Approach to the pruritic cat (Proceedings)

Diagnosing and treating the pruritic cat can be difficult due to the variations in clinical presentation and the large number of possible underlying causes. In addition, many cats over- groom and lick in hiding, i.e. they exhibit their symptoms in secret. A detailed history and clinical examination are critical in the proper diagnosis and management of a pruritic cat.

History

Signalment: Breed and age of onset can give clues to underlying disease etiologies. Pruritus with an onset before 6 months of age is most often seen in parasitic diseases (notoedric mange, cheyletiellosis, otodectes, fleas), allergies (especially flea and food allergy) and dermatophytosis. The differentials for pruritus in middle-aged cats would include the above, but also tend to emphasize allergies (food and atopic dermatitis). When older cats with no history of skin disease develop pruritus, conditions such as epitheliotropic lymphoma, pemphigus foliaceus, Bowen's disease and paraneoplastic syndromes should to be considered. Certain breeds are also at risk for specific pruritic skin diseases, such as Persians for dermatophytosis and Siamese for food allergies.

Environment

Cats that go outdoors are more at risk to be exposed to parasites (such as fleas) and infectious agents (such as dermatophytes and viral infections). When the pruritus seems seasonal, then flea allergy and atopy are the top differentials. Environmental changes such as a recent move into a new home, introduction of a new pet or person in the household, or construction/remodeling can produce a psychogenic pruritic syndrome. Siamese and their crosses seem to be slightly more at risk for psychogenic disorders. Knowing if any other animals or people in the household are affected is important, as contagious or zoonotic conditions causing pruritus include dermatophytosis, cheyletiellosis, notoedric mange and otodectes.

Previous Drug and Disease History

When cats have a history of inflammatory bowel disease (IBD) and pruritus, food allergy should be diligently pursued. Cats with concurrent airway disease/asthma and pruritus are more likely to be atopic. There may be a higher suspicion of a viral dermatoses in a pruritic cat with facial lesions and a history of upper airway virus infection. Because drugs and vaccines can trigger erythema multiforme and pemphigus foliaceus, a detailed drug history is important. Responsiveness to glucocorticoids and antibiotics can also provide information as to what the underlying disease is.

Physical Examination

      Lesion distribution

Distribution of lesions, especially at the initial stages of the disease, can be very helpful in narrowing a differential diagnosis list.

In general (although exceptions occur):

Head and neck: food allergy, atopy, flea allergy, notoedric mange, mosquito bite hypersensitivity, Otodectes, viral dermatoses

Dorsum and Lumbar Regions: flea allergy and Cheyletiellosis

Ventral Abdomen: Demodex gatoi, flea allergy, atopy, food allergy, psychogenic dermatoses

Generalized: flea allergy, food allergy, atopy, epitheliotropic lymphoma, drug reactions, paraneoplastic syndromes

      Lesion type

The most common lesions indicative of pruritus include excoriations, erosions, ulcers, papules, crusts, plaques and alopecia.

Excoriations are characterized by being linear and are most prevalent on the head and neck. They are typically linear as they are usually caused by self-trauma with the cat's claws.

Erosions are similar to excoriations but are often wider and the damage does not extend completely through the epidermis. When erosions are due to scratching they tend be linear, whereas those associated with licking tend to be oval or circular. Many of these lesions are often associated with eosinophilic plaques.

Ulcers, in addition to occurring from self-trauma, can occur as focal non-pruritic lesions on the upper lip area, known as indolent, or rodent, ulcers. These lesions are one of the components in the eosinophilic granuloma complex (EGC). EGC lesions, including indolent ulcers, have multiple underlying etiologies, including allergic disease, genetics, and insect bite reaction. Many other diseases can also create ulcers on the body that can have varying degrees of pruritus, including vasculitis, autoimmune diseases, drug reactions, and neoplasia.

