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August 2023
Skin Diseases in Cats
Although it has been said in many different contexts that “cats are not small dogs,” this is particularly true with regards to the manifestation of skin disease in cats.
Why They Are (Once Again) Not Small Dogs
Although it has been said in many different contexts that “cats are not small dogs,” this is particularly true regarding the manifestation of skin disease in cats. The feline patient presents a unique clinical entity in terms of dermatologic abnormalities. In general, their clinical lesions are often more striking, more severe, and more marked regarding level of improvement compared to their canine counterparts. That said, the diagnostic work-up for a pruritic cat can be a bit more confusing than a pruritic dog due to the fact that they will often present with one or more cutaneous reaction patterns associated with dermatologic disease. Additionally, treatment options for whichever condition is identified needs to be weighed and considered differently compared to if the same disease was seen in a dog.
Trying to figure out what is making a cat itchy starts with identifying which reaction patterns are present. Cutaneous reaction patterns are commonly recognized lesions in the feline patient that reflect the skin’s response to various inflammatory stimuli. The reaction patterns themselves are just that: patterns. They do not represent a definitive diagnosis. Although feline cutaneous reaction patterns are often indicative of underlying allergic skin disease, other differentials need to be considered. This should include parasitic and infectious etiologies based on history, other clinical signs present, and core dermatologic diagnostics; other differentials including genetic and neoplastic conditions may also necessitate consideration in various circumstances.
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This is very different than what is seen in dogs. With certain pruritic dermatoses, very distinct patterns of lesion distribution emerge to guide practitioners on what should be considered first. A good example is flea allergy dermatitis—caudal hind end pruritus is the classic presentation. In cats, however, with any of the reaction patterns, all potential differential diagnoses may be fair game depending on history and other clinical signs.
Identifying Common Cutaneous Reaction Patterns
The common cutaneous reaction patterns in the feline patient include head, neck, and pinnal pruritus (cervicofacial pruritic dermatitis); self-induced alopecia; miliary dermatitis; and eosinophilic lesions (granuloma, plaque, indolent/rodent ulcer).
With head, neck, and pinnal pruritus, lesions are by and large restricted to the front portion of the cat. From the neck back, the cat will generally appear normal. However, the face, ears, and neck may be marked with excoriation, crusts, alopecia, and erythema. In some cases, pruritus can be so severe that obvious self-trauma is apparent. Many cats with cutaneous adverse food reactions of food allergies will manifest with head, neck, and pinnal pruritus; however, as mentioned, other allergic and nonallergic causes should be considered prior to initiating a strict hypoallergenic diet trial.
Many cats with cutaneous adverse food reactions of food allergies will manifest with head, neck, and pinnal pruritus; however, other causes should be considered prior to initiating a strict hypoallergenic diet trial.
Also termed “symmetrical alopecia,” “barbering,” or “fur mowing,” cats with self-induced alopecia will overgroom to the point of partial to near-complete alopecia of the affected body region. On close inspection, hairs will often appear broken (barbered) where the cat has chewed them off. The skin in the alopecic areas may or may not be erythematous and may or may not have concurrent excoriations. Excessive hairball production may also be reported as cats will often ingest a large amount of hair that they remove; in fact, the owners may initially present them for a vomiting work-up. This is the reaction pattern historically overdiagnosed as feline psychogenic alopecia. When these historically reported cats were evaluated more closely though, the majority of them responded favorably to antipruritic agents as opposed to behavior modifying medications. Based on the location of the self-induced alopecia, other nondermatologic conditions should be considered. For example, when self-induced alopecia is centered on the ventral abdomen, this may be indicative of bladder discomfort, abdominal pain, and even radiating spinal or back pain. A complete, thorough physical examination is recommended particularly for these patients.
Termed after “millet seeds” (small grains), miliary dermatitis lesions in the cat will often best be felt as opposed to visualized. When they are present, the lesions are small, pinpoint erythematous, crusted papules. The sparsely haired region of preauricular skin can be the best location to visualize miliary dermatitis in the feline patient without having to clip the hair coat. On palpation, the lesions will feel like small grits or grains under the skin, as if petting coarse sandpaper. This reaction pattern is present commonly in flea allergic cats (especially when distributed along the dorsal topline), but again, other etiologies should be considered. Mastocytosis and Bowenoid in situ carcinoma should be considered differentials particularly if lesions do not respond to appropriate antibiotherapy, antiparasitics, or antipruritic interventions.
