allergy dermatitis symptoms vary depending on the

  • Severity of the allergy - as with humans, repeat 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

Testing and Diagnosis

A vet will check the dog for evidence of a flea infestation by combing through the dog’s fur to look for 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 a chance to create a large population. 

One common approach for mild symptoms is a therapeutic trial of flea medication. In many cases, itchy dogs show improvement by simply taking a monthly flea control pill.  The basic diagnostic workup for an itchy dog includes three diagnostic tests: 

  • A skin scrape, where cells and debris from the surface of the skin are collected and examined under a microscope. This is primarily performed to look for mites (which also can be very itchy), but negative results do not rule mites out. 

  • A fungal culture, which involves collecting a small sample of superficial skin cells and hair in an effort to identify the pathologic fungi that cause ringworm (dermatophytosis). As with mites, false negatives can occur. 

  • Skin cytology, where cells and debris from the surface of the skin are stained and examined under the microscope for signs of secondary bacterial and fungal skin infections. In some cases, a therapeutic trial of allergy medication may be recommended. It’s worth noting that many types of symptomatic allergy treatment suppress the immune system and this can make conditions that look similar to FAD (like mites or skin infections) worse.

Steps to Recovery

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 FAD Common in Dogs?

Flea bites are the leading cause of allergic reactions in dogs. Flea allergy dermatitis is the most common skin disease in American dogs. While a dog may develop an allergy at any point in its life, most allergies appear in dogs between ages two and five years old. Dogs with atopic dermatitis are likely to also be allergic to flea saliva.

Typical Treatment

Flea bite allergies cannot be cured, but injections and oral medications are available for temporary relief of allergy symptoms. When secondary bacterial and/or fungal skin infections have developed, treatment with antibiotics and/or antifungal agents will also be necessary before the dog will experience relief. The best way to reduce or eliminate the symptoms of FAD is to keep the dog on a veterinarian-approved flea product year-round in an effort to minimize the number of flea bites a dog receives in the first place.

Treatment for flea allergy dermatitis is targeted at:

  • Eliminating fleas

  • Relieving the allergic itch

  • Treating any secondary infection

A variety of options for flea treatment exist and veterinary guidance is critical for successfully navigating the available choices:

  • Oral flea medications

  • Topical flea products

  • Environmental treatment

Note: Many of these treatments can be toxic if used incorrectly, especially to other animals (like cats). Only use treatments recommended by a vet. While undergoing treatment to kill the fleas, shampoos and medications might help reduce the symptoms. Frequent bathing may also decrease the effectiveness of some types of flea control, so check with your vet before bathing.

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