Atopic Dermatitis

Description

Atopic dermatitis is a heritable disorder in which animals are hypersenstive to common environmental allergens. It is one of the most common skin diseases of dogs worldwide.

Susceptible produce allergen-specific IgE to normally-innocuous environmental allergens. This IgE then binds to receptors on cutaneous mast cells to facilitate degranulation on further allergen exposure¹. It is thought that allergen exposure occures via percutaneous absoprtion. Degranulation of mast cells in response to allergen-specific IgE is a type 1 (immediate) hypersensitivity reaction, which results in the release of histamine, cytokines, chemokines, and other chemical mediators. The cytokines involved are biased towards a Th2-type response, with particularly high production of IL-4. IL-4 is a key cytokine for further production of IgE by B-cells. . bacterial superantigens, autoantigens release via keratinocyte samage plays a role in perpetuating inflammation.

Signalment

Atopic dermatitis is a disease of dogs, although it can occur sporadically in the cat. The typical age of onset of atopic dermatitis is between 6 months and 3 years of age and signs are hardly ever seen in animals under 6 months of age. Atopy is heritable and so breed predispoitions occur. Susceptible breeds include the : Beaceron, Boston Terrier, Boxer, Cairn Terrier, Cocker Spaniel, Dalmation, English Bulldog, English Setter, Fox Terrier, Sealyham Terrier, Setters, Shar-Pei, West Highland White Terrier, Wire Hiared fox Terrier, and Yorkshire Terrier^beale. Certain breeds such as the Cocker Spaniel, Dachshund, Doberman Pinscher, German Shepherd, German Short-haired Pointer and Poodle appear to have a decreased risk of atopy. There is no sex predilection.

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. The primary reason for pursuing intradermal or serologic allergy testing is to identify the offending allergens in an animal, and to formulate specific immunotherapy. Allergy testing (intradermal or serologic) is a diagnostic aid that measures elevated levels of tissue-bound or circulating IgE; alone, it is not sufficient to diagnose atopy. Test results are significant only if the offending allergens identified are compatible with the history or seasonality of pruritus.

Clinical Signs

Signs are often, but not always, seasonal. Pruritus is the hallmark of atopic dermatitis and may be the only complaint. This gives rise to self-trauma, causing lesions. Lesions commonly include alopecia, erythema, scaling, crusting, excoriations and salivary staining. Macular-papular eruptions are occasionally seen^beale. With time, lichenification, and hyperpigmentation develops. Because the route of allergen contact is thought to be percutaneous absorption^beale, it follows that hairless regions are most frequently affected: the face, ears, axillae, feet and inguinal regions are predilection sites. Secondary infections such as superficial staphylococcal pyoderma and Malassezia are common, and otitis externa often occurs concurrently^{merck, beale, willemse}. A small number of cases exhibit only chronic or recurrent otitis externa. Another uncommon presentation is allergic rhinitis, manifesting as sneezing, nasal discharge or allergic conjunctivitis^beale.

Laboratory Tests

In-Vitro Allergy Tests

Serum based allergy tests are widely used in the evaluation of canine atopic dermatitis. However, they are often used prior to ruling out other disorders with similar clinical signs (scabies, food allergy/intolerance). Measurement of total serum IgE levels is not a useful tool for diagnosing atopic dermatitis in dogs. IgE levels are not significantly different in dogs with atopic dermatitis than in normal healthy dogs. In addition, things like routine vaccinations and the presence of ecto or endoparasites may influence serum IgE levels. Finally, there may be breed variations in serum IgE levels.

Allergen-specific IgE measurement may be used to identify allergens for immunotherapy once a clinical diagnosis of atopic dermatitis has been established. In these assays, the patient's serum is evaluated for IgE antibody directed against a panel of regional allergens (pollens, mold, dust, epidermals). Methodologies vary between laboratories, but in general, the principal is the same: the serum is allowed to react with an individual allergen extract . The unreacted antibodies are washed away, and then the allergen-bound antibody is detected using an IgE specific reagent (coupled to an enzyme or radioisotope). The IgE bound reagent is then quantified which should be proportional to the amount of allergen-specific IgE. Measurement depends on the assay utilized, but may use radiometric, fluorometric, or colorimetric measures.

