Allergic skin disease is the most common reason a client seeks veterinary care for an animal in the United States. In some parts of the country, at certain times of the year, more than half of the animals a small animal practitioner examines will have a form of allergic skin disease. Though common, treatment of allergic skin disease can be frustrating for the veterinarian and the client. Recently, there have been new advances in the diagnosis and treatment of allergic skin disease that will be of great benefit to veterinarians and small animal patients. When a patient is presented with allergic dermatitis, success is often achieved when the correct diagnosis is made and specific therapy is instituted. However, because of the multifactorial nature and financial constraints, non-specific therapy may be more practical and affordable. Overall, a combination of both specific and non-specific therapy is what is most commonly used, and yields the greatest benefit.
Photo 1: A cat with food hypersensitivity.
Flea Control. In recent years, significant strides have been made in the safety and efficacy of flea control. Though it is beyond the scope of the article to address flea control in depth, a few points can be made to ensure success. The major emphasis of flea control should be focused on the animal. Several products that are widely available are both efficacious and safe. Some commonly dispensed products include: imidacloprid, fipronil, selamectin and lufenuron. In areas where fleas are prevalent, flea control must be instituted first and foremost before addressing other aspects of allergy. Without proper flea control in endemic areas, all other therapies and protocols for the diagnosis and treatment of allergic dermatitis in animals will be unsuccessful.
Immunotherapy. Atopic dermatitis (AD) is a genetically programmed disease of an individual in which IgE and IgG antibodies are formed resulting in pruritic dermatitis. In humans, researchers have identified polymorphism in the coding region of the IL-13 gene locus of atopic individuals. Both allergen-specific and non-specific mechanisms contribute to the disease development. AD skin lesions are proposed to be initiated by activation of allergen-specific Th2-type cells, potentially influenced by local cutaneous factors. In many cases of atopic dermatitis in the dog, hyposensitization or immuno-therapy is the best therapeutic option available for treatment. The mechanism of action for hyposensitization is multifactorial and not clearly understood. Hypotheses include reducing the levels of IgE through humoral desensitization, generation of suppressor cells, cellular desensitization, induction of "blocking antibodies" and, or a combination of the above. Several key factors should be kept in mind when immunotherapy is used; 1) Response to therapy takes time and very few cases respond immediately. Most cases take between four to 12 months (average seven months) for the animal to become less pruritic. 2) Response may be partial rather than total. Not all patients will have total resolution of pruritus. However, some decrease in pruritus may be beneficial. 3) Response to immunotherapy may fail. Some patients will not respond to immunotherapy to any degree. 4) Successful immunotherapy is not a "cookbook" procedure. Many patients need adjustment in the administration of their antigens to ensure a decrease in pruritus. Modifications need to be made for these individuals based on his/her response to antigens and environmental conditions. Communication of these facts with owners will help small animal practitioners when developing an immunotherapy protocol (Photo 2).
Elimination diets. Food allergy or food hypersensitivity is the subject of constant debate among veterinary dermatologists. Most agree that food hypersensitivity is a type-I allergic response with both immediate and late-phase reactions. One hypothesis for the pathogenesis of the disease includes a by-pass of secretory IgA by food antigens and subsequent lack of tolerance by failure to activate suppressor T-cells or IL-4, IL-10 and TGF-beta from Th2 and Th3 lymphocytes. Testing animals for food hypersensitivity by either scratch, prick, intradermal or serologic methods is virtually useless. Feeding animals with suspected food hypersensitivity can be both therapeutic and/or diagnostic. Depending on which theories a practitioner ascribes to, there are several approaches to treating a patient with a food hypersensitivity. One approach is to feed the patient a "hypoallergenic" or elimination diet for six to 12 weeks. The diet may be a "novel" type of protein (to which antibodies have not been formed), a hydrolyzed diet, in which the protein may have limited antigenicity due to the small molecular weight, or a "home-cooked" diet, which may have limited fillers, expanders or preservatives to which the animal may be allergic. Several key factors need to be emphasized to owners that when feeding a hypoallergenic or elimination diet to allergic animals; 1) An elimination diet is not the same as a weight loss diet, it must be followed in a strict manner with no "cheating". Feeding even small amounts of food which are not part of the diet may result in failure of the whole program. 2) Since the diet is only fed for a relatively short period of time, it does not need to be nutritionally complete. 3) Results, as seen in a decrease in pruritus, may take weeks, if not months to observe. Decreases in pruritus may be subtle. 4) Partial response to dietary manipulations may be viewed as a success when used while integrating with other therapies for the overall management of an allergic patient.
Photo 2: An intradermal skin test in a dog with atopic dermatitis.
Non-specific therapy. In some instances, specific therapy for patients with allergic dermatitis is either impractical, costly or has failed for one reason or another. In addition, non-specific therapy is usually needed for allergic patients while waiting for specific therapy (i.e. immunotherapy) to become effective. Overall, non-specific therapy is often necessary and useful in treating patients with allergic dermatitis.
