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Flea Allergy Dermatitis or Flea Bite Hypersensitivity in Cats
Veterinary & Aquatic Services Department, Drs. Foster & Smith
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Studies have shown that there are over 15 different antigens in the saliva of the flea. Each one of these is capable of causing an allergic response in a sensitive cat or dog. Despite recent advances in flea control, flea bite allergies and flea bite dermatitis still continue to be common problems.

Preventing flea bites is critical for pets with flea allergies.

Cats and dogs rarely become desensitized to flea bites once they develop an allergy. Cats and dogs that are not allergic to flea bites rarely develop lesions from the bites, but may bite or scratch at the flea when it bites them. There does not appear to be a breed or sex predilection for this allergy.


flea dirt which has dissolved into a red color on a wet white paper

Flea bite allergy is characterized by being a seasonal allergy that is worse during peak flea times in the summer and fall. Even in temperate areas or in cases with home infestations, the symptoms of flea bite allergies appear to worsen in the summer and fall. Cats that have flea allergies will bite at the base of their tail and scratch frequently. Even a few fleas can cause hours and days of intense itching. Many cats have a characteristic loss or thinning of hair above the base of the tail. In addition, fleas or flea dirt (feces) can be found on the cat the majority of the time. The feces, or flea dirt will dissolve into a red color when moistened; this is because it is primarily digested blood. However, if the cat is bathed or treated regularly, very little evidence of fleas may be found. Severely affected cats may itch over their entire bodies, have generalized hair loss, and red inflamed skin.

Flea bite allergy is characterized by being a seasonal allergy that is worse during peak flea times in the summer and fall.

Cats with flea allergy dermatitis can have a wide spectrum of symptoms including miliary dermatitis, which is characterized by crusty papules (small, red, raised skin lesions), symmetrical hair loss (alopecia), and eosinophilic plaques and linear granulomas.


Diagnosis can be made by visual signs in combination with the presence of fleas or through intradermal skin testing. Intradermal skin testing is a very effective diagnostic tool for this particular allergy, although some false negative results can occur. Since many affected cats also suffer from other allergies, many times the flea bite antigen is incorporated into a broader intradermal skin testing program. Some cats with flea allergy dermatitis do not have a positive intradermal test.


Treatment primarily involves preventing the fleas from coming into contact with the cat.

Flea Treatment of Cats: A number of topical and oral preparations are available to use as flea control. It is best use an adulticide, which kills the adult fleas, plus an insect growth regulator (IGR) as well. IGRs help to kill immature forms of the flea, preventing them from developing into biting adults. Talk to your veterinarian about what flea product(s) will work for you. In choosing the product(s) your veterinarian will consider the severity of the flea allergy dermatitis, the severity of the flea infestation, whether your cat goes outside, whether there are multiple cats in the household, how easy it is for you to treat your cat, etc.

Treatment of Environment: Environmental treatment for fleas involves treating the house, outside areas the cat may frequent, and especially sleeping areas of the cat with a product that kills the adults (adulticide) and with an insect growth regulator as well. Another alternative is to use sodium polyborate powder (eg., Fleabusters). Vacuuming, and proper disposal of the cleaner bag are also very beneficial. Other pets in the home should also be treated as they could continue to bring fleas into the environment. While injudicious use of pesticides and growth regulators is never recommended, an effective flea prevention program is much safer and easier than dealing with a full blown flea infestation.

Hyposensitization: Hyposensitization of cats with a series of injections does not appear to be very effective.

References and Further Reading

Jacobs DE, Hutchinson MJ, Ryan WG. Control of flea populations in a simulated home environment model using lufeneron, imiadacloprid, or fipronil. Medical Veterinary Entymology 2001;25:73-77.

Medleau L, Hnilica KA, Lower K, et al. Effect of topical application of fipronil in cats with flea allergic dermatitis. Journal of the American Veterinary Medical Association 2002;221:254–257.

Scott DW, Miller WH, Griffin CE. Muller and Kirk's Small Animal Dermatology 6th edition. WB Saunders Co, Philadelphia PA, 2001;490-500.

Sousa CA. Fleas, flea allergy, and flea control. In: Ettinger SJ, Feldman EC. Textbook of Veterinary Internal Medicine, Elsevier, St. Louis MO 2005:63-65.

Cat Allergies: Symptoms and Treatments for Cat Allergy Relief
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