Paul B. Bloom, DVM, DACVD, DABVP (Canine and Feline Specialty)
Allergy, Skin and Ear Clinic for Pets, Livonia, MI; Department of Dermatology, Michigan State University, East Lansing, MI, USA
When dealing with the pruritic cat there is a tendency for veterinarians to diagnosis the reaction pattern of the pruritus rather than the true etiology. This is especially true for the (in) famous eosinophilic granuloma complex (EGC). Some people have referred to the EGC as the eosinophilic granuloma confusion.
The first step is reviewing the signalment. Kittens are most likely to have ectoparasites (e.g., otoacariasis, notoedric acariasis) or dermatophytosis as their underlying disease. Young adult cats are more likely to have atopy or cutaneous adverse food reactions (CAFR) than kittens, but remember they too can have ectoparasites or dermatophytosis as their underlying disease. Next step is to obtain a detailed history. If the cat has had previous skin or ear disease, getting a copy of the medical records may help tremendously in developing a differential diagnosis list. Then, question life style, diet, other animals in the household and any medications they are currently using. Also ask, if appropriate, what the response to the previous medications and treatment was. Ask about the course and symptoms of the disease.
The next step is to do a complete dermatologic examination, including an otic and oral exam. Cats have some very unique cutaneous reaction patterns. These reaction patterns are not specific for any etiology. They include eosinophilic granuloma complex, miliary dermatitis, symmetrical alopecia and head and neck pruritus.
Eosinophilic granuloma complex (EGC) consists of the eosinophilic plaque (EP), eosinophilic granuloma (EG) and indolent ulcer (IU). Since EP, EG and IU are most commonly manifestations of a hypersensitivity reaction, differentiating among these, clinically or histologically, is not essential in establishing the underlying cause, nor is it helpful in choosing a treatment plan. Using histopathology to rule in or rule out other similar appearing diseases (e.g., neoplasia, infection) may be necessary since treatment and prognosis for these other diseases would be drastically different. Histopathology should be performed in cases that clinically appear atypical or for those cases that fail to respond to appropriate treatment.
Eosinophilic plaque (EP) may be singular or there may be multiple lesions. Grossly, these are alopecic, erosive to ulcerative erythematous patches or plaques most typically involving the ventral abdomen, perianal and medial thigh regions. Cytology should be performed and if bacteria and inflammatory cells are seen, a 21-day course of amoxicillin trihydrate-clavulanate potassium may be beneficial. As mentioned previously, it is important to identify and treat any underlying hypersensitivity. This will be discussed under diagnosis and treatment.
Miliary dermatitis (MD) consists of pruritic, multifocal and erythematous papulocrusts that are frequently found on the trunk, neck and face. There are group of cats with MD that are reported to be non-pruritic. It is unclear whether these cats are 'closet' itchers or that the papulocrusts are truly a primary lesion of the disease and not a result of self-trauma. Diagnosis and treatment will be discussed later.
Indolent ulcer (IU) appears as an erosive to ulcerative lesion with slightly raised edges occurring most commonly on the midline of the upper lip or adjacent to the upper canine tooth. IU may be a precancerous lesion that may progress to a squamous cell carcinoma. Please note that dermatophytosis needs to be on the rule-out list for a cat with IU to include ringworm. It has been reported that infection with Microsporum canis may be responsible for lip ulcers. Diagnosis and treatment will be discussed later.
Eosinophilic granuloma (EG) (linear granuloma or collagenolytic granuloma) is non-pruritic and appears as a mildly erythematous, alopecic, eroded or ulcerated nodule or plaque. It is an off-white to yellow to pink lesion that may occasionally have white granules found in the middle of the nodule. The nodular form is most commonly found in the oral cavity (tongue, hard palate or glossopharyngeal arches) or on the chin. When EG involves the chin ('chin edema'), affected cats have a 'pouty' appearance due to the swelling of the area. EG may also involve the footpad or the conjunctiva. Lesions frequently appear on the caudal thigh as a linear plaque that is non-pruritic. The linear plaque is frequently detected only by 'accident' when the owner is petting the cat or during a physical examination. Diagnosis and treatment will be discussed later.
Symmetrical alopecia that occurs in cats is, as opposed to in dogs, not due to an endocrinopathy but due to a pruritic skin disease. It is rarely psychogenic. This pattern may be seen on the ventral abdomen, caudal thighs, medial or caudal aspect of the forelegs or along the caudal 1/3 of the dorsal trunk. Close inspection of the hairs (best done by performing a trichogram) reveals broken hairs consistent with a post traumatic lesion. These hairs do not easily epilate as they would with an endocrinopathy. The diagnosis of psychogenic alopecia is a diagnosis by exclusion.
