Contributors: David Godfrey, Rosanna Marsella

 Species: Feline   |   Classification: Diseases

Introduction Pathogenesis Diagnosis Treatment Outcomes Further Reading


  • Although the hallmark of allergy is pruritus, this is not always readily apparent in cats.
  • Signs: very variable   →   excoriation, miliary dermatitis, alopecia and eosinophilic granuloma complex disorders are all common.
  • Allergens may be derived from ectoparasites, airborne particles, drugs and chemical, ingested substances and possibly some other sources.
  • Diagnosis: often by trial treatment although ancillary diagnostic tests area important.
  • Treatment: pharmaceuticals: ectoparasiticides and anti-inflammatories, with avoidance of the allergen rarely being possible.
  • Prognosis: guarded for cure - most cats require on-going therapy.

Presenting Signs

  • Skin allergies often present with identical skin lesions without regard to the nature of the underlying allergen.
  • The following is a list of common signs of flea-allergic dermatitis, atopy and food allergy:
    • Miliary dermatitis.
    • Alopecia.
    • Excoriation.
    • Otitis externa.
    • Eosinophilic granuloma complex.
  • Secondary pyoderma or Malassezia-associated dermatitis may be a feature.
  • Pruritus may be generalized but is often localized to the pre-aural area, ears or ventral abdomen,
  • Indirect evidence of pruritus:
    • Plucked hair in the house.
    • Furballs.
    • Constipation.



  • Flea allergic dermatitis Flea bite hypersensitivity Dermatitis: flea allergic  Flea feces test  Flea feces: microscopic appearance :
    • the allergen is substances found in flea saliva.
  • Food allergy Food hypersensitivity Food allergy testing  Skin: food allergy 01  Skin: food allergy 02  Skin: food allergy 03 :
    • This condition may be a mixture of true allergic disease and the ingestion of excessive amounts of pruritus-inducing substances, eg histamine.
  • Atopy Skin: atopic dermatitis  Atopy: positive intradermal skin test :
    • Probably a Type I hypersensitivity to air-borne allergens either inhaled or entering the body percutaneously.
  • Drug reactions Drug eruption  Drug eruption: due to FeLV vaccination :
    • Part of the pathogenesis in reactions to drugs and vaccines can be hypersensitivities.
  • Contact allergy Skin: allergic contact dermatitis  Allergic contact dermatitis :
    • Is not well documented in cats.
  • Insect bite hypersensitivity:
    • Well documented to occur with mosquito bites.
  • Angiedema:
    • Usually caused by hymenoptera stings, also reported with flea and drug allergens.
  • Urticaria:
    • Probably reported with drugs.
  • Ectoparasites Parasitic skin disease: overview:
    • Although these are often usually classified as etiologically different from allergy, the pathogenesis of parasitic dermatoses often involves allergic disease, eg infestation with Otodectes cynotis Otodectes cynotis.
    • An allergic reaction may occur in those cats which are pruritic with dermatophytosis   Dermatophytosis.
  • Miscellaneous:
    • Allergies to intestinal worms and protozoa, and to anal sac material have been postulated but little has been reported about these purported conditions.


  • Allergic skin disease is the subject of much of the research in veterinary dermatology although relatively little is directed at feline disease.
  • Different pathological processes are involved in different allergic conditions and there is probably always a mixture of processes in any one condition.

Coomb's classifications

Current concepts

  • Some more recent ideas about the pathogenesis of allergies include the concept of the balance among CD4 helper T cells being awry, such that the T helper 2 cells are more active than in a non-allergic individual.
  • Cytokines, especially interleukin-4, induce excessive IgE.


Presenting Problems

  • Pruritus.

Client History

  • In a suspected allergic dermatosis it is vital to take a good history.
  • Important aspects are:
  • Age of onset:
    • Atopy is said to start most often between 6 months of age and 2 years.
  • Seasonality:
    • Fleas or atopy may be seasonal.
  • Drugs:
    • Response to corticosteroids or progestagens is consistent with an allergy.
    • Chronic flea allergies may become steroid resistant and some food allergies are not steroid responsive.
    • Concurrent infections may cause steroid resistance.
    • Drug or contact allergies should be ties in with exposure.
  • Flea treatment:
    • Most cats will have a flea allergy and most of these will have had some flea treatment but it will have been inadequate.
    • Unless all the household animals have been treated with a modern adulticide at a frequency and dose appropriate to their bodyweights for at least 2 months, and all the carpets and furnishings have been treated with an appropriate, long-acting, agent then flea allergy should be the prime diagnosis.
  • In-contact lesions:
    • Both animal and human.
    • Fleas, Cheyletiella and dermatophytosis.

