Sonya V. Bettenay, BVSc (Hons), MACVSc, DED, FACVSc, DECVD
In feline dermatology, certain cutaneous reaction patterns are very common. Those patterns most commonly seen in practice are:
Eosinophilic granuloma 'complex'
Head and neck pruritus
These are referred to nowadays as 'reaction patterns', because two cats may present with an identical clinical picture, but it will be identified to have a different underlying cause. In this case, the skin is reacting in an often relatively predictable fashion or pattern to the underlying stimulus. This predictable reaction can even be similar histopathologically!
Miliary dermatitis is characterized clinically by numerous, small, localized or generalized papules and crusted papules. It is the most common feline problem in small animal dermatology.
The list of differential diagnoses for miliary dermatitis is lengthy. Allergies such as fleabite hypersensitivity, atopy or food intolerance, ectoparasites such as Otodectes cynotis, Notoedres cati, or Cheyletiella blakei, infections with dermatophytes or bacteria, neoplastic diseases such as mast cell tumors, immune-mediated diseases such as pemphigus foliaceus and nutritional shortcomings such as essential fatty acid deficiencies may all cause a papulo-crustous dermatitis.
A thorough history will often give clinical clues with information on the presence and/or extent of pruritus (worse with allergies and ectoparasites), pruritus and dermatitis in contact animals or humans (dermatophytosis or ectoparasites), seasonality (atopy, flea bite hypersensitivity), response to previous medications and so on.
My initial approach will depend on the historical information, but typically includes a thorough flea combing as the very first step. More than 50% of miliary dermatitis cases are suspected to be associated with a flea bite hypersensitivity and it must be one of the first conditions to be considered diagnostically and to be 'ruled-out' when the miliary lesions are present on the trunk and especially on the caudal half of the trunk. If the flea combing is done casually, while speaking to the owner, then the combings can be placed on a wet white paper towel and observed for the tell-tale presence of blood. Skin scrapings, cytology (to evaluate for infection, to look for the presence of eosinophils and also to act as a double check for the rare possibility of pemphigus foliaceus) and a Wood's light are the next essential diagnostic tests.
If these tests do not point to a specific diagnosis, then the next step will depend on the owner's wishes, the severity of the symptoms and the clinician's judgment. However, typically trial therapy will be the next step, to rule out ectoparasites and food allergy as the possible causes.
Fungal culture using the McKenzie toothbrush technique can be performed at the first visit, in which case the negative result can likely be shared with the client at the 3 week recheck visit, as this is an uncommon cause of miliary dermatitis; or it can be delayed, to see whether there is a clinical response to any of the trial therapies. The choice will typically depend on the owner, but the advice of the veterinarian must consider the home demographics. In households with small children, pregnant women, aged or immunosuppressed individuals there is a more urgent need for an early diagnosis of dermatophytosis than in households with healthy adult humans. A Persian cat, a young cat, or a cat with an extensive history of corticosteroid use, which presents with miliary dermatitis has an increased risk for dermatophytosis and these individuals should be cultured earlier in their diagnostic workups.
Flea control or rather an 'intensive flea bite treatment trial' can these days be combined with an ectoparasite treatment trial. One of the 'all-purpose' spot-on treatments*, applied 3 times at two weekly intervals, will help treat all of the mites (Cheyletiella, Otodectes, Notoedres) and in many areas of the world will also do an adequate job for flea control. In those areas of the world where there is an intense flea pressure (Los Angeles, Hong Kong, Spain, Sydney) additional measures will probably need to be undertaken. Frontline spray is a useful flea trial agent when applied every two weeks. When I use this for a flea bite trial, it is applied by my staff in the clinic, as it is unreasonable to expect many owners to be able to apply it properly. For those owners who can pill their cats, nitenpyram tablets, daily for 6 weeks with a concurrent ovicidal product is another alternative.
Environmental control is important using a growth regulator or development inhibitor such as methoprene, fenoxycarb or pyriproxyfen at the beginning of the trial. Of course, contact animals need to be treated for all ectoparasite treatment trials to assure maximal efficacy.
*Most newer ectoparasiticidal agents are effective for a number of cutaneous parasites. Imidacloprid q 2 weeks for 3 treatments is effective for cheyletiellosis and flea infestation, fipronil spot-on has been reported effective for the treatment of flea infestation as well as Otodectes cynotis (as a single treatment with one drop applied in each ear and the rest between the shoulder blades). I prefer to give this treatment 3 times 2 weeks apart. It also has been reported to be effective in canine scabies. Selamectin administered q 2 weeks for 3 treatments is effective for Notoedres cati, Otodectes cynotis and Cheyletiella blakei as well as fleas.
An elimination diet is easier said than done for many cat owners, although the fact that the cat has evolved as an obligate carnivore removes the need for extensive meal preparations. Commercial foods with unusual protein sources such as rabbit or duck or hydrolyzed diets with proteins of lower molecular weight of between 6 and 12 kd are convenient, balanced and used commonly.
I recommend preferably, for the first part of the elimination diet, an exclusive 'pure' and non-commercial protein source such as elk, duck or venison meat. A 'home cooked diet' for a cat consists of 100% meat, preferably cooked to reduce the risk of toxoplasmosis. Ultimately the cat's previous diet, the availability of alternative protein sources and the cat itself (!) will determine the ideal elimination diet. Whatever method is chosen, commercial or 'home cooked', it should be fed exclusively for a minimum of 6 weeks. Exclusively means nothing else but the elimination diet. Long-term, this type of a 'home-cooked' diet is not ideal unless it is formulated appropriately. Advice from a veterinary nutritionist or switching to commercial elimination diet is needed if a food hypersensitivity has been established by rechallenge. A dedicated owner may be convinced to perform a sequential rechallenge to identify the offending protein, where one new protein is added to the diet every two weeks. Once the offending protein is identified, it must be avoided in the future, but any diet not containing that particular protein may be fed.
