Manifestations of Feline Allergy

Annual Dermatology Continuing Education Conference

Joseph A. Bernstein, DVM, Resident

Dermatology Service



  • Review the clinical reaction patterns associated with allergic dermatologic diseases in the feline
  • Present an organized approach to diagnostic decisions based on an awareness of differential diagnoses.
  • Review therapeutic modalities available to the small animal practitioner


Key Points

  • Combine a recognition of feline reaction patterns with awareness of a prioritized differential diagnosis list.
  • Infectious and parasitic causes must be ruled out before proceeding with an allergy work-up
  • Consistently performing diagnostics rather than relying on empirical therapies allows the small animal practitioner to improve the level of patient care


Feline Reaction Patterns

  • Self- inflicted alopecia
  • Miliary dermatitis
  • Eosinophilic granuloma complex
  • Head and neck pruritus


Self-inflicted alopecia

A common and frustrating presentation of the feline is hair loss as a result of overgrooming.  The client may not report the cat to be licking or grooming excessively.  Additional history taking should include questioning regarding the vomiting of hairballs, hair in the feces, or large tufts of hair in the environment.  When self-inflicted alopecia is suspected, it can be proven in two ways:

  • Trichogram
  • The E-collar test

When it is clear the cat’s alopecia is self-created, the next question is whether the origin of the pathology is pruritus or psychogenic.  In general, the ruling out of pruritic causes should be performed prior to empirical use of behavior modifying psychopharmacologic therapies.  Differential diagnoses include: Flea allergy, ectoparasites (Demodex species, Cheyletiella, Otodectes, Notoedres etc.) dermatophytosis, food allergy, and atopy and idiopathy.  Initial diagnostics include: Woods lamp, KOH prep, DTM culture, combing, skin scrapes (deep and superficial), fecal exam, and flea control rule out.  Second-line diagnostics include lime sulfur dips to rule out Demodex gatoi followed by food allergy and atopy work-ups.  Treatments for this reaction pattern should be based on the underlying etiology.


Miliary dermatitis

Feline military dermatitis is a commonly seen multifactorial reaction pattern consisting of papules surmounted by crusts.  Pruritus is variable and often does not correlate with the severity of the papulocrustous dermatitis.  The lesions are generally multifocal to diffuse and are often noted on the dorsal or ventral torso and the cervical region.  Other reaction patterns may be seen concurrently including self-inflicted alopecia and eosinophilic plaques and ulcers.  The differential diagnoses for the underlying cause include: flea allergic dermatitis (#1), ectoparasites (demodicosis, cheyletiellosis, otodectic acariasis, trombiculiasis etc.), dermatophytosis, bacterial folliculitis and malassezia dermatitis, food allergy, and atopy.  More unusual causes include drug eruptions and feline hypereosinophilic syndrome.  First-line diagnostics include: Woods lamp, KOH prep, DTM culture, skin scrapes, combing, fecal exam and flea control rule out.  Second line diagnostics include lime sulfur dips, and food allergy and atopy work-ups.  Treatment is based on underlying etiology.


Eosinophilic Granuloma Complex

Cats respond to a variety of antigenic stimuli with eosinophilic cell infiltrate.  Though the eosinophil is typically associated with parasitic or environmental antigenic exposure, it can be associated with other causes.  There are four recognized clinical lesion types within the eosinophilic granuloma complex: eosinophilic plaques, indolent ulcers, granulomas, and mosquito-bite hypersensitivity syndrome.  


Eosinophilic plaques

The characteristic lesions consist of alopecic, edematous or eroded, bright pink to salmon colored papules and plaques.  The lesions often occur on the ventral abdomen and medial thigh.  Initially the lesions may appear as mild variably pruritic erythema, but as they progress, they become thickened, elevated, eroded and markedly pruritic.   


Indolent ulcers

Also known as “rodent ulcers” or eosinophilic ulcers, this common, clinically distinctive subtype within the complex is typically located on the mucocutaneous junction of the upper lip, opposite the
canine.  The lesions are non-exudative, well-circumscribed ulcerated plaques that are neither painful or pruritic.  These ulcers can progressively enlarge, becoming disfiguring as they cross the midline.  These lesions
may not be eosinophilic.


Eosinophilic granulomas

These lesions appear most commonly as an eroded or ulcerated plaque or nodule.  They can occur anywhere in the oral pharynx and as a linear form on the caudal thighs over the popliteal fossa (linear granuloma).  They may appear as non-ulcerated swelling on the chin, lower lip, nose or around the footpads (“fat lip”, “fat nose”, “fat chin” syndromes).  The eosinophilic granuloma was formerly termed “collagenolytic” due to the histopathologic appearance of the brightly eosinophilic collagen flame figures, but recent evidence suggests that the collagen itself is not degenerated.  


