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.
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
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†