Cats with skin disease will also present with head and neck pruritus. Differential diagnoses include atopic dermatitis, cutaneous adverse food reactions, flea bite dermatitis, Notoedres cati or Otodectes cynotis, dermatophytosis, cutaneous drug reaction (e.g., methimazole) and autoimmune diseases such as pemphigus foliaceus.
Other causes of feline pruritus include Malassezia dermatitis (MaD) and bacterial pyoderma, both of which occur much less frequently in cats than in dogs and will not be discussed further.
As with most dermatologic diseases a diagnosis is made by using a combination of signalment, history (including response to therapy), clinical findings, laboratory testing (see below) and response to therapy.
Since EGC are fairly distinctive lesions, it is tempting to forget that other diseases can appear similar. For IU differentials would include squamous cell carcinoma (SCC) and infectious ulcers (Herpesvirus, Calicivirus, FeLV infection and Cryptococcus). For EP or EG rule-outs would include cutaneous epitheliotropic T-cell lymphoma, infectious granulomas (demodicosis, bacterial (including Mycobacterium), fungal, mast cell tumor or SCC.
The minimum data base for a cat presented with pruritus and/or EGC include:
1. Skin scrapings, clear acetate tape preps and combing the hair coat thoroughly with a fine tooth comb.
2. Wood's lamp examination and dermatophyte culture.
3. Cytology of the lesions. As opposed to dogs, bacterial pyoderma and Malassezia dermatitis are less common but do occur.
4. If the lesions appear atypical, or don't respond to therapy, skin biopsy should be performed.
If bacteria (especially intracellular) and inflammatory cells (neutrophils, eosinophils or macrophages) were seen on cytology it would be appropriate to treat with antibiotics. If focal, topical therapy may be adequate. First line topical antibiotics include mupirocin or clindamycin (available as a pad, solution, foam or lotion). I prefer the pad or solution. Antiseptics such as chlorhexidine or benzoyl peroxide are also effective for focal lesions.
Frequently either because of the widespread nature of the lesions or the temperament of the cat, systemic antibiotics are used. If cocci are seen on cytology, first-line antibiotics would be amoxicillin trihydrate/clavulanate, cephalexin or clindamycin (11–15 mg/kg BID).
If MaD has been diagnosed then treatment a topical azole (miconazole) for a focal lesion is effective. For more generalized disease or for Malassezia paronychia, systemic treatment is necessary. Fluconazole, itraconazole or terbinafine are effective antifungal agents.
An aggressive therapeutic trial for ectoparasites (especially directed toward fleas) should be instituted. Initially, while awaiting response to this insecticidal treatment, a 14–21 day course, using a tapering dose, of oral prednisolone may be used for symptomatic relief if there is no active bacterial or Malassezia infection.
If a positive response is seen continue aggressive flea control. Be aware that if there is a response to therapy, it may be that flea allergy (or some other ectoparasite) is the trigger, or, the cat has seasonal environmental allergen induced disease and the season has changed.
If the diagnostics already performed have not identified the cause of the pruritus and the treatment for ectoparasites is unsuccessful, the next step depends on the severity of the disease and the frequency of the occurrence.
When diagnosing environmental allergen induced atopic dermatitis remember that it is a diagnosis by exclusion. Blood testing or even intradermal testing does not diagnose environmental allergen induced atopic dermatitis as the trigger for the pruritus. These tests are used to select antigens for allergen specific immunotherapy once the diagnosis has been made.
When all the previously mentioned diagnostics and therapies have failed to 'cure' the problem, and the symptoms are nonseasonal, a food trial should be performed. Dietary change (preferably home cooked) is needed to confirm the diagnosis. Blood testing or intradermal testing is worthless! The food trial should up to 12 weeks.
Regardless of which method is used for the elimination diet trial, it should be fed for up to 12 weeks. Failing this, it is appropriate to test for allergen specific immunotherapy. The author prefers intradermal testing over blood tests.
If the owner declines allergen specific immunotherapy (ASIT), if symptomatic treatment is needed while awaiting response to a food trial or ASIT, if the cat's symptoms has failed to respond to ASIT or the symptoms are infrequent, options for symptomatic relief include (please note that these therapies may be used concurrently with ASIT) - systemic glucocorticoids, oral prednisolone, antihistamines & Omega 3 or Omega 3/6 fatty acid combinations, cyclosporine, doxycycline or oral alpha-interferon.