Eosinophilic granulomas, plaques, and indolent ulcers fall under the subheading of eosinophilic skin lesions in cats, previously termed “eosinophilic granuloma complex (EGC).” Granulomas will often present on the caudal thigh with or without concurrent alopecia and will feel “like a pencil shoved under the skin.” These lesions may or may not be associated with pruritus. Granulomas may also be noted on the ventral chin; these may be referred to as “fat chin” or “pouty cat” lesions. Oral granuloma lesions may also arise; cats may initially present with clinical signs of dysphagia, drooling, decreased appetite, or even dyspnea depending on the size of the lesion present. Eosinophilic plaques, on the other hand, are typically severely pruritic and will commonly be seen concurrently with self-induced alopecia.
Allergic cat with large eosinophilic granuloma on the caudal aspect of the tongue. The feline also has a large bilateral indolent ulcer on the upper lips.
These erythematous plaque-like lesions have a glistening and often moist surface. They will often be noted on the ventral abdomen; occasionally, multiple lesions will coalesce to form a large eosinophilic plaque. Indolent or rodent ulcers are often present in the absence of any clinical signs; rather, this may be an incidental finding on physical examination. Lesions may be unilaterally or bilaterally present on the lips; extension up the philtrum to the nasal planum is not uncommon. It is generally accepted by most dermatologists that these eosinophilic lesions (granulomas, plaques, indolent ulcers) in cats are often a manifestation of feline bacterial pyoderma (as opposed to the papules, pustules, epidermal collarettes, and crusting seen in canine bacterial pyoderma). These lesions will often respond to antibiotic therapy alone; “Convenia-responsive skin disease” in the feline patient is more common than what has historically been reported.
Diagnostics and Therapeutics
Once the cutaneous reaction pattern has been documented, diagnostic work-up should proceed as for most pruritic patients, be it canine or feline (or other species). In general, infectious and parasitic causes should be ruled in or out prior to investigating underlying allergy or other causes. Core dermatologic diagnostics for the feline patient include flea combing, skin scrapings (superficial, deep), impression cytology/acetate tape preparations, Woods lamp evaluation, dermatophyte test medium (DTM) fungal culture/dermatophyte polymerase chain reaction (PCR) test, and antimite/antiparasitic treatment trial. It is important to remember that with certain mite species (e.g., Demodex gatoi), a negative skin scraping does not rule out the external parasite contributing to skin disease; treatment trials are often indicated when these parasites are suspected. Additionally, most dermatologic abnormalities in cats should be considered as possible dermatophytosis until proven otherwise, given the sheer commonness of this disease (especially in Persians and other long-haired breeds); this is particularly true for alopecic lesions in cats.
Allergic cat with severe head and neck pruritus. The patient has a large amount of self-induced excoriations. These lesions were found to be secondarily infected with bacteria.
Additional diagnostic work-up or therapeutic trials should be initiated once parasitic and infectious etiologies have been definitively ruled out. Flea allergy dermatitis is the most common allergic skin disease in cats worldwide. Any of the cutaneous reaction patterns can be manifestations of flea allergy dermatitis. This possibility should be critically evaluated and appropriate therapy instituted. This is true even for indoor-only cats who do not go outside—this can be a difficult conversation to have with some clients. Discussion often of “insect bite allergy” and comparison to how various individuals may respond to a mosquito bite (e.g., minimal reaction versus development of a welt) can help with acceptance of preventative recommendations.
In patients where head, neck, and pinnal pruritus is the only cutaneous reaction pattern present and when other parasitic and infections etiologies have been investigated and flea allergy dermatitis ruled out, then a hypoallergenic diet trial should be pursued. This involves feeding a novel protein/hydrolyzed diet for at least six to eight weeks to evaluate efficacy for controlling pruritic skin disease. It is important that during this time the following should be avoided: other types of cat food, treats, table scraps, and flavored medications. The diet should be chosen based on the animal’s previous dietary exposure; ideally, the cat should be fed something that it has never eaten before. The most common food allergens in cats are fish, chicken, and milk. Home-cooked diets are generally not recommended for feline patients due to their dietary need for taurine. Diet trial can, however, be a rather difficult venture to pursue given the limitations of current veterinary diets; we are lacking in many good options in cats, especially those with a wet food option. The novel protein sources tend to be polarizing as to whether a cat will eat the diet. The discerning nature of the species when it comes to food can make an elimination diet trial incredibly challenging.
