Pruritus is defined as an irritating, nonadapting cutaneous sensation that evokes the impulse to scratch.
Pruritus is defined as an irritating, nonadapting cutaneous sensation that evokes the impulse to scratch. A wide variety of stimuli and noxious agents are believed to liberal peripheral chemical mediators that stimulate the itching sensation. These include cytokines, leukotrienes, prostaglandins, histamine, peptides and proteolytic enzymes. Cats have a pruritic threshold mediated by both allergic and nonallergic factors that initiate pruritus. The severity and distribution of the pruritus is modulated by the interaction of the various mediators of pruritus, The more definitive the diagnosis of the dermatoses with specific etiologies, the more success we have in managing the pruritic symptoms.
Feline skin has a limited spectrum of primary eruptive processes resulting in similar clinical findings with many different causes. Following a systematic approach to diagnosis is essential in the work up of the pruritic cat. This includes obtaining a thorough history, completing a comprehensive general physical and dermatologic examination in order to make an initial differential diagnoses list. Once the initial differential diagnoses list is completed, the list should be prioritized as to most likely to least likely. Then appropriate screening diagnostic tests including scrapes, smears and fungal cultures are performed. Trial therapy with antibiotics, hypoallergenic diet, and parasiticides on the initial differential diagnoses list are completed to help "rule in" or "rule out" specific diseases. The initial differential diagnoses list is then modified based on all of the historical, clinical and diagnostic data. This process is continued to eliminate differential diagnoses until a definitive diagnosis or diagnoses is made.
History can identify various factors that help the clinician establish initial differential diagnoses. If a Persian cat is presented with generalized pruritus with broken hairs and variable scaling, dermatophytosis must be considered. Pruritus in the kitten <6 months of age is most likely to be caused by fleas, Cheyletiellosis or dermatophytosis. Scaling with minimal alopecia will support Cheyletiella infestation. Partial or patchy dorsal trunk alopecia with miliary crusts would be suggestive of flea allergy. Partial alopecia with broken hairs, often with minimal inflammation is most suggestive of dermatophytosis. If the onset of pruritus with variable excoriation around the head and neck regions is observed between 1 and 3 years of age, an allergic dermatitis should be considered. If the older cat develops generalized exfoliative (scaly or crusting) dermatoses cutaneous lymphosarcoma or pemphigus foliaceus should be on the initial differential list. If there is a history of upper respiratory signs that preceded pruritic lesions on the face or trunk, viral dermatoses may be the cause. If there is a zoonotic or contagion history dermatophytosis, cheyletiellosis, ear mites or Notoedric mange would be most likely. The drug history is important. If there is no preventative flea control, pruritus associated with a flea allergy is more likely. If pruritus is temporarily responsive to glucocorticoids, allergies (food, atopy, flea) would be likely. A poor response to antiinflammatory doses of glucocorticoids may be observed with dermatophytosis, demodicosis, pyoderma, autoimmune diseases, some psychogenic alopecia cases and some cutaneous neoplasms.
Lesions associated with pruritus can be extremely variable. There are four lesions most commonly associated with pruritus in cats. In many cases multiple lesions will be observed. First is a self-induced symmetrical alopecia characterized by diffuse hair loss and broken, jagged hairs observed on a trichogram. The distribution is often limited to regional areas such as ventrolateral abdomen or medial forelegs. Dermatoses associated with these lesions include flea allergy dermatitis, behavioral disorders and rarely allergic dermatitis. Miliary lesions (small crusted papules) are a second type of lesion seen in pruritic cats. The most frequent diagnoses associated with miliary dermatitis are allergies (flea, atopy, food), parasitic (fleas) and dermatophytosis.
Eosinophilic infiltrated lesions are commonly associated with a history of pruritus. These frequently present as plaques, granulomas and/or ulcers. The surface may be covered with crusts, moist or eroded. The lesions tend to be well-demarcated and either solitary or multiple. These eosinophilic lesions are usually included a diagnosis of eosinophilic granuloma complex. Allergies are the most common etiology. The fourth type of gross presentation is an exfoliative dermatitis. Cats may present with either crusts or scales. If crusts predominate, autoimmune diseases (pemphigus foliaceus), bacterial folliculitis although uncommon, and parasites associated with severe pruritus (Notoedric mange) may be considered. If scaling is the predominant presentation, dermatophytosis or demodicosis are initial differential diagnoses.
