Clinical Findings

There is no sex predilection in canine AD. There are breed predilections, but the prevalence within a breed is largely dependent on the genetic pool and region. Breeds predisposed to developing AD include Chinese Shar-Peis, Wirehaired Fox Terriers, Golden Retrievers, Dalmatians, Boxers, Boston Terriers, Labrador Retrievers, Lhasa Apsos, Scottish Terriers, Shih Tzus, and West Highland White Terriers. The age of onset is generally between 6 mo and 3 yr. Clinical signs usually occur on a seasonal basis but may be seen year-round with time. Pruritus is the characteristic sign of AD and ALD and may be the only complaint. The feet, face, ears, flexural surfaces of the front legs, axillae, and abdomen are the most frequently affected areas. Lesions develop secondary to self-trauma and include alopecia, erythema, scaling, salivary staining, hemorrhagic crusts, excoriations, lichenification, and hyperpigmentation. Superficial staphylococcal pyoderma, Malassezia dermatitis, and allergic otitis externa with secondary infections are common complications. Chronic or recurrent otitis is the only complaint in a small number of animals.

Diagnosis

The diagnosis is based on the signalment, a thorough history, appropriate physical examination findings, and ruling out of other causes of pruritus. Differential diagnoses include food allergy (nonseasonal), flea allergy (seasonal), contact allergy, and scabies. Allergy testing (intra-dermal or serologic) is a diagnostic aid that measures elevated levels of tissue-bound or circulating IgE; alone, it is not sufficient to diagnose AD. The primary reason for pursuing intradermal or serologic allergy testing is to identify the offending allergens in an animal and to formulate specific immunotherapy. Test results are significant only if the offending allergens identified are compatible with the history or seasonality of pruritus. Animals with classic clinical signs but negative allergy tests are given the diagnosis of ALD. Immunotherapy would be difficult if not impossible in these animals.

Treatment and Control

AD and ALD are lifelong conditions that cannot be cured. There are 3 therapeutic options available for management of AD: avoidance of the offending allergen(s), symptomatic therapy to control pruritus, and immunotherapy (ie, hyposensitization, desensitization, allergy vaccine). A good management plan for atopic dermatitis requires the use of several different treatments. Clear and frequent client education ensures that the owner has reasonable expectations of response. Frequent progress evaluations are required so that the plan can be adjusted if needed.

Avoidance of allergens is the best choice but is often difficult, especially with allergens such as pollens, molds, and dust. Reduced exposure can be achieved by keeping the pet indoors during peak seasons, using a high efficiency filter on heating and air conditioning systems, covering and laundering bedding weekly, replacing bedding annually, and using hypoallergenic shampoos and cool water rinses to mechanically remove accumulated allergens from the skin and coat. In addition, removing or controlling other allergens such as fleas or foods will help control pruritus.

Symptomatic therapy is directed at controlling both primary and secondary signs that accompany AD. All superficial pyodermas and Malassezia dermatitis should be treated aggressively with an appropriate antibiotic or antifungal agent until complete clinical resolution is achieved. Most require a minimum duration of 3 wk; treatment should extend 2 wk beyond clinical resolution.

Omega-3 and omega-6 essential fatty acids (EFA) may be of some benefit. Omega-6 fatty acids (linoleic acid) help decrease transepidermal water loss by reinforcing the epidermal lipid barrier. Omega-3 fatty acids (eicosapentaenoic acid) disrupt the arachidonic cascade and its production of inflammatory mediators. EFA are reported to work synergistically with antihistamines to reduce pruritus; antihistamines alone are of modest benefit. The best response may be obtained when antihistamines and EFA are given together, but it is difficult to ascertain whether improvement is the result of the sedative effects of the antihistamine or true antihistaminic effects of the drug.

The calcinurin inhibitors, such as cyclosporine and tacrolimus, have demonstrated reliable reduction in pruritus and overall lesion improvement in dogs and cats with AD. A reported 50% reduction in pruritus in 70% of dogs with AD given cyclosporine A parallels the response seen with glucocorticoids but with fewer adverse effects. Some animals can be maintained comfortably with this drug alone. Tacrolimus ointment provides a benefit in animals with more localized lesions. Drugs in this category tend to be more expensive than other symptomatic treatments, however.

Pentoxifylline, a phosphodiesterase inhibitor, has immunomodulatory properties and had been shown to reduce erythema and pruritus in some dogs.