If parasites and infections have been ruled out, flea and food allergy has been evaluated, and the cat remains pruritic or lesional, then feline atopic skin syndrome (environmental allergies) should be considered.
If parasites and infections have been ruled out, flea and food allergy has been evaluated, and the cat remains pruritic or lesional, then feline atopic skin syndrome (environmental allergies) should be considered. Remember that allergy tests (e.g., serum immunoglobulin E [IgE] testing, intradermal testing) do not diagnose allergy; rather, this is a clinical diagnosis of exclusion and the tests are used strictly to confirm diagnosis and formulate allergen-specific immunotherapy. Again, any (or all) of the cutaneous reaction patterns can be seen in feline patients with environmental allergies. As with dogs (and humans and other species), this is a condition that can be managed but not cured. Management options in cats are far more limited than what is available in dogs. Bathing and topical therapy tends to be challenging in cats. However, some of the leave-on formulations (e.g., mousse, spot-on) can at least have adjunct benefit for managing allergies.
There is not yet a targeted monoclonal antibody (e.g., Cytopoint) for managing allergic skin disease in the feline patient. Indeed, due to the highly species-specific nature of the molecule, this therapeutic should not be used in cats due to the potential for severe or even fatal reactions. Apoquel is also not a good option in cats with allergies for a number of reasons: there is a higher potential for side effects (especially with long-term administration), cats metabolize the drug more rapidly than dogs (often necessitating twice daily administration), and dosing frequently needs to be higher than what is labeled for dogs. Atopica is a great option for managing allergic skin disease in cats given the ability to decrease frequency of administration over time and the efficacy of therapy in most allergic patients; however, palatability can impact the ability of an owner to consistently administer the drug.
While effective, steroids come with the higher chance of side effects especially in obese patients (e.g., development of diabetes) or underlying cardiac disease (congestive heart failure). This is especially true with injectable formulations. Allergen-specific immunotherapy (either allergy injections or oral allergy drops) can be highly effective and safe in cats with environmental allergies. Recommendation for referral to a veterinary dermatologist when this option is pursued tends to be ideal given the intricacies of formulation and manipulation.
Allergic cat with miliary dermatitis lesions along the ventrum. Lesions were initially much more readily palpated, however when patient was shaved for abdominal ultrasound, the crusted papules were able to be observed.
When antiparasite, antibiotherapy, and antipruritic or antiallergy therapy are not beneficial or when the cutaneous reaction pattern does not appear as “classic” as clinicians are used to seeing, then biopsy of lesions may be beneficial. Indeed, if there is question as to what is seen, biopsy can be helpful to at least rule out more “scary” differential diagnoses. It is important to choose lesions that are representative of the disease state and submit them to a dermatopathologist for evaluation; a thorough physical examination and history will help a pathologist determine what differentials can be more likely ruled in or out. When neoplasia and autoimmune diseases are considered differentials, specifically note this on a submission form; this will alert the pathologist to pay special attention and remark on the presence or absence of these supporting characteristics.
The itchy cat presents not only a diagnostic but also a therapeutic challenge for many practitioners, but a solid approach can make the process less daunting. That said, we need to do better by this species in veterinary dermatology. Research and development for managing the itchy, allergic cats is lacking. It’s time for cats to come into the light with consideration and interventions of their own.
Alison Diesel, DVM, DACVD, is a dermatologist in Austin, Texas. She graduated from Kansas State University College of Veterinary Medicine in 2005, then worked as an emergency clinician for one year prior to beginning a three-year residency in dermatology at the University of Wisconsin-Madison School of Veterinary Medicine. She taught at Texas A&M University College of Veterinary Medicine and Biomedical Sciences for 12 years prior to transitioning into private dermatology practice. Her main clinical interests lie in feline dermatoses, particularly feline allergic skin diseases, and expanding knowledge of the cutaneous microbiome in companion animals. She is heavily invested in improving and promoting wellness in the veterinary profession. |
Photo credits: Vonschonertagen/iStock via Getty Images Plus, Photos courtesy of Alison Diesel, DVM, DACVD, FatCamera/E+ via Getty Images