Canine Hot Spots - Environmental Allergens
(Continued article ... page 2 of 6)
| Definition / Causes | Parasitic Allergens | Allergy Testing |
| Environmental Allergens | Other Causes | Prevention |
Atopy
Atopy is a disorder by which dogs have a predisposition for developing antibodies to environmental allergens. Atopy is the most common disorder causing hypersensitive skin reaction in non-flea allergic patients presenting with dermatitis and accounts for up to 70 to 90% of all hypersensitive conditions. Although it has been found that up to 10% of dogs with atopy may also have food allergies, up to 80% of dogs diagnosed with food allergies also have atopy, thus accounting for the high rate of failure to treat food allergy patients through manipulation of diet alone.
Cause: Dogs that develop atopic dermatitis have a predisposition for excessive production of immunoglobin E (IgE) antibodies. IgE antibodies are believed to be the primary immune defense against parasitic organisms. However, in humans and other animals, including the dog, these antibodies are also responsible for producing allergic reactions. When a hypersensitive dog is first exposed to an environmental allergen, such as pollen, mold, or dander, its immune system will begin to produce high levels of IgE antibodies that will accumulate in the tissues of the body. (In humans, IgE is found predominantly in the respiratory and conjunctival mucus membranes, while in dogs, IgE antibodies locate predominantly to tissues comprising the skin.) When the dog is re-exposed to the same allergen at a later time, IgE antibodies bind to the allergen and activate the release of histamines (chemicals that attract other immune surveillance cells to the site of infection) from specialized, blood-derived immune cells called mast cells, which are involved in the inflammatory response. Histamine release results in reddened and itchy skin consistent with symptoms of dermatitis. In some instances, atopic dogs have been found to have non-reactive IgE antibodies, but demonstrated elevated levels of immunoglobin Gd (IgGd) antibodies. IgGd is a subset of IgG antibodies, a separate class of antibodies that predominantly circulates in the serum and is responsible for producing such conditions as anaphylactic reactions. Current studies are aimed at exploring the role of IgGd antibodies in the atopic disease process.
Symptoms: The most common symptom of atopy is pruritis (itching), usually beginning around the face and paws and in some cases eventually becoming more diffuse over other areas of the body, particularly the ears, armpits, elbows, and groin. Recurrent ear infections are present in up to 75% of dogs diagnosed with atopy. Skin lesions are not usually apparent, unless resulting from excessive scratching, but a raised, pustular rash with or without hair loss may occur as a result of secondary pyoderma. Some dogs may develop conjunctivitis.
Diagnosis:Because many other dermatologic disorders may present with similar symptoms to atopy, certain criteria have been established to select symptomatic patients for further in vitro testing and to identify the causative allergen:
- Member of a breed with a known predisposition for atopy
- Clinical symptoms manifesting between 6 months and 4 years of age
- Waxing and waning symptoms associated with seasonal changes
- Positive response to glucocorticoid treatment
Though symptoms during seasonal changes are the best indicator of atopy in a dog, a diagnosis of atopy should not be excluded in the absence of seasonal symptoms, since it has been found that up to 80% of dogs with atopy will demonstrate continual, year-round symptoms. When these criteria are met and other differential diagnoses are ruled out, then allergy testing to identify the responsible allergen(s) becomes the next consideration.
Principles of Allergy Testing. In general, mixed allergen testing (combining antigens from different sources together, in the same way a human might be tested, say on his or her back) may be performed. Since the purpose of testing is to identify a specific allergen or allergens for immunotherapy, testing each allergen individually is the preferred method. Common testing allergens include: grasses, trees, shrubs, weeds, molds, house dust mite bodies, mite eggs, mite larvae, mite feces, fleas, ants, flies, cockroaches, mosquitoes, moths, feathers, nylon, wool, silk, and tobacco. Food allergen testing is rarely effective for identifying food-related allergens; therefore, food elimination testing is the preferred method for screening for food allergies (to be discussed in an article in the next series). Selection of allergens for the purpose of treating dogs with immunotherapy (hypo sensitization treatment) may be done either through intradermal testing or in vitro testing. Each method has its strengths and weaknesses in terms of laboratory analysis. With intradermal testing, a commercially available antigen is selected and injected under the skin and the site of the injection is observed for signs and intensity of an allergic response. Many dermatologists utilize intradermal testing because it has a high level of specificity and thus positive results are more likely to be true-positives. The main limitation with this test is the occurrence of false-negatives because of poor sensitivity associated with this form of allergen testing.
An alternative to intradermal testing is in vitro allergen testing. This second method requires reacting serum from the dog with a commercially-available antigen that has been bound to a solid substrate in a radioallergosorbent test(RAST) or enzyme-linked immunosorbent assay (ELISA), which measures the amount of IgE in the patient's serum that binds to the allergen. This test is particularly useful in evaluating dogs that have already undergone or are continuing treatment with glucocorticoids (since these drugs will inhibit intradermal test reactions yielding false-negative results). However, in vitro allergen testing has a higher degree of non-specificity and, therefore, false-positive results. Recent modifications to methods of in vitro allergen testing by the Veterinary Allergy Reference Laboratory (VARL) have provided a means to increase the level of specificity of these in vitro tests aimed at detection of IgE through the use of monoclonal anti-IgE reagents. Additionally, this testing method also seems to provide comparable specificity while providing greater accuracy compared to intradermal testing for detecting insect-associated allergies.
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