If none of these gets us a diagnosis, skin biopsy is the final test in most practices. If the biopsy reveals an inflammatory dermatitis, and an elimination diet and flea control have not improved the dermatitis, then atopic dermatitis is the most likely diagnosis and skin or serum testing may be offered to allow allergen-specific immunotherapy.
This frequently presents as miliary dermatitis. Atopy is a diagnosis of exclusion. That means that the diagnosis is reached when the cat has compatible clinical signs and all other possible causes have been ruled out. Once other hypersensitivities and ectoparasites have been ruled out, the client has the choice of either pursuing symptomatic therapy alone or (at least initially) in combination with allergen-specific immunotherapy. Once the allergy shots successfully control the symptoms, other therapies can be tapered and often discontinued. To formulate the shots, offending allergens need to be identified. At this point in time, intradermal skin testing is recommended as the test of choice. Success rate of allergy shots is the same as in the dog; approximately 60-75% of patients have a good to excellent response to this therapy.
Symptomatic therapy includes fatty acids, antihistamines and glucocorticoids. Fatty acid supplementation is much more successful in the cat than in the dog, in general products based on omega-3 fatty acids are preferable due to the δ-6 desaturase deficiency of the feline species. Antihistamines are also much more successful in the cat with approximately 60-70% of the cats responding to 2-4 mg chlorpheniramine per cat q 12 h. Other antihistamines may be used successfully, if chlorpheniramine does not control clinical signs and include hydroxyzine at 2mg/kg q 12h or diphenhydramine at 10 mg per cat q 12h. Glucocorticoids are used frequently in various formulations and are better tolerated in the cat than the dog, however, long-term use of glucocorticoids may cause skin fragility syndrome, and diabetes mellitus has also been observed with chronic steroid use.
This may be caused by a variety of conditions. If the cat licks the hair out, fleabite hypersensitivity and/or food intolerance and/or atopic dermatitis may be involved. Very rarely is psychogenic alopecia the cause of non-inflammatory, typically symmetric alopecia. Nevertheless, a combination of psychogenic and allergic factors are frequently seen, particularly in highly-strung breeds such as Siamese or Abyssinians. Hair loss is associated with dermatophytosis, anagen defluxion (where severe metabolic diseases or chemotherapy interfere with hair production and leads to structurally weak hair shafts, that may break, leading to alopecia), telogen effluvium (where a severe stress 4-12 weeks prior to the hair loss pushed the hair from the growth or anagen into the resting or telogen phase and now the new hair growth pushes out the old hair leading to clinical alopecia) or hormonal disease such as hypothyroidism or hyperadrenocorticism, both of which are extremely rare diseases. If the owner does not observe any licking, a trichogram is performed, pulling out some hair in the affected area and evaluating the tips. If these are broken off, the cat licks the hair out when not being observed ('closet lickers' or 'hidden groomers') or it breaks off due to dermatophytes (identified with the trichogram, a fungal culture or biopsy) or anagen defluxion (easily identified by a thorough history). If the tips are tapered, telogen effluvium (again should be identified with a thorough history) or hormonal diseases may be a consideration.
Eosinophilic Granuloma 'Complex'
Indolent (or eosinophilic or rodent) ulcer commonly affects the upper lip unilaterally or bilaterally, but may occur in the oral cavity or elsewhere on the body. The well-circumscribed ulcers with raised borders are rarely painful or pruritic; frequently the owner is more bothered by the lesions than the cat. The differential diagnoses of the feline eosinophilic ulcer are neoplastic diseases such as squamous cell carcinoma and infectious ulcers (eosinophilic ulcers are often secondarily infected as well). Diagnosis is confirmed by biopsy, prior antimicrobial treatment is recommended if cytology is indicative of infection.
Eosinophilic plaques occur typically on the abdomen or medial thighs, are well circumscribed and severely pruritic.
Eosinophilic (linear) granulomas are nonpruritic, raised, firm, yellowish, and clearly linear plaques and occur most commonly on the caudal thighs. Differential diagnoses of both eosinophilic plaques and granulomas include neoplasia, bacterial and fungal granulomas.
Diagnostic procedures of choice are cytology and biopsy. After the diagnosis has been confirmed, the underlying cause needs to be identified (if possible) and treated. If elimination diet and flea control do not lead to remission, atopic dermatitis is most likely and skin testing may be an option, if owners wish to pursue allergy shots which have been reported to be effective in the majority of patients with eosinophilic granuloma due to atopic dermatitis.
Head and Neck Pruritus
Head and neck pruritus in the cat is often associated with significant self-trauma, alopecia, erosions, crusts and ulceration. Differential diagnoses include atopic dermatitis, food adverse reactions, fleabite hypersensitivity as well as ectoparasites such as Notoedres cati or Otodectes cynotis. Often the eroded or ulcerated lesions are secondarily infected with bacteria or yeast organisms, which need to be identified by cytology and treated appropriately. Elimination diet, ectoparasite treatment trial and flea control are diagnostic tools frequently used. If no improvement is seen, biopsy or skin testing may be considered.