Ralf S. Mueller, MAVSc, DACVD, FACVSc, DECVD, DrMedVet, DrHabil, FAAAAI
Miliary dermatitis is characterized by numerous, small, localized or generalized papules and crusted papules. Frequently, papules can be felt on palpation rather than being appreciated visually. Miliary dermatitis is the most common feline problem in small animal dermatology.
The list of differential diagnoses for miliary dermatitis is lengthy. Allergies such as flea-bite hypersensitivity, atopic dermatitis or adverse food reaction, ectoparasites such as Otodectes cynotis, Notoedres cati or Cheyletiella blakei, infections with dermatophytes or bacteria, neoplastic diseases such as mast cell tumours, immune-mediated diseases such as pemphigus foliaceus and nutritional shortcomings such as essential fatty acid deficiencies may all cause a papulocrustous dermatitis.
A thorough history will often give clinical clues with information on the extent of pruritus which is worse with allergies and ectoparasites. If pruritus and dermatitis in in-contact animals or humans occurs, one may think of dermatophytosis or ectoparasites. Seasonality indicates atopic dermatitis or flea-bite hypersensitivity. Response to previous medications may also be helpful in providing clues on the pathogenesis. Nutritional problems should not cause miliary dermatitis in cats presented to small animal practitioners in the developed countries, only very severe biotin or fatty acid deficiencies are able to cause clinical signs. If crusting occurs on the inner pinnae, pemphigus foliaceus is a highly likely diagnosis. Additional clinical clues may be the more yellowish colour of the crusts (versus dark red to purple in miliary dermatitis of allergic, infectious or ectoparasitic origin) and concurrent paronychia.
The author's initial approach will depend on historical information, but typically includes superficial skin scrapings and cytology to evaluate infection. If skin scrapings are positive, you have your diagnosis. However, negative skin scrapings do not exclude the possibility of ectoparasitism, and an ectoparasite treatment trial may still be indicated. Next steps commonly are flea control (particularly if it is summer) and elimination diet. If flea control products are well chosen, the trial can serve as an ectoparasite treatment trial as well. If none of these get us a diagnosis, biopsy and/or fungal culture are the final tests in most practices.
A negative fungal culture rules out dermatophytosis. Biopsy will reveal the unusual neoplastic or immune-mediated pathogenesis. If the biopsy specimen shows chronic dermatitis and diet and flea control have not improved the cat, you deal with atopic dermatitis and skin testing may be offered to allow allergen-specific immunotherapy.
Ectoparasite Treatment Trial
Most newer ectoparasiticidal agents are effective for a number of cutaneous parasites. Imidacloprid applied once every 2 weeks for three treatments is typically effective for flea infestation; in combination with moxidectin it is also effective for many superficial mites. Fipronil spot-on has been reported to be 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). However, aural application constitutes off-license treatment in the UK. The author prefers to give these treatments two to three times 2 weeks apart.
Selamectin administered once every 2 weeks for three treatments is effective for Notoedres cati, Otodectes cynotis and Cheyletiella blakei as well as fleas. Treatment intervals of less than 4 weeks are off-license in the UK. However, if the trial is used to identify suspected flea-bite hypersensitivity, environmental control may be needed as well using a growth regulator or development inhibitor such as methoprene, fenoxycarb or pyriproxifen at the beginning of the trial. Of course, in-contact animals need to be treated for all ectoparasite treatment trials.
Elimination diets are aimed at identifying food intolerance in a given patient. Commercial foods with unusual protein sources such as rabbit or duck or hydrolysed diets with proteins of lower molecular weight of between 6 and 12 kd are convenient, balanced and used commonly. In some cats a home-cooked diet is needed to identify food intolerance; in these patients the author recommends an exclusive protein source such as horse, emu or venison meat. Ultimately the cat's previous diet and availability of alternative proteins will determine the ideal elimination diet. The only criterion for selecting the food source is that the chosen protein should not have been fed before. It should be fed exclusively for 6-8 weeks. If after 6-8 weeks there is a good response, we will re-challenge with the old diet and food intolerance is confirmed if the animal relapses within 2 weeks (typically within 2 days).
Treatment of Miliary Dermatitis
A diagnosis of the underlying disease is essential for optimal treatment of any patient. If there is no response to an ectoparasite/flea control and non-allergic diseases are ruled out, symptomatic treatment with antipruritic drugs is also possible. Antihistamines, fatty acids, glucocorticoids and ciclosporin are discussed in more detail below.
Antihistamines are useful adjunctive agents in the management of atopic cats. We combine antihistamines and fatty acids on a regular basis to increase our chances of controlling an allergic cat without corticosteroids. The success rate for a single antihistamine in cats is much higher than in dogs and may be as high as 50-70%. We use either Histacalmine (Virbac, France, a combination of hydroxyzine and chlorpheniramine) or chlorpheniramine alone at 2-4 mg/cat once to twice daily most commonly.
Drowsiness is the most common side effect but it may decrease after 2-3 days on therapy. Thus, it may be worthwhile continuing treatment for several days before ceasing the drug due to this side effect. Less common are gastrointestinal signs. However, the major problem with antihistamine treatment in cats is the difficulty most owners have with administering tablets to cats on a regular basis.
Essential Fatty Acids
Essential fatty acid (EFA) supplementation has been advocated in the recent past as sole or adjunctive anti-inflammatory therapy in cats with hyper-sensitivity disorders. EFAs are polyunsaturated fatty acids. Important players in the feline game are gamma linoleic acid found in high concentrations in evening primrose oil and eicosapentaenoic acid/ docosahexaenoic acid in fish oil.
EFA supplements are of potential value in inflammatory, pruritic skin disease as well as dry, scaly skin (seborrhea sicca). The correct dose, type and ratio of fatty acid supplementation is still unclear. We aim at 50 mg/kg of EFA daily (although nobody knows if this is the best dose) and usually prefer fish oil. If cats like fish, this oil can be expressed from the capsules directly on to the food, facilitating the application. If cats are not fond of fish, evening primrose oil may be tried and can also be mixed into the food. We typically use 250 mg capsules (1 capsule daily/cat).
Glucocorticoids are very commonly used in the treatment of skin conditions. Corticosteroids are effective in most patients with atopic disease and resolve the symptoms at least initially on reasonably low dosages. In cats, we usually administer prednisolone at 1 mg/kg/day and gradually taper to the lowest effective dose. Every other day therapy is definitively preferred over daily drug administration because as it is thought to lower the chances of iatrogenic hyperadrenocorticism.
Adverse effects include polyuria, polydipsia, polyphagia, increased susceptibility to infection and other well known symptoms of iatrogenic hyperadrenocorticism. However, in cats iatrogenic hyperadrenocorticism is much less likely than in dogs.
A new drug recently introduced for the treatment of atopic dermatitis in humans and animals is ciclosporin. Ciclosporin inhibits the release of interleukin 2, the cytokine predominantly responsible for T-cell activation, but has several other mechanisms of anti-inflammatory action as well. Several studies have evaluated this drug in atopic humans and dogs and results have been very encouraging. Similarly, a pilot study in cats provided good results. The dose used is 5 mg/kg daily. Administration may be decreased to every other or every third day therapy if daily therapy leads to remission within the first 4-6 weeks, however, the dose should not be decreased to below 5 mg/kg in animals.
The most common adverse effects with this drug include diarrhoea and vomiting. In rare cases increased infections, hepatotoxicity or gingival hyperplasia have been reported in dogs. Renal disease and changes to blood pressure so far have not been observed in dogs and cats at this dose to the author's knowledge. Ciclosporin is rather costly, but may be a reasonable treatment alternative.
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