Introduction
Identifying pruritus in cats can be difficult due to their independent nature. Owner assessment can be influenced by the amount of time the owner spends with the cat and how the owner defines itchiness. Overgrooming is often overlooked as being a sign of “itch”. A trichogram can be very informative to help determine if a cat is licking excessively. Hairs may be plucked from regions of hypotrichosis and microscopically evaluated for signs of trauma. Hairs with tapered ends are normal. Broken and blunted follicular tips suggest overgrooming.
Diagnostic testing for a pruritic cat can include skin scraping, flea combing, cytology, dermatophyte culture, Wood’s light examination, and skin biopsies. Finally, if all other pruritic disorders have been ruled out, appropriate work-up for allergic skin disease should be pursued.
It is important to remember multiple causes of pruritus can be present in the same patient and pruritic effect can be additive. For example, a food allergic cat infested with fleas will be more pruritic than a cat with only one of those conditions. To achieve maximum comfort, all possible causes of pruritus must be managed including secondary infection.
Collecting History
History is one of the most valuable diagnostic tools in dermatology. Sometimes it provides greater diagnostic significance than the physical/dermatologic exam because no lesion pattern in cats is pathognomonic for any specific diagnosis. Taking the time to record and evaluate a thorough history helps you make the diagnosis more quickly and accurately. Signalment, seasonality, travel history, pruritus level, and response to previous medications can provide significant diagnostic information. History questionnaires can be a time effective way to collect important information prior to the appointment.
The Dermatologic Examination
Clinicians can gain critical diagnostic information without expensive and/or invasive tests, just by looking at the skin. The type and distribution of lesions provide valuable etiologic information. When pruritus is demonstrated in the exam room, the clinician can gain a clearer understanding of the severity of this patient’s condition. Keeping records on size of lesions such as eosinophilic plaques and areas of self-induced alopecia can help assess the response to therapy on recheck. Do not forget to perform otoscopic and oral examinations on cats (if able). Due to the unique appearance of allergies in come cats (such as rodent ulcers), these can provide important information.
Secondary Infections
Superficial bacterial pyoderma is underdiagnosed in cats. Crusted papular eruptions or miliary dermatitis are the most common presentation of folliculitis in the cat. However, large areas of erythematous and erosive dermatoses can also be associated with large numbers of bacterial organisms. Bacterial infections are generally considered a secondary complication of underlying disease, but management of the bacterial component can be a critical factor in achieving control and remission of the primary disease. It is essential that cutaneous cytology is routinely performed on skin lesions to appropriately diagnose and treat the pyoderma while working up the underlying primary cause.
Case Presentation #1: Bald Belly Cat
Hints in the history: Ectoparasites (such as Ctenocephalides felis) are usually a primary differential to rule out. It is important to ask about parasiticides used on the presenting cat AND all the other animals at home. Do not assume a client is administering parasiticides just because they have purchased them in the past. Travel, seasonality, gastrointestinal signs, and dietary history can provide useful clues to decipher between food allergy, flea allergy, and atopic syndrome.
Paraneoplastic alopecia may be a higher consideration in older cats (especially with a “shiny belly”). Has the cat ever had skin issues prior to this? How is the cat systemically feeling? Psychogenic alopecia is far less common than hypersensitivity dermatitis, but since these conditions may be clinically indistinguishable. It is important to rule out a hypersensitivity disorder prior to diagnosing psychogenic alopecia.
Exam tips: It is important to differentiate between hair falling out and hair being pulled out. As discussed above, a trichogram can help differentiate. Pruritic diseases such as hypersensitivity disorders cause self-induced alopecia (pulled out). Regions such as the caudal abdomen are easily accessible to the cat’s mouth, thus alopecia at these regions may be more commonly self-induced.
Cats with lesions also affecting the rump or tail and with miliary dermatitis are more likely to be flea allergic. Look for evidence of fleas and flea dirt. However, these do not have to be identified for a flea allergy to be the cause of pruritus.
The workup: Diagnosing a hypersensitivity disorder can often occur with history alone. But identifying the exact type of hypersensitivity disorder can be difficult, especially when more than one allergy is present. Keep in mind that atopic dermatitis is typically a diagnosis of exclusion.
Besides cytology to identify secondary infection, a parasiticide trial with a quality product is very important and often the first step in pruritic rule outs. Ingredients such as spinosad (Comfortis®; Elanco), selamectin (Revolution®; Zoetis) and dinotefuran (Vectra® (with pyriproxifen); Ceva) can provide great speed of kill for fleas. However, the new class isoxazolines have been beneficial since they are also effective against several types of lice and mites. This is very helpful during the work up of a pruritic cat. Commercially available isoxazolines labeled for cats in the United States include sarolaner (Revolution Plus® (with selamectin); Zoetis), lotilaner (Credelio™; Elanco) and fluralaner (Bravecto® and Bravecto Plus® (with moxidectin); Merck).
