Medical management of the allergic cat (Proceedings)

Pruritus and the various aberrations of skin and hair coat that it provokes are, by far, the most common reasons for which cats are presented to veterinarians for dermatologic diagnosis. Although many different dermatoses can be pruritic, and the differential diagnosis of pruritus is complicated, allergic (hypersensitive) skin diseases are certainly the most common causes of pruritus in cats.

Pruritus is defined as a sensation that elicits the desire to scratch. The pathophysiology of pruritus is complicated and poorly understood for most diseases in most species. The literature is plethoric with information on various mediators and modulators of pruritus. However, the relative importance of these mediators and modulators in any given species, disease, or individual is rarely known.

In our practice, the most common reason for having difficulty in managing the allergic cat is failure to frequently reconsider the "summation of effects". Any "allergic" cat that is difficult to control or suddenly "comes out of control", needs to be reassessed for other problems (secondary bacterial pyoderma, secondary Malassezia dermatitis, flea infestation, dry skin, contact dermatitis, etc.) before its allergy medicine is adjusted.

There are several categories of therapeutic agents, and many cats do better on combinations of these.

Systemic Therapy

Glucocorticoids are, without a doubt, the most used and abused compounds in veterinary dermatology. They are also the most consistently effective drugs in the management of allergic pruritus in cats, and can be used effectively and safely in many patients (Table 1). All glucocorticoids are not created equal. Thus, if a cat does not do well with one glucocorticoid, a different one may be more acceptable. Some cats do not appear to be able to convert prednisone to prednisolone; hence using the latter is more effective. Situations do arise wherein the use of glucocorticoids is undesirable or contraindicated. Examples would include: (1) objectionable acute or chronic side effects, (2) certain concomitant diseases (e.g., cardiac disease, diabetes mellitus, pancreatitis, renal failure), (3) concurrent infections (bacterial, fungal, viral), (4) concurrent immunodeficiency states (e.g., FIV, FeLV), and (5) owners who are "cortisone"- or "steroid"-conscious. For these reasons, clinical and research interest in nonsteroidal antipruritic agents has "exploded" in the last several years. Although nonsteroidal antipruritic agents are often useful in the management of allergic cats, they do not have an immediate antipruritic and anti-inflammatory effects. Hence, it is often necessary to give glucocorticoids along with the nonsteroidal agents for the first 3 to 7 days.

Table 1. Glucocorticoid Therapy in Cats

Antihistamines

All "traditional" H1-blockers have antihistaminic, anticholinergic, sedative, and local anesthetic effects. They must be used with caution, if at all, in the presence of liver disease, glaucoma, urinary retention, gastrointestinal atony, seizures, pregnancy, and nursing queens. Responses are notoriously individualized and unpredictable. Thus, one often has to try several before the one that is "right" for the patient is found (Table 2). Each antihistamine should be tried for at least two weeks. Concurrent antihistamine administration often allows reduced glucocorticoid doses. Antihistamines are often synergistic with omega-3/-6 fatty acids.

Table 2. Antihistamine Therapy in Cats†

Heterocyclic ("Tricyclic") Antidepressants

In addition to poorly-defined behavior-modifying properties, these agents are very potent H1-blockers (Table 3). In addition to classic antihistamine side effects, these agents can also cause cardiac arrhythmias, lower seizure thresholds, and potentiate side effects of monoamine oxidase inhibitors (amitraz). Like antihistamines, heterocyclic antidepressants often act synergistically with glucocorticoids and omega-3/-6 fatty acids.

Table 3. Heterocyclic Antidepressant Therapy Cats

Omega-3/omega-6 Fatty Acids

Fatty acid supplements containing omega-3/omega-6 fatty acids are potent modulators of prostaglandin and leukotriene synthesis. They rarely cause side effects. Numerous clinical trials have shown that these agents are useful in many allergic cats, and may also act synergistically with glucocorticoids and antihistamines. The literature is very confusing as concerns the "correct" dosage, ratio, and type of omega fatty acid to be used. Most of this is directly attributable to a failure to consider the patient's base diet. These products should be given for at least 3 weeks.

If the clinician wants to know if a cat will benefit from omega-6/-3 fatty acids, a commercial diet with controlled amounts and ratios is preferable. Otherwise, selecting a commercial supplement without knowledge of the cat's base diet is fraught with misinterpretation and frustration.

Cyclosporine

Cyclosporine is a potent inhibitor of T lymphocyte-dependent immune responses.Its various effects include decreased IL-2, IL-3, IL-4, IL-5, TNF-α, and IFN-α production; inhibition of antigen presentation, eosinophil and mast cell production, histamine release from mast cells, neutrophil adherence, and growth and differentiation of B lymphocytes. The microemulsified forms (Atopica,® Novartis: capsules; Neoral,® Novartis: capsules and emulsion; generic) are preferred (better absorption) over the original forms (Sandimmune,® Novartis). Drugs that inhibit cytochrome P-450 enzymes (macrolides, azoles, tetracyclines, large doses of glucocorticoids) increase cyclosporine blood levels. Cyclosporine is not registered for use in cats. Recommended initial dosage is 5 mg/kg q24h PO for cats. Side effects are common (especially gastrointestinal) but usually mild. Cyclosporine was reported to be as effective as prednisolone in atopic cats (prednisolone dose only half of what I use).

Atopica® is not approved for use in dogs less than 6 months of age, dogs weighing less than 4 pounds, during pregnancy or lactation, or in malignant neoplasia. Patients on cyclosporine should probably receive only killed vaccines, and NOT be treated with ectoparasitic doses of avermectins. Food does NOT reduce the clinical efficacy of cyclosporine in dogs. Cyclosporine is reported to be effective for allergic pruritus, idiopathic pruritus, eosinophilic plaques, and eosinophilic granulomas in cats. Cats should be checked for FIV, FeLV, and Toxoplasma infection prior to therapy.

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