Papules are small raised 1-5mm lesions that are often associated with crusts and are the most common lesions seen in miliary dermatitis cases. Like EGC lesions, miliary dermatitis is not a specific disease, but rather a lesion indicating the presence of underlying etiology. These lesions can be seen in flea allergy dermatitis, atopic disease, food allergy, bacterial folliculitis, cheyletiellosis, dermatophytosis, pemphigus foliaceus and drug reactions. The distribution and intensity of lesions can help formulate a differential list. Flea allergy cases are typically the most severe over the lumbosacral, groin and pericervical areas, whereas food allergy often, although not exclusively, focuses on the head. Atopic dermatitis can have highly variable presentations, but often mimics food allergy presentations. Cheyletiellosis tends to favor a dorsal distribution with scale and papules and crusts, although asymptomatic carriers are possible. Dermatophytosis can be localized to a specific site or can also present more generalized. Pemphigus foliaceus, although it can be generalized in the feline, usually targets the pinnae, bridge of the nose, nail beds and peri-mammary areas.

Plaques are characterized by lesions that appear as moderate to well-defined elevations of the skin with erythema. One of the EGC variants, the eosinophilic plaque, results from cellular infiltrates within the dermis, edema and fibrosis. Plaques may represent coalescing papules, wheals or result from areas of trauma due to excessive licking.

Alopecia can present as broken-off, barbered hairs from over-grooming, secondary to excessive pruritic scratching, complete alopecia due to poor hair growth, or epilation from the hair follicles. Alopecia due to pruritus is often associated with a poor dry brittle coat that does not readily epilate. Commonly, other lesions of pruritus are associated with the alopecia, making it easy to recognize as a self traumatized etiology, although, in some cases, broken/barbered hair is the only clue that the cat is pruritic.

Diagnostic Tests

Skin scrapings: one of the easiest and most important tests in dermatology. Parasites that can be identified include Cheyletiella blakei, Otodectes cynotis, Notoedres cati, Demodex cati and gatoi, Trombicula autumnalis, Felicola subrostratus and the cat fur mite Lynxacarus radovsky.

Scotch tape preps: Clear scotch tape preps can be helpful in picking up ectoparasites, especially cheyletiella. The tape is impressed onto the affected surface of the hair and skin and then placed on a glass slide and examined for parasites.

Hair coat collections: Combing the coat and collecting dander, scale and hair with a flea comb and examining on a glass slide with mineral oil is an effective method for identifying many parasites. It has been shown to be the most successful method of cheyletiellosis identification, and can be helpful in flea and flea feces detection.

Trichogram and direct hair exam: Plucking hairs from affected areas of partial alopecia can help to determine if the hairs are being removed by self-trauma by examining the tapered tip of the hair. In cases where self-trauma is causing the alopecia, the tip of the hair will appear fractured and jagged. Direct hair exams can also be a method of identifying dermatophytosis, as fungal hyphae and spores can often be seen when the condenser is turned down-although false negatives are certainly possible and a fungal culture is needed for definitive diagnosis.

Woods light examination: This test is often misinterpreted as many forms of debris, medication and follicular and glandular secretions can cause a false positive reaction resulting in the characteristic apple green glow. The Wood's light fluorescence is positive in only a small percentage of Microsporum canis cases. Positive fluorescing hairs should be plucked for DTM culture or for direct microscopic examination for definitive diagnosis.

Cytology: Cytology is perhaps the single most important test in dermatology. It is a rapid tool to assess the presence of pyoderma or bacterial overgrowth, and can give clues to many underlying etiologies. True pyoderma cases should demonstrate intracellular bacteria, usually within neutrophils and, in the cat, sometimes within eosinophils. Eosinophils are a very common inflammatory cell seen in a variety of feline disorders, but most commonly with ectoparasites, allergies and in some forms of eosinophilic granuloma complex (granulomas and plaques). It is not uncommon to visualize fungal spores or hyphae in dermatophyte cases. It can also be of value in some forms of cutaneous neoplasia, and on rare occasion can identify ectoparasites. Acantholytic cells are suggestive of pemphigus foliaceus (PF), although they can also be seen in some dermatophyte cases and biopsy is needed for definitive diagnosis of PF.

Dermatophyte test media (DTM): The DTM is one of the most accurate ways of identifying a dermatophyte infection. Dermatophytes utilize protein and produce an alkaline by-product that produces a red color change. However, after all the carbohydrates are utilized, saprophytic contaminants can utilize the protein and turn the media red as well over time. For this reason DTM cultures should be inspected daily, and any suspected fungal growth should be lifted with clear plastic tape and stained with lactophenol cotton blue to look for characteristic macroconidia and fungal identification. Speciation of the dermatophytes should always be performed to determine the source of infection to help prevent future re-infection.