Mosquito-bite hypersensitivity

A distinct form within the complex, it is Popularly referred to as “ears, nose and toes syndrome.”  The most commonly affected site is the nose, with poorly defined swelling or erosions and crusting; but the poorly haired areas of the pinnae and foot pads may have papules and plaques associated with erosion, crusting and depigmentation as well. Histopathology helps rule out other differentials like squamous cell carcinoma, mast cell tumor or Cryptococcosis.  The prognosis is guarded if the cat cannot be brought inside to avoid the mosquitoes.  An alternative treatment choice is cyclosporine.


Biopsy for histopathology may be necessary as other disorders that can mimic eosinophilic granulomas include: neoplasia (squamous cell carcinoma, mast cell tumor, Bowen’s disease etc.), Herpes virus, and bacterial and fungal granulomas.  The clinical entities of the eosinophilic granuloma complex are most commonly associated with an allergic etiology, however an approach to the differential diagnosis list is similar to other reaction patterns.  Infectious and parasitic etiologies should be ruled out initially and 100%flea control instituted before moving on to food allergy and atopy work-ups.  As with the other reaction patterns, treatment aimed at the underlying etiology is preferred to indefinite symptomatic therapy.


Head and Neck Pruritus 

This extremely pruritic reaction pattern can be very frustrating for the small animal practitioner as the degree of pruritus often results in a significant amount of self-trauma and tends to be less steroid responsive than the other reaction patterns.  Self-excoriation can result in extensive ulceration with secondary infection.  The most likely differentials include: otodectes, notoedres, food allergy, flea allergy, atopy, dermatophytosis, demodicosis (especially D. gatoi), and other ear disease.  Initial diagnostics include: ear examination, skin scrapes, Woods lamp, DTM culture, combing and flea control rule-out.  Because food allergy is higher in the differential diagnosis list in head and neck pruritus than it is in the other reaction patterns, earlier institution of a hypoallergenic dietary trial may be considered.  However, a lime sulfur trial to rule out Demodex gatoi is often preferred prior to allergy work-up.  These cases can be so severe (and clients so desperate) that definitive diagnosis may be difficult due to the need to move quickly with diagnostics and therapy.  Glucocorticoid administration and E-collar may be needed initially in attempting to stop the self-trauma.


Diagnostic approach to the feline reaction patterns

The basic differential list for the majority of feline pruritic dermatoses is the same with some variation in the prioritization of the list from pattern to pattern.  Ultimately as previously discussed, ruling out infectious and parasitic causes before moving to allergy work-up is the best way to proceed. Diagnostics may be loosely ordered based on a first, second and third-line model.


First line- initial diagnostics

  • Woods lamp, trichogram and DTM culture
  • Skin scrapes (deep for D. cati, superficial for D. gatoi and Notoedres)
  • Combing/ Scotch tape preps
  • Fecal float
  • Surface cytology
  • Institution of 100% flea control


Second line (Assuming all first visit diagnostics negative and no response to the flea control)

  • Weekly lime sulfur dips to rule out D. gatoi – 5 week trial


Third line- Allergy work-up begins

  • If non-seasonal- hypoallergenic food trial
  • Atopy work-up: In-vitro testing or intradermal skin testing vs. empirical therapy


Simple Diagnostic Tests

Trichogram (hair examination)

Examination of hairs from an alopecic area can inform in two ways.

  1. In the self-inflicted alopecia case, the barbered hair has a blunted tip, instead of the normal tapered tip.  The bulb and shaft are normal.  
  2. In dermatophytosis, visualization of the hair shaft may reveal distortion, with hyphae and arthrospores seen within the shaft.  Use of 20% KOH (let sit one hour under coverslip) will help break down the keratin and make visualization easier (especially in dark hairs).


Skin scrapes/ Combing/ Scotch tape preps/ Fecal flotation

Skin scrapes may help identify Demodex and Notoedres. Otodectes and Cheyletiella may also be revealed with scrapes, but flea combing, scotch tape preps and concentration with fecal flotation solutions may be useful. Response to trial miticidal therapies may also be useful (i.e Revolution 2x for a month).  Because Demodex gatoi may not necessarily be discovered in superficial scrapes, trial lime sulfur dips can be diagnostic.


Surface cytology

Cytology of surface exudates and/or the contents of papules and pustules may reveal the presence of bacteria or yeast.  Unlike in the canine, pyoderma is uncommon.  However, a significant secondary bacterial component may exist, especially in the excoriated feline patient.  This makes antibiotic trials along with E-collar protection valuable.


Woods Lamp/ DTM

Examination of hairs with a Woods lamp is a useful screening test, but a negative is not a rule-out of ringworm as less than 50% of M. canis fluoresce.  A positive consists of an apple green on the hair shafts. False positives include:

  1. Crusts and scale- these always glow a dull green
  2. Medications
  3. Cotton and carpet fibers- blue-white fluorescent debris floating on the surface of the hair coat. DTM culture is the definitive diagnostic test for dermatophytosis.  