Case Presentation #2: Facial Excoriations/Ulceration
Hints in the history: Several types of feline parasitic diseases including feline scabies (Notoedres cati), ear mites (Otodectes cynotis), and feline demodicosis (Demodex gatoi) can present as pruritic facial dermatitis. If humans or other cats in the home have also experienced pruritus it is important to pursue a parasiticidal trial first.
Cutaneous adverse food reactions (CAFR) often present as head and neck pruritus in the cat. Although the cat is presenting for skin disease, don’t forget to ask about gastrointestinal upset. Cats with CAFR present significantly more often with digestive signs compared with those affected by other types of hypersensitivity dermatitis.
History of any exacerbation of symptoms caused by stress and/or glucocorticoid administration can be helpful in assessing the possibility of viral infections such as feline rhinotracheitis (feline α-herpesvirus-1). Upper respiratory symptoms including sneezing and conjunctivitis may also be present.
Exam tips: No lesion pattern is pathognomonic for any specific diagnosis in the cat. However, one multicenter study out of Europe found that cats with food allergy presented significantly more frequently with lesions affecting the head and neck (Hobi et al 2011). Look for clinical signs such as excoriations and from secondary infection. Otoscopic examination should be performed routinely to evaluate for signs of otitis which is common in food allergic cats.
Feline herpesvirus-1 is most commonly characterized by erosion and ulceration of the face including the dorsal and lateral muzzle and the nasal planum. Non-cutaneous signs of herpes viral infection would include rhinitis, keratoconjunctivitis, anterior uveitis, ulcerative stomatitis, and regional lymphadenopathy.
The workup: Luckily, studies have supported the off-label use of the isoxazoline family of anti-parasitics against most mites and can be used as a rule out for most pruritic parasites.
While food allergic cats may present more commonly with lesions to the head and neck compared with other types of allergic dermatitis, the diagnosis of CAFR cannot be made by dermatologic examination alone. The only reliable diagnosis is a strict, prescription or home-made diet trial lasting at least 8 weeks. Serum or salivary antibody tests are notoriously unreliable and correlate poorly with clinical signs. Food trials should be tailored to the individual patient and household and should take into account dietary history.
Several different diets are available for exclusion of CAFR. Hydrolyzed diets have become the preferred choice for many dermatologists. The purpose of hydrolyzing proteins is to disrupt the protein structure within the diet to remove any existing allergens and allergenic epitopes. Options include Purina® Pro Plan® Veterinary Diets HA with hydrolyzed soy, hydrolyzed chicken liver, and rice starch; Royal Canin Veterinary Diet® Hydrolyzed Protein (HP) with brewers rice, hydrolyzed soy protein, and chicken fat; Royal Canin Veterinary Diet® Ultamino® with corn starch and hydrolyzed poultry-by-products aggregate (chicken feather); Hill’s® Prescription Diet® z/d® with brewers rice and hydrolyzed chicken liver; and Blue Natural Veterinary Diet® HF with hydrolyzed salmon, potato starch, and pea fiber.
There is a variety of novel protein diets available including rabbit, venison, alligator, kangaroo, etc. A thorough history must be obtained to decide what types of proteins and carbohydrates are in the pet’s history. There is also the potential for cross reaction. The closer the taxonomic relationship between meat sources, the higher the risk of cross-reactivity. It is important to use veterinary prescription-based diets during a diagnostic food trial when using a commercial diet to avoid contamination with other ingredients.
If there is improvement during the diet trial, then the cat can be challenged with the previous diet to confirm diagnosis. Symptoms usually return within 1-3 days, but delayed reactions up to 7 days may be noted. If CAFR is confirmed, then more in-depth evaluation can be performed through individual ingredient challenges to identify the particular allergens.
Diagnosis of FHV-1 should be considered if respiratory and conjunctival signs are also present. Virus isolation from oropharyngeal or conjunctival swabs or detection of basophilic intranuclear inclusion bodies on histopathology may confirm the diagnosis.
Case presentation #3: Eosinophilic Granuloma Complex
Hints in the history: Eosinophilic granuloma complex (EGC) is a clinical presentation, not a final diagnosis. Hypersensitivity disorders are the most common underlying cause, so questions relevant to allergic disorders are appropriate in these cases. In addition to standard inquiries relevant to hypersensitivity including signalment, age of onset, seasonality, response to medications and gastrointestinal signs, questions regarding exposure to fleas and mosquitos are particularly relevant as insect bite hypersensitivity is a common underlying cause of EGC lesions. Also, signs of feline asthma (wheezing, sneezing, etc.) can be suggestive of environmental hypersensitivities.