Immunotherapy

Immunotherapy is arguably the best treatment option for AD because it is the only therapy that potentially leads to the remission of signs without the addition of other medications. It remains the treatment of choice of most dermatologists and allergists. Hyposensitization or immunotherapy attempts to increase an animal's tolerance to environmental allergens (subjectively measured when an individual is exposed to an identified allergen without developing clinical signs). Although the mode of action is not completely understood, the primary theory states that IgG increases during the first few months of hyposensitization and exerts a blocking effect on circulating allergens by binding them and preventing mast cell degranulation. Another theory states that immunotherapy causes a shift away from TH₂ toward TH₁ by enhancing γ-interferon expression. After an injection, however, allergen-specific IgE levels may also increase when immunotherapy is initiated due to a response to the additional allergen load from the immunotherapy injections. This may result in increased pruritus in some animals. Reducing the amount of allergen given will often alleviate this reaction, and with time, allergen-specific IgE levels decrease. Decreased IgE levels and clinical improvement are not always directly correlated, however.

Immunotherapy is best considered for animals with problematic clinical signs that occur for several months during the year. The animal must also be cooperative enough to receive allergy injections. The criteria for successful hyposensitization include appropriate interpretation of test results, careful selection of allergens, adequate control of secondary infections, control of other allergies (food or flea), systematic administration of immunotherapy injections, and periodic communications between the owner and veterinarian. The longterm commitment needed from both the owner and the veterinarian for successful immunotherapy cannot be overemphasized. The owner must be willing to follow instructions accurately, be patient, and be able to communicate effectively with the veterinarian. The veterinarian must be able to recognize and treat other primary or secondary causes of pruritus (eg, otitis, pyoderma, Malassezia dermatitis, insect hypersensitivity) as they occur and gently guide the owner through these challenges until improvement is seen. Symptomatic therapy is required in almost every case during the induction period and at various times of the year. Symptomatic therapy includes not only antipruritic medications (ie, glucocorticoids, essential fatty acids, antihistamines, oral cyclosporine, topical shampoos and rinses) but also specific antimicrobial therapy.

Vaccine preparation involves selection of individual allergens for a particular animal. The allergen selection is determined by correlating the positive allergens on the test results with the prominent allergens during the time of year when the animal is symptomatic. If the test shows positive results for pollens that have no clinical relevance (eg, high pollen count during a period when the animal has no pruritus, positive reaction to an allergen not in the geographic area), then either the allergic reaction is mild (subthreshold) or it is a false-positive reaction. Either way, the allergen should not be included in the vaccine. Most veterinary vaccines are aqueous extracts. Development and manufacture of allergen extracts has not been standardized; thus, ragweed pollen from one manufacturer is not necessarily equivalent to ragweed pollen from another. Allergen supply companies are required to culture each allergen or vaccine to ensure sterility before release to a veterinarian. To maintain sterility, vaccines are preserved with either phenol or glycerin and are kept refrigerated. Phenol-preserved vaccines lose potency faster than glycerinated vaccines, but glycerin-preserved vaccines can cause local reactions in animals. Most vaccines and antigens today are preserved with phenol. Vaccine concentrations are measured either as protein nitrogen units (PNU) per mL or weight to volume (w/v). Neither accurately measures biologic potency, but the PNU measurement is generally preferred. Allergen extracts should be refrigerated to preserve shelf life. Enough vaccine should be made to last up to 6 mo. The potency of most vaccines is considered inadequate after 1 yr.

The main variables involved with hyposensitization immunotherapy, other than allergen selection, are the frequency of the injections and the dosage of allergens given. Allergens are administered by SC injection. The number of allergens in an individual allergy vaccine should be limited to 10–12 because too many allergens in one vaccine may dilute the concentration of each individual allergen, yielding an inadequate response.

Vaccine protocols vary but usually have induction and maintenance periods. During the induction period, the dosage of allergen gradually increases until an arbitrary maintenance dosage is reached. Once the maximum dosage is given, this maintenance level is continued. The interval between maintenance dosages may vary from 3–4 days to 3 wk. Adjustments in the interval are based on the animal's response. Owners are advised not to expect much response for 6 mo and are asked to commit to at least 1 yr of therapy before deciding the usefulness of immunotherapy. The best assessment of response is to compare the degree of disease or discomfort between similar seasons. Most owners learn to administer the allergy injections very well, while others may need assistance from a capable friend or veterinary staff member.

Last full review/revision July 2011 by Patricia D. White, DVM, MS, DACVD