100% flea control

The demonstration of adult fleas and/or flea dirt with combing is very helpful, but the flea allergic cat may have little or no direct evidence of fleas.  This is because hypersensitive patients may require only a few bites to incite the reaction, and the excessive grooming and scratching of the pruritic cat may effectively remove the fleas.  Recent bathing by the clients may also have removed the evidence.  Therefore response to a trial of excellent flea control is commonly used by the veterinary dermatologist to diagnose flea allergy.  Here at LSU, this consists of application of Advantage® (Imidacloprid) once every two weeks in addition to ensuring that all in contact animals are on flea control.  Regular monitoring with a flea comb by the clients at home is recommended


Hypoallergenic food trials

Intradermal skin testing and in-vitro testing have not proven helpful in the diagnosis of food allergy.  A food elimination diet trial is currently the only way to diagnose food allergy.  As in the dog, a diet composed of ingredients the patient has never ingested must be given for a minimum of 8 weeks.  If response is noted, a challenge at the end of the trial period is used to confirm the diagnosis.  Food trials can be more difficult in cats because of their often finicky eating habits. Care must be taken, especially in the overweight feline patient, not to induce the development of hepatic lipidosis.  An important thing to note is the impossibility of performing a food trial in a cat that has access to the outdoors.  


Testing for atopy

Intradermal skin testing for atopy is preferred by many veterinary dermatologists, but is  often financially unfeasible for the general small animal practice.  The use of in-vitro tests for atopy is possible (i.e Heska).  The goal is the formulation of allergen specific immunotherapy.  The information can also be used for avoidance of the significant allergens reported (i.e house dust mite avoidance and environmental control).


Feline Antipruritic Therapeutics Update

Glucocorticoids (GCS)

Although cats tend to tolerate GCS better than dogs and people, adverse effects due occur including: obesity, diabetes mellitus, alopecia, skin fragility, aggravation of heart failure, behavioral alterations and predisposition to infection. Therefore diagnosis and therapy of underlying etiologies is highly preferred to reliance on empirical GCS use in the pruritic feline.

  • Methylprednisolone acetate (Depo-Medrol)- 4-5 mg/kg or 20mg/cat SQ or IM
  • Methylprednisolone (Medrol)- 0.8-1.5 mg/kg q 12-24 hrs PO then wean
  • Prednisone/ prednisolone- 2 mg/kg q 24 hrs PO then wean. Some believe prednisolone to be more effective than prednisone in cats
  • Triamcinolone- 0.3-0.75 mg/kg q 24 hrs PO then taper to q 48-72 hrs
  • Dexamethasone- 0.25 mg/cat q 12-24hrs then 1-2x/week


The use of antihistamines has been reported anecdotally to be more effective in the cat than the dog, but no controlled trials have demonstrated this.  Cats may be at greater risk of paradoxical behavioral reactions to antihistamines (i.e hyperexcitability).  A 7-21 day trial period is recommended .

  • Fexofenadine (Allegra)- 10 mg/cat q 12 hrs PO 
  • Chlorpheniramine maleate (Chlor-Trimeton)- 2-4 mg/cat q 12-24 hrs PO
  • Clemastine fumerate (Tavist)- 0.34-0.68 mg/cat q 12 hrs PO
  • Cetirizine (Zyrtec)- 5 mg/cat q 12-24 hrs PO
  • Cyproheptadine (Periactin)- 2 mg/cat q 12 hrs PO
  • Diphenhydramine (Benadryl)- 0.5 mg/kg q 12 hrs PO
  • Hydroxyzine (Atarax)- 1-2 mg/kg or 5-10 mg/cat q 8-12 hrs PO
  • Loratadine (Claritin)- 5 mg/cat q 24 hrs PO

Fatty Acids

Anecdotally there have been reports of better efficacy of fatty acid therapy in the cat than the dog, but no well-controlled trials have been performed.  The FAs are administered in the hopes of shifting the eicosanoids to less inflammatory mediators.  Some suggest that there is synergistic effect when used with corticosteroids or antihistamines.  Examples of these products include 3V Caps, DermCaps and Eicosaderm.  Dose recommendation is 500mg daily.


Behavior-Modifying Drugs- Psychotropic agents

  • Amitriptyline (Elavil)- 5-10 mg/cat q 12-24 hrs PO
  • Fluoxetine (Prozac)- 1 mg/kg q 24 hrs PO



Cyclosporine (Atopica, Neoral)  binds to specific intracellular receptors in T-lymphocytes and has been investigated for treating  a number of skin diseases in veterinary dermatology.  It appears to be tolerated well by the feline patient at a dose of 25 mg/cat q 24 hrs PO.  Good response has been noted in eosinophilic plaques and granulomas, atopic dermatitis, and idiopathic facial dermatitis of Persian cats (anecdotal).  Ideally the drug should be given on an empty stomach (1hr before or 2 hrs after a meal).  The most common adverse effect is nausea and loss of appetite.