Examination tips: There are three reaction patterns typical of EGC: 1) rodent ulcer 2) eosinophilic plaque and 3) eosinophilic granuloma. Affected cats may exhibit one or more of these reaction patterns simultaneously. Lesions may be variably pruritic and range in severity.
Eosinophilic granulomas most often occur on the caudal thighs, face (especially lips and chin), and in the oral cavity including the tongue and soft palate. Lesions are characterized by papular, nodular oval to linear plaques which are typically firm and vary in color from erythematous to orange-yellow.
It is likely that licking is a common contributing factor to the formation of eosinophilic plaques. So, lesions occur most commonly to parts of the body accessible to the cat’s tongue including the abdomen or medial thigh. Plaques may be single or multiple, well circumscribed, erythematous, eroded, oozing, and ulcerated.
Rodent ulcers most commonly occur on the upper lip near the philtrum or adjacent to the canine teeth. They may be unilateral or bilateral. Early lesions are usually crusted, erythematous, and depressed. Ulcers can enlarge to become well-circumscribed, red-brown in color, alopecic, and glistening. The enlarged lesions have a concave, ulcerated center of granulation tissue that may have areas of necrotic tissues. Raised borders may develop and become very swollen, firm, proliferative ulcerated masses.
Because EGC lesions are most commonly due to hypersensitivity, other evidence of pruritus may be noted on dermatologic exam including self-induced alopecia, miliary dermatitis, erythema, and excoriation.
The workup: Cytology is an inexpensive and minimally invasive means to support a clinical diagnosis of EGC. Cytologic findings from these cases exhibit eosinophils in addition to other white blood cells including neutrophils. Intracellular and extracellular coccoid bacteria are also commonly found as EGC lesions are frequently secondarily infected.
While biopsy confirms the diagnosis of EGC, it will not identify the underlying cause. In cases which appear clinically atypical or fail to respond to appropriate therapy, histopathology should be performed to rule out similar appearing conditions including neoplasia (SCC, MCT, lymphoma, metastatic mammary carcinoma) or infection (bacterial or fungal granuloma).
If flea allergy and CAFR have appropriately been ruled out, then a diagnosis of feline atopic syndrome (FAS) can be made. Anti-inflammatory therapies include corticosteroids, cyclosporine (Atopica®; Elanco), or off-label use of oclacitinib (Apoquel®; Zoetis).
Allergen specific immunotherapy (ASIT) is the safest long-term therapy for atopic dermatitis but can take several months up to a year for full effect. There is no single protocol for ASIT that works for all patients, and each pet’s therapy is individualized. Therapy is continued for at least 12-18 months before maximum benefit can be seen. When effective, therapy is usually life-long.
Case Presentation #4: Paw Lesions
Hints in the history: A key difference between the typical lesion distribution of allergic dogs versus cats is that lesions to the paws are rare in allergic cats. When the paws of a feline patient are affected, diseases such as pemphigus foliaceus (PF) and plasma cell pododermatitis are higher on the differential list depending on the location.
Both diseases can have a waxing and waning course and may respond transiently to injectable corticosteroids. In particular, pemphigus foliaceus is often misdiagnosed as allergic skin disease. Look closely at previous exam notes for the use of long-acting injectable corticosteroids. Owners may think the paw lesions are seasonal and not realize their cat was receiving a steroid injection every couple of months.
Exam tips: PF is the most common autoimmune disease in cats. While primary lesions in the cat are vesicobullous or pustular, these lesions are fragile and transient. Crusting is the most predominately recognized sign. In fact, crusting and purulent discharge around individual or multiple claw folds is a common manifestation of this condition in cats. Crusted lesions can also focus on the face, pinnae, and mammary region.
Clinical signs of plasma cell pododermatitis include soft swelling of multiple paw pads and lameness. The skin of the pad may develop a purplish hue and may even ulcerate. The central metacarpal or metatarsal pads are usually affected. However, all pads may be involved.
The workup: Cytology can be suggestive of PF with the finding of acantholytic keratinocytes, however histopathology is needed for definitive diagnosis in most cases. Acantholytic cells in the cat can be challenging to isolate on cytology or histopathology and evaluation of multiple sectioning samples may need to be requested. Focusing on lesions with heavy crusting can increase the diagnosis yield. Also, refrain from prepping the biopsy site as this can remove crusts that may be needed for diagnosis. Treatment with corticosteroids, cyclosporine, or chlorambucil is the most commonly used therapy for this condition. Prognosis is generally good although some cases can be challenging to achieve remission.
To diagnose plasma cell pododermatitis, fine needle aspirate of predominantly plasma cells and clinical presentation may be enough. However, histopathology may be needed for definitive diagnosis. Treatment options include doxycycline (which should be given with water or in a liquid form with cats), corticosteroids, cyclosporine, or surgical excision.
References
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