Flea control trials: The response to intense flea control can be of diagnostic value to support flea allergy when noticeable clinical improvement occurs without concurrent long-term anti-pruritic therapy to mask symptoms. The anti-chitin product, lufenuron (Program®, Novartis) has some value in households where cats are predominantly indoors, although it works to sterilize female fleas that bite the cat and is not an adulticide. More impressive are the topical formulations of imidacloprid (Advantage®, Bayer), fipronil (Top Spot®, Frontline spray®, Frontline Plus®, Merial) and selamectin (Revolution®, Pfizer). These products are often applied more frequently than manufacturer recommendation in flea allergic cats. The author will routinely apply Advantage, Frontline or Revolution every 2-3 weeks to produce better clinical results. Nitenpyram (Capstar®, Novartis) is an orally administered product that eliminates live fleas in a very short time. It lasts for 24 to 36 hours, and must be re-administered to provide continued elimination of newly acquired fleas. This product is ideal for cats with a high flea exposure or that acquire fleas in situations such as kenneling, trips to the groomer or cat shows. The product also works well in cases of extreme moisture or very frequent bathing. The use of other topical sprays and dips are very limited due to the advent of these newer products. Cats are very sensitive to many pyrethrins and pyrethroids and especially permethrin, with toxic reactions and even death if such products are used improperly at the recommended canine concentrations. Even at the recommended concentrations for cats, there are still cases of toxicity seen; therefore, the author recommends avoiding these products. Environmental flea control is occasionally recommended, depending on the flea burden in the indoor and outdoor environment of the pet. The author will occasionally recommend the use of indoor and outdoor premise spraying in multiple pet households or densely populated pet areas. Insect growth hormone regulators (methoprene or pyriproxyfen) are very effective, as are the borate-based products. Organophosphates like dursaban or diazinon, are rarely recommended as these products need to be used with extreme care, especially in cat households, due to their increased sensitivity to these products. Flea collars containing insect growth hormone regulators such as methoprene and pyriproxyfen can also be effective in cats, although they are rarely used.

Food elimination diets: To evaluate for food allergy in a cat, an elimination food diet should be exclusively fed for an 8-12 week period. Switching from one commercial food to another is not acceptable since many of these contain similar ingredients. The author prefers an eight to twelve week course consisting of home-cooked or limited protein based commercial diets, although hydrolyzed diets can be effective in some food allergic animals. Examples include: Royal Canin® (Innovative Veterinary Diets IVD) duck, rabbit, venison or lamb and green pea, Hills® (Feline Z/D low allergen, a hydrolyzed chicken and chicken liver diet, Feline D/D – canned lamb and rice). When the owner is willing or eager to home cook, they can be directed to www.balanceit.com, where they can purchase recipes and supplements formulated by the UC Davis nutrition department. Because of the unique nutritional needs of felines, balanced home cooked diets are required.

Limitations of these foods include palatability and gastrointestinal problems, particularly diarrhea or anorexia, and the simple fact that cats tend to be more finicky than dogs. At the end of the 8-12 week period, the cat is re-challenged with the original diet and observed for exacerbation of clinical signs.

Allergy testing: Atopic dermatitis in the cat is diagnosed based on history, physical findings and ruling out all other pruritic diseases. Allergy testing can be of some diagnostic value, but does not replace a good history, physical findings and ruling out other differentials. It should be emphasized that allergy testing is NOT used to diagnose atopic dermatitis, but rather is used to determine what specific allergens the patient is allergic to after the diagnosis has been made. An experienced veterinary allergist should perform intradermal skin testing in cats, since feline skin test reactions are very subtle and, to an untrained eye, a positive reaction may go unnoticed. Feline in-vitro allergy tests are also available. The in-vitro tests offer an alternative to skin testing, and both tests can be used to select allergens for allergen specific immunotherapy (ASIT).

Skin biopsies: Histopathology can be very useful in the diagnosis of pruritic skin diseases in the cat. The pattern of inflammation and cell types can be very helpful in limiting differential diagnoses, giving classes of diseases to pursue, and, in many cases, provide a final diagnosis. Many specific infectious, parasitic, autoimmune and neoplastic diseases will be diagnosed with skin biopsies. Multiple samples should always be taken and samples should be sent to a dermatopathologist who has expertise or special interest in skin disease.