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Is Your Pet Allergic to Fleas?

Flea allergy dermatitis is the most common allergy in dogs and cats and is caused by flea bites. It is a very itchy disease and predisposes to the development of secondary skin infections. Adult fleas are blood-sucking parasites that require a host, such as a dog or cat, for protection, food and survival. When a flea bites an animal, a variety of irritating and/or allergy-causing substances from its saliva are injected into the skin. Pets are not allergic to the fleas themselves. They are allergic to the protein in the flea saliva when the fleas bite the pet.

In an allergic pet, the bite of only one flea may cause a serious problem and the pet will itch for over five days. Extensive skin damage may result from the pet's biting, licking and scratching. Though flea bite allergy can be treated, it often recurs since:

a) Flea eggs may hatch up to 1 year after being deposited in the environment.

b) Most of the flea's life cycle occurs off the pet.

c) Your pet's environment may be contaminated with fleas.

d) Even an occasional flea bite can trigger the allergic reaction.

Pets with flea allergy have very few fleas because they are so itchy, they groom themselves excessively, eliminating any evidence of fleas. However, a couple of flea bites every two weeks are sufficient to make a flea allergic pet itchy all the time.

Fleas are bloodsucking insects with a life span of 6 to 12 months. This life span is influenced by environmental conditions and can vary from two to three weeks up to a year. The adult flea spends most of its life on the host, while the immature stages (eggs) are found in the environment. The worst time of the year for fleas is from spring to late fall. Warmer climates may experience a more prolonged flea season. Animals residing in southern states may have fleas and the resulting allergic reaction year round.

The abdomen and rump are the most commonly affected areas in the dog. These itchy areas may develop oozing scabs. Cats, on the other hand, typically overgroom; they may have hair loss but not develop skin lesions.

Flea Life Cycle Flea Life Cycle


  • Severe itching
  • Chewing and biting of the tail, rump, back legs and occasionally front legs
  • Oozing lesions (lick granuloma) from chewing
  • Hot spots on the hips or face, which is severe skin damage from scratching
  • Excessive hair loss
  • Small red papules with reddish-brown crusts


Flea allergy dermatitis is a common cause of itchiness and scratching in pets, but other medical problems can lead to similar symptoms. Other disorders that must be excluded are:

  • Food allergy
  • Atopy
  • Trauma or other cause of local skin irritation
  • Sarcoptic mange
  • Cheyletiellosis (a mite infestation)
  • Primary keratinization defects

Diagnosis of a flea allergy is made based on pet's history, examination of the skin, presence of fleas, and response to treatment. Fleas may be seen crawling on the dog or cat. Fleas in hiding may be hard to spot, so veterinarians examine affected areas looking for flea excrement, also known as flea dirt, which looks like small, black to rust colored, comma-shaped dandruff. Unfortunately the absence of fleas or flea dirt does not rule out flea allergies. To confirm that the particles found on a pet are flea dirt, place the substance on a wet paper towel. Flea dirt will turn a red or reddish-brown color signifying digested blood.

Some pets may have more than one medical problem. For example, scratching or biting due to flea irritation can cause a “hot spot” (acute moist dermatitis) and secondary bacterial skin infection (pyoderma) can follow.

Preventative Care

  • Prevention of flea bites. The most important part of treatment is preventing flea bites with aggressive flea control on your pet and in the environment.
  • Treatment of secondary skin infections. Antibiotics and antifungal drugs may be necessary to treat secondary skin infections triggered by the flea allergy.
  • Breaking the itch cycle. If your pet is intensely itchy, a short course of steroids may be necessary to break the itch cycle and make your pet more comfortable.
  • Frequent Bathing.
  • All bedding and rugs must be cleaned to make sure that there are no flea larvae. Frequent vacuuming and carpet cleaning strategies can help remove eggs and larvae from the pet’s indoor environment. Professional cleaning or exterminating service in difficult cases can be very helpful.

Use an effective safe flea control product on your pet on a regular basis. Flea Preventive are the simplest way to treat animals. If your pet has flea allergies, a topical adult Flea Preventive is better than an oral product. The goal is to kill adults before they bite so the animal is not exposed to saliva.

See your veterinarian promptly if your pet develops acute skin lesions (acute moist dermatitis) as a result of biting or scratching at fleas. Frequent grooming of your pet with a “flea comb” may be helpful to remove fleas.


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