Unfortunately, the laboratories are not subject to any standardization or quality control procedures. Reliability and repeatability of some of these commercial tests are unacceptable. Because there is no external regulation of these diagnostic laboratories, we must rely on the ethics of the companies performing the tests.

Besides laboratory inaccuracies, there are other factors which may influence test results: age, season during which the patient is tested, prior or current corticosteroid therapy. Intradermal Skin Testing

Intradermal skin tests (IST) detect the ability of allergens injected intradermally to cause mast cell degranulation leading to a subsequent wheal and flare reaction. Aqueous allergens are used for testing. Allergens are selected based upon the region in which the patient lives. Tree, weed, and grass pollens as well as molds, house dust mites, insects and epidermals should be included in the test. In theory, testing should be performed utilizing the highest concentration of extract that does not cause an irritant reaction. Recommended test strengths for allergens are as follows: 1000 PNU/ml for pollens, 1:50,000 W/V for house dust mites, 250-500 PNU for epidermals and 1000 PNU/ml for insects.

Aqueous allergen extracts lose potency over time, particularly when in their more dilute form for testing. Diluted allergens should not be kept for longer than 2 weeks. They should be stored at 4C. Prior to testing, allergens should be allowed to reach room temperature. Once testing is completed, allergens should be returned to the refrigerator as soon as possible.

Test results can be affected or inhibited by numerous factors: medications, sedatives, stress, etc.

Antihistamines: Hydroxyzine has been shown to inhibit skin test reactivity, and it is presumed that other antihistamines will have the same effect. A withdrawal period of 10 days is recommended, and longer for antihistamines with longer half-lives (ceterizine, etc).

Tricyclic Antidepressants: Histamine reactivity is affected by doxepin in people for up to 11 days.

Glucocorticoids: Withdrawal of topical glucocorticoids for 3 weeks has been suggested, oral glucocorticoids a minimum of 3 weeks, and repositol steroids for a minimum of 8 weeks. However, the optimal times have not definitively been established. Patients that have been receiving glucocorticoid therapy for extended periods of time may require a longer withdrawal period, and some dogs may have positive results with a shorter withdrawal period.

Inflamed or infected skin makes IST difficult. Ideally, patients should be treated for their pyoderma or Malassezia dermatitis prior to testing.

Sedation is recommended for IST as testing is more easily performed. Sedatives that have been demonstrated to not affect test results include xylazine HCL, Medetomidine, and Tiletamine/zolazepam. Diazepam, oxymorphone, acepromazine and propofal may all adversely affect test results. The lateral thorax is routinely used for testing. The site is clipped with a #40 blade.

Intradermal injections are made with 26-27 g needles. Sharp needles ensure less bruising and easier injections. Positive (histamine phosphate 1:100,000) and negative (0.9% buffered saline) control injections are made to help ensure test accuracy. Injections are made utilizing approximately 0.05-1.0 ml solution. Injections are made based on "bleb" size. The injection blebs should be of approximately the same size at each injection site. Reactions are most commonly evaluated subjectively on a scale of 0-4 (compared to the positive and negative controls). Reactions are read at approximately 15 minutes. False positive reactions may occur with irritants, or allergens used at too high a concentration. False negatives may occur as well with drug interference, host factors, time of year, and improper technique.

Treatment

cyclosporin

References

1. Tilley, L P and Smith, F W K (2004) The 5-minute Veterinary Consult (Fourth Edition),Blackwell.
2. Beale, K M (2006) Atopic Dermatitis: Clinical Signs and Diagnosis. Proceedings of the North American Veterinary Conference 2006.
3. Merck & Co (2008) The Merck Veterianry Manual (Eight Edition), Merial.
4. Willemse, T (2007) The Newest on Canine Atopic Dermatitis. Proceedings of the Southern European Veterinary Conference & Congreso Nacional AVEPA.