February 12, 2018

ACVC 2017: Understanding Skin Disease in Cats

Although much remains unknown about feline dermatology, one thing is certain: Many skin conditions are related to internal disease.
By JoAnna Pendergrass, DVM
Facial herpes.
When it comes to skin disease, cats are not just small dogs. This was a key point from a presentation on skin disease in cats by Candace A. Sousa, DVM, DABVP (Emeritus; Canine & Feline Practice), DACVD, at the 2017 Atlantic Coast Veterinary Conference in Atlantic City, New Jersey. “Cats are cats,” she said, “and their disease processes need to be approached from a cat point of view.”

Feline skin diseases may involve a severe underlying illness and require a thorough diagnostic evaluation, including a detailed history and physical examination. Getting a diagnosis generally makes therapy and prognosis “easy,” Dr. Sousa said.

Unlike internal medicine, veterinary dermatology has limited diagnostic tests (eg, skin cytology, trichogram, biopsy and analytic tools (eg, Wood’s lamp, microscope) available; trichograms, in particular, are performed more often in cats than in dogs. Veterinarians should charge for each skin diagnostic test, including simple scrapings, Dr. Sousa said, emphasizing that clients should pay for the expert opinion and knowledge. In addition to obtaining a history and performing a physical exam, organizing possible skin diseases into categories can help in making a diagnosis. Categories include allergic, bacterial, congenital/ hereditary, endocrine, fungal, immune-mediated, miscellaneous, neoplastic, nutritional, parasitic, and viral diseases.

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After making the diagnosis, Dr. Sousa recommended that veterinarians ask owners what they want to do regarding treatment—“What would you like?” or “What are your goals for your cat?”—being careful to avoid a paternalistic tone. She then provided details on congenital, endocrine, neoplastic, parasitic, viral, and miscellaneous skin diseases that are unique to cats.

Congenital Diseases
Idiopathic Facial Dermatitis of Persian Cats
Also known as “dirty face syndrome,” this chronic disease starts when Persian cats are young and causes black crusts on the face, particularly in the folds and periocular area. Affected cats may traumatize themselves. In addition, ceruminous otitis may be present concurrently. Treatment response is typically poor.

Proliferative and Necrotizing Otitis
Proliferative and necrotizing otitis is rare, has an unknown etiology, and typically affects 3- to 6-month-old kittens. Physical examination of the ears reveals large tan or dark coalescing plaques on the concave surface of the pinnae and external ear canals; the ears may also contain comedones. Histopathologic findings include acanthosis, follicular keratosis, and hair follicle outer sheath hyperplasia and keratinocyte necrosis.

This skin condition can be treated with topical 0.1% tacrolimus, which is expensive. Oral prednisolone can also be used, but its effectiveness in treating this disease remains unknown. The prognosis is poor.

Endocrine Diseases
Feline Skin Fragility
Typically a marker of feline hyperadrenocorticism, feline skin fragility generally occurs in older cats. With several potential causes (eg, progestin therapy, excessive administration of corticosteroids), this condition results in a collagen deficiency that makes the skin extremely thin.

Examining an affected cat requires very gentle handling to avoid tearing the skin. Interestingly, Dr. Sousa noted, not much bleeding occurs when the skin tears. Also, unlike in dogs, alopecia is typically not present in affected cats. Biopsies, although not practical because of the skin’s thinness and fragility, often show collagen deficiency and severe epidermal, dermal, and follicular atrophy. Dr. Sousa recommended that adrenal function be tested during the diagnostic evaluation. The prognosis is grave, especially if the underlying cause cannot be identified.

Neoplastic Diseases
Paraneoplastic Alopecia Associated With Visceral Neoplasia
This skin disease has an acute onset and generally affects cats aged 10 years and older. There is complete alopecia of the ven- trum; the skin appears shiny, potentially due to stratum corneum exfoliation. Scaly footpads are a hallmark of this disease.

Clinical signs (weight loss, anorexia, and lethargy) suggest an underlying systemic disease. Abdominal ultrasound may reveal visceral neoplasia, which is usually a pancreatic adenocarcinoma. Skin biopsy findings—severe follicular and adnexal atrophy; alopecia-associated epidermal thickening—can also aid in tumor identification. It is thought that the tumor releases factors, such as cytokines, that cause follicular shrinkage. The prognosis is grave.

Exfoliative Dermatitis and Thymoma Exfoliative Dermatitis
This rare form of dermatitis, secondary to an underlying thymoma, causes scaly and erythematous dermatitis that begins on the head and neck and eventually becomes generalized. The dermatitis is nonpruritic and affects middle-aged and older cats. It is thought that defective thymic lymphocyte selection results in an abnormal immune response to keratinocyte antigens, causing keratinocyte apoptosis.
 
Top left: Bowen disease on the inner right pinna. Bottom left: Skin fragility. Right: Plasma cell pododermatitis.

Histopathology findings include cell-poor interface dermatitis (dermatitis at the dermo-epidermal junction), mild hydropic degeneration of basal cells, and keratinocyte apoptosis. This feline skin disease is treatable with surgical resection of the tumor.

Metastatic Bronchogenic Adenocarcinoma
Older cats with lung tumors may develop this condition, also known as “lung digit.” Amazingly, Dr. Sousa said, affected cats typically show no signs of respiratory disease before skin abnormalities appear. The tumor metastasizes to the capillaries in the distal extremities and toes, causing an acute onset of hemorrhage in the digits. Without confirmation of a lung tumor, this skin disease may be mistaken for inflammatory pododermatitis.

Topical or systemic corticosteroids can be used to relieve edema and general discomfort, but the prognosis is poor.

Parasitic Diseases
Mosquito Bite Hypersensitivity
This seasonal disease is uncommon in cats but worth knowing about. Mosquitoes may bite thinly haired areas like the bridge of the nasal planum, the pinnae, and footpads, creating immediate and late-phase hypersensitivity reactions to the mosquito salivary antigens. Typically, the initial bite will develop into a wheal within 20 minutes; papules appear in 24 hours and become encrusted within about 48 hours.

Cats that are chronically affected by mosquito bite hypersensitivity develop lesions such as scaling, alopecia, and pigment changes. Histopathology, which is needed to confirm this skin disease, demonstrates highly eosinophilic dermatitis. Cats that become hypersensitive to mosquito bites are usually affected repeatedly. However, Dr. Sousa noted, not all cats that get bitten by mosquitoes develop a hypersensitivity reaction.

Management options include confining the cat in a mosquito-free environment and using topical mosquito repellent on areas susceptible to bites. Systemic corticosteroid treatment is indicated for cats that cannot be confined.

Viral Diseases
Feline Immunodeficiency Virus–Related Dermatitis
Cats with either feline immunodeficiency virus (FIV) or feline leukemia virus may get skin disease. FIV-related skin problems include abscesses, skin and ear bacterial infections, and mycotic infections. Some cats with FIV develop nonpruritic, generalized, papulocrustous lesions with concurrent alopecia and scaling, which are most severe on the head and limbs. On histopathology, FIV-related dermatitis demonstrates hydropic interface dermatitis and giant keratinocytes.

To date, treatment options for this skin disease have not been successful.

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