Clinical Signs

Lesions site

  • The cause of the allergy cannot be diagnosed by the lesion site but this does allow prioritization of the differential diagnoses.
  • Food allergies are said to most commonly affect the cranial one-third of the body.
  • Lesions along the dorsal trunk are more likely to be flea allergy than other allergies.
  • Contact allergies will be at the site of contact, eg paws, if it the allergen is something on the ground or at the application site for topical medications, eg ear treatments.

Concurrent disease

  • Conjunctivitis is recorded in cats with atopy and food allergy.
  • Gastroenteritis is recorded in cats with food allergy but is said to be rare.

Presence of fleas

  • Cats with flea allergic dermatoses characterized by alopecia, may often have little of no evidence of fleas found on combing due to excessive grooming.
  • It may be more rewarding to examine other cats or dogs from the same household.

Diagnostic Investigation

  • It is more important to rule out non-allergic diseases in long-standing cases, or where there are strong indications of other conditions on the history and physical examination.
  • Skin scrapings are important for most cases.
  • Dermatophyte cultures, blood, urine and feces tests, and diagnostic imaging are important in only in a small proportion of first opinion cases.


  • Trial treatments: treatment with modern adulticide and environmental agents for 2 months to rule out flea allergy.
  • An exclusion diet for 2 months to diagnose a food allergy.
  • A trial treatment with corticosteroids may provide evidence that an allergic-type disease is occurring.
  • This is subjective to false positives and false negatives and there are possible side-effects for the cat.
  • It is imperative that this is planned well in advance so that reliable information is obtained.
    Do not start a corticosteroid trial, a food trial and flea treatment all at the same time as it is difficult to see which trial has helped.

Do not start a corticosteroid trial if pyoderma might be present. Perform an antibiotic trial first and continue this until there is no further improvement.

  • If the disease worsens with the corticosteroids, review the case from the beginning and rule out pyoderma, dermatophytosis, Malassezia and demodicosis.
    It is more manageable to use oral steroids than injections.
  • A high dose must be used, the equivalent of prednisolone Prednisolone  2 mg/kg per day divided into 2 doses is a minimum dose for initiating treatment.
  • Eosinophilic granuloma complex disease  Eosinophilic granuloma complex may need up to 6 mg/kg per day divided and it is often sensible to start at 4 mg/kg per day divided.
  • Intradermal skin testing Intradermal skin test
  • Intradermal testing is well recognized in cats for aiding the diagnosis of atopy Skin: atopic dermatitis and flea allergy.
  • Cats react less distinctly than dogs, possibly due to stress-related factors and so the tests can be difficult to read.
  • Many normal cats react to intradermal allergens so other diseases should be ruled out before using this test for atopy and flea allergy should be confirmed by a response to treatment.
    Food allergy cannot be diagnosed with this test.


  • Blood tests for allergen identification: most dermatologists are still not convinced about the reliability of blood test to detect excessive IgE.


  • Larger numbers of eosinophils and basophils are consistent with a diagnosis of allergy.
  • Degenerate neutrophils with intracellular bacteria and consistent with pyoderma.


  • Is most useful to help diagnose pyoderma and to rule out non-allergic diseases.
  • Does not reliably distinguish between psychogenic alopecia and allergic overgrooming.
  • Allergies are likely to show a perivascular dermatitis or gross reaction pattern with eosinophilic granuloma complex disease and miliary dermatitis.
  • The source of the allergen cannot be diagnosed using histopathology.


  • Plucking hairs from a hypotrichotic lesion can be useful to identify broken hair ends to demonstrate evidence of self-trauma to the owner.

Differential Diagnosis


Initial Symptomatic Treatment

  • Prevent exposure to the allergen in contact allergy and drug reactions.
  • In some cases of atopy allergen avoidance is possible Skin: atopy - allergen avoidance.
  • See relevant disease component for specific therapy.