The pruritic cat may present with focal or multifocal erosive or ulcerative lesions associated with cutaneous neoplasia (mast cell tumor) or deep or intermediate fungal infection (Sporotrichosis) or blistering immune mediated diseases. Pruritic nodular lesions may be deep abscesses, tumors, granulomas or infection.
The distribution of lesions in the early stages of development can be helpful in selecting initial differential diagnoses. If there is exclusive involvement of the head and neck region, differential diagnoses may include atopy, food allergy, mosquito bite hypersensitivity, ear mites, Notoedric mange and demodicosis. Lesions limited to the dorsal trunk are most often associated with a flea allergy or dermatophytosis. Cheyletiellosis and dermatophytosis must be considered if the lesions are scales on the dorsal trunk. If the lesions associated with pruritus are generalized consider cutaneous lymphosarcoma, immune mediated disease or dermatophytosis. Alopecia, whether focal, multifocal, regionalized or generalized is a common finding. Since alopecia in the pruritic cat is assumed to be the result of self-trauma, it is not very useful in formulating the initial differential diagnoses list.
Screening diagnostic tests that should be considered early in the diagnostic workup include skin scrapings, cutaneous cytology, and trichograms. Skin scrapings are indicated for any pruritic cat with a history consistent with parasitic disease, scales, crusts or alopecia. Tape preparations are a simple diagnostic test to look for Cheyletiella and yeast. Surface scrapes stained with new methylene blue or similar stain help to identify Cheyletiella, fungi, types of inflammatory or neoplastic cells, and bacteria. A Woods lamp examination will help identify up to 50% of M. canis infections. Complete blood counts and chemistry profiles may be indicated.
Therapeutic trials with glucocorticoids, cyclosporine, psychogenic drugs, parasiticides (flea control products, ivermectin, lime-sulfur), hypoallergenic diet and antibiotics should be considered if a definitive diagnosis has not been determined after the initial in-office screening tests.
Most definitive diagnostic tests will take several days to obtain results. These include fungal cultures (7 to 21 days), skin biopsies (4-7 days), serum allergy tests (7-14 days) and thyroid tests (1-3 days).
The specific diagnostic tests to be pursued initially will be based on the differential diagnosis list. Although a parasitic disease may be low on the differential list, scrapings are a rapid, inexpensive way to definitively rule out some diseases. If there are alopecic lesions, a trichogram will help determine if the hair is broken (self trauma or dermatophytosis) or if it is being removed intact. If the most likely diagnoses are conditions that require a microscopic evaluation (autoimmune, neoplasia), a biopsy may be performed early in the diagnostic workup. If an immune-mediated disease is high on the differential list and demodicosis and dermatophytosis are either ruled out or very unlikely, an immunosuppressive dose of glucocorticoids may be administered while a biopsy is pending. Similarly if food allergy and atopy are high on the differential list, the client may wish to pursue both simultaneously. A blood sample may be taken for serum allergens, a diet trial started to rule out a food allergy, parasitic control initiated and a limited glucocorticoid or cyclosporine trial to evaluate effectiveness in stopping the self-trauma. If the glucocorticoids or cyclosporine can be discontinued without a relapse of the pruritus while on the hypoallergenic diet, either a food allergy, flea dermatitis or seasonal air-borne allergy would still be considered. The serum allergen test will help identify atopy. Provocative exposure to components of the regular diet will be needed to confirm or rule out a food allergy.
If a systematic approach is pursued for all pruritic cats by obtaining a complete history, performing a thorough general and dermatologic exam and recognize specific gross lesions and distribution patterns, an differential diagnosis list will be relatively easy to make. The screening and definitive diagnostic tests should be carefully selected to yield the most information in a cost-effective and timely manner to help narrow the selections on the differential diagnoses list.