Topical therapy. Topical therapy can be an effective adjunct to both specific and non-specific therapy for the allergic patient. Topical therapy may include shampoos, sprays, lotions, creams and ointments. Shampoos and the very act of shampooing (hydrating) can be soothing and beneficial to patients with allergic dermatitis. Cool water should be used when owners are bathing animals. In addition, blow drying should be avoided if possible. Many owners are wary of bathing animals too frequently, causing dry skin (xerosis) and thereby increasing rather than decreasing pruritus. This can be avoided by applying topical moisturizers and emollients immediately after bathing. Sprays such as Genesis™ Topical Spray (0.015 percent triamcinolone acetonide) or Relief® Spray (1 percent promoxine) can be very beneficial if applied frequently. When using topical medications, contact time is important.
Antihistamines. First generation, non-sedating antihistamines are H1 blockers. They act as physiologic or receptor antagonists of histamine and reduce histamine formation by preventing inflammatory mediator release from basophils and mast cells. Besides the effects on histamine and inflammatory mediators, first generation antihistamines often have a sedative effect which may decrease pruritus. The efficacy of antihistamines is often questionable and unpredictable. In addition, they must be avoided in patients with hepatic disease, glaucoma, urinary retention, gastric atony and pregnant animals. In some instances however, antihistamines may be useful as adjunctive therapy to immunotherapy or corticosteroids. In a small number of cases they may be tremendously beneficial in reducing pruritus. When prescribing antihistamines to patients with allergic dermatitis, practitioners should advise owners of the possible lack of efficacy, potential adverse side effects (sedation, excitation) and screen patients for contraindications.
Photo 3: Whirlpool therapy can often soothe the skin of pets suffering from allergies.
Essential fatty acids. Fatty acids are often used as adjunctive therapy in treating pruritus in patients with allergic dermatitis. The mechanism of action is proposed to be inhibition of portions of the arachadonic acid cascade, leading to a decrease or elimination in the formation of LTB4. In most cases, successful administration of fatty acids requires dosages much higher than recommended by manufacturers. This can be accomplished by feeding a diet rich in essential fatty acids along with oral supplementation. Adverse side effects include increase in body weight, diarrhea and in rare cases, pancreatitis. Few cases of allergic dermatitis respond solely to fatty acid therapy. In most instances, the supplements are used more successfully when given concurrently with other forms of therapy.
Corticosteroids. Many cases of chronic administration of corticosteroids result in significant liver pathology along with concurrent urinary tract infections, calcinosis cutis or eventual steroid tachyphylaxis. Though cats have fewer steroid receptors than other species, chronic administration can also result in adverse conditions such as diabetes mellitus.
Though it is preferable not to use corticosteroids to treat patients with allergic dermatitis, at times it seems almost cruel not to. A few points are useful to remember when prescribing corticosteroids. 1) Judicious use of corticosteroids should be closely monitored. Often problems occur if owners are left unchecked to administer corticosteroids for long periods of time. 2) Communication with owners as to the potential for adverse side effects is critical. 3) Not all corticosteroids are created equal. Though many practitioners use only prednisone or prednisolone, oral dexamethazone and methylprednisolone may also be used. 4) Remember potential side effects and physiologic interaction with other diseases and drugs. 5) If possible, when using corticosteroids for allergic dermatitis, concurrent administration of non-steroidal treatment may help decrease the dosage of corticosteroid necessary to achieve patient comfort.
Cyclosporine. Cyclosporine will soon be a significant drug used to treat allergic dermatitis in small animals. In some instances cyclosporine may replace many other forms of therapy. The drug has been used for treatment of atopic dermatitis in humans. For use in dogs, a dose of 5-10 mg/kg, q24h, PO is useful for the treatment of allergic dermatitis. There is a significant lag period (four to eight weeks) before a decrease in pruritus may be observed. In approximately 50 percent of dogs the dose may be reduced after the initial period (four to eight weeks). Ten percent of dogs may experience vomiting, which may be reduced or eliminated by concurrent administration of food or discontinuation of the drug. In cats, 25mg given once daily may help decrease pruritus after four to eight weeks. Due to immunosuppression, dogs and cats need to be monitored for potential secondary diseases (demodicosis, dermatophytosis etc).
Successful management of patients with allergic skin disease will occur when a clinician uses an integrated approach. Many cases are multifactorial in etiology and pathogenesis, therefore many forms of therapy can and should be used. In addition, patients with allergic skin disease and environmental conditions are constantly changing so constant monitoring is needed. Changes in therapy should be expected. Communication with clients about expectations is essential. Though difficult and frustrating, successful management of patients with allergic skin disease can be very rewarding.