Symptomatic treatment

  • At least the equivalent of prednisolone Prednisolone (2 mg/kg divided dose per day).
  • Increased dose required with eosinophilic granuloma complex, including plaques (up to 6 mg/kg divided per day).
  • Continue with the effective dose until lesions completely resolve before starting to reduce the dose or the frequency.
  • This may take 1-4 weeks.
  • Then reduce the dose as far as possible.
  • If the dose cannot be reduced to 1 mg/kg EOD of prednisolone (or equivalent) then further diagnostics and/or treatment options should be explored.
    It is a common mistake to give corticosteroids at too low a dose in cats.
  • Some cats respond to one steroid and not another.
  • The longer acting injections can be useful but an attempt should be made to use them like the tablets; starting with a high dose and then reducing the dose and frequency as far as possible.


  • May be useful in some cases of flea bite allergy but the main indication is atopy.
  • Any one histamine may help a particular cat.
  • Trial treatment is the only way to find an effective antihistamine.
  • A beneficial effect should be seen within a few days: a common approach is to try a different drug each week until the most efficacious has been identified.
  • Antihistamines are more often useful to enable a reduction in the corticosteroid dose , but in some cats they can be effective on their own.
  • Most common recommendation is chlorpheniramine Chlorphenamine  (204 mg/catBID).
  • Published doses that are not based on published trials are:
    • Clemastine (0.67 mg/catBID).
    • Diphenhydramine  Diphenhydramine  (10 mg/catTID-BID).
    • Hydroxyzine  Fluoxetine hydrochloride  (10 mg/catBID).
    • Amitriptyline (5 mg/catBID).
    • Cyproheptadine (2-4 mg/catBID).

Essential fatty acids (EFAs)

  • Some studies suggest that EFAs can reduce pruritus in allergic cats.
  • This may also help in reducing the dose of steroids that controls the pruritus.
  • Generally, a high dose should be used.
  • There is no conclusive evidence that one form of oil is better than another.
  • Treatment should be tried for 4-8 weeks before being discontinued as ineffective.

Topical treatments

  • Occasionally, topical treatments containing steroids and antibiotics can be useful, especially for local lesions on the head and neck, and for otitis externa.
  • Most often, systemic treatment is necessary as well.
  • Shampoos can be helpful for those cats that enjoy being bathed. Antibacterial, anti-Malassezia, antiseborrheic and antipruritic shampoos can all be used as appropriate.
    Progestagens do not need to be used. Virtually all cats can be managed without them and they should not be tried except as an alternative to euthanasia.



  • Generally poor for a cure as most cats will have recurrent skin diseases or require on-going treatments and preventions.
  • Rarely should it be a contributing factor in euthanazing a cat:
    • For most cats, the diagnosis is flea bite allergy and modern flea control regimes used thoroughly should enable the majority of cases to stay free of clinical signs.
    • Food allergies potentially have a good prognosis if a suitable food can be found - but performing an exclusion diet can be difficult.
    • Drug and contact allergies should be avoidable.

Expected Response to Treatment

Reasons for Treatment Failure

Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • O'Dair H A & Foster A P (1995) Focal and generalized alopecia. Vet Clin North Am Small Anim Pract 25 (4), 851-870 PubMed.
  • Wills J & Harvey R (1994) Diagnosis and management of food allergy and intolerance in dogs and cats. Aust Vet J 71 (10), 322-326 PubMed.
  • Scott D W, Miller W H Jr. (1993) Medical management of allergic pruritus in the cat, with emphasis on feline atopy. J S Afr Vet Assoc 64 (2), 103-108 PubMed.
  • Miller W H Jr. (1989) Nutritional considerations in small animal dermatology. Vet Clin North Am Small Anim Pract 19 (3), 497-511 PubMed.
  • Schultz K T (1982) Type I and type IV hypersensitivity in animals. JAVMA 181 (10), 1083-1087 PubMed.
  • Thoday K L (1981) Investigative techniques in small animal clinical dermatology. Br Vet J 137 (2), 133-154 PubMed.
  • Scott D W (1978) Immunologic skin disorders in the dog and cat. Vet Clin North Am (4), 641-664 PubMed.

Other sources of information

  • Bevier D E (1997) Atopy. In: Consultations in Feline Internal Medicine. pp 214-220. 3rd edn. Ed. August J R. W B Saunders. ISBN 0 7216 5814 8.
  • Scott D W, Miller W H and Griffin C E (1995) Hypersensitivity disorders. In: Small Animal Dermatology. pp 497-556. 5th edn. ISBN 0 7216 4850 9.
  • Moriello K A (1994) Allergic skin diseases. In: The Cat - diseases and clinical management. pp 1924-1931. 2nd edn. Ed. Sherding R G. W B Saunders. ISBN 0 443 08879 9.

Other Sources of Information