In cats, there are three eosinophilic skin diseases.
An eosinophilic ulcer is a well-circumscribed erosive to ulcerative lesion most commonly present on the dorsal lips. It is most commonly associated with allergic skin diseases in cats, particularly flea bite hypersensitivity, but can be observed in some cats with environmental and/or adverse food reactions. This lesion can also develop as a result of inflammation (eg, trauma).
Diagnosis is based on the clinical signs, and its presence should trigger a work-up for parasitic and/or environmental allergies. Lesion evaluation should include a hair trichogram for Demodex mites, an impression smear, Wood's examination, and in at-risk cases, dermatophyte culture. Biopsy is indicated if there is no response to appropriate treatment and/or if the lesion develops suddenly and is destructive. When present unilaterally, other oral neoplasia should be considered..
Eosinophilic plaque is an intensely pruritic exudative lesion that can occur anywhere on the body of the cat. It is the result of self-trauma from pruritus. Skin cytology shows eosinophilic and neutrophilic exudation. The most common trigger is flea bite hypersensitivity and/or allergy skin disease due to environmental or adverse food reactions. This is one manifestation of feline pyoderma; treatment is with systemic antibiotics based upon culture and susceptibility. Flea control should be instituted or re-evaluated. Recurrent lesions are the hallmark of underlying allergic skin diseases. Histology shows a diffuse eosinophilic dermatitis with marked epidermal inter- and intracellular edema and vesicles containing eosinophils. Mast cells may also be present in the dermis. Peripheral eosinophilia is common.
Focal eosinophilic granuloma is characterized by firm nodules in the skin. Lesions can occur anywhere on the body. and the most common trigger is insect bite hypersensitivity. Lesions on the ears are common in cats that have mosquito bite hypersensitivity. Chin and lip lesions are common in cats with flea bite hypersensitivity. Pencil-like thickenings on the caudal thighs are rarely seen anymore due to the widespread use of flea preventives; the underlying trigger was flea bite hypersensitivity. Histologically, a granulomatous inflammatory response surrounds collagen fibers. Tissue and peripheral eosinophilia are marked when the lesions are in the mouth but vary when lesions are on the skin. Lesions will resolve when the underlying trigger is treated.
These conditions are not diseases, per se, but rather clinical signs. The presence of these lesions should alert the clinician to look for common causes of allergy in the cat, the most common being flea bite hypersensitivity. Thorough treatment with flea preventive medication should be started and maintained for 2–4 months. Lip ulcers and exudative lesions often improve with concurrent antibiotic therapy based upon culture and susceptibility, and this should be the first treatment approach. Empirical antibiotic therapy should not be used. If lesions do not resolve with aggressive flea control and antibiotic use, then anti-inflammatory dosages of prednisolone or triamcinolone acetate can be used. Resolution may take several weeks. Cats with recurrent lesions may have feline hypersensitivity disorder, and testing for environmental allergens and/or a food trial may be indicated. Rarely, an underlying trigger cannot be identified, and feline cyclosporine liquid suspensions (7 mg/kg, PO, once daily) may be helpful.
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