Allergy is a common and frustrating problem in veterinary practice.  A wide variety of breeds suffer from allergic dermatitis and otitis.  The typical age of onset is between 2 and 6 years of age.  Pure breed dogs will often exhibit clinical signs at an earlier age.

How does an allergic reaction occur?

• The body is constantly exposed to environmental allergens (molds, pollen, dust mites, fleas).  Both healthy and allergic dogs will produce antibodies (IgE, IgG, IgM) to combat these foreign proteins. 
• The allergic dog is thought to produce significantly higher levels of IgE (considered the most important antibody in producing allergic signs / symptoms) than the clinically normal dog. 
• Although there is no specific level of IgE that will produce pruritus (itching) … each dog has its own “threshold”  (an allergic dog is often thought to have lower threshold or “tolerance” which makes it more susceptible). 

The allergic reaction "pathway"

• Exposure to an allergen causes the body to produce IgE specific for that allergen
• IgE binds to the mast cells in the skin and “coats” the cell
• Re-exposure to the allergen
• Allergen binds to the allergen-specific IgE
• Allergen - IgE - Mast cell binding causes the Mast cell to degranulate (rupture)
• Mast cells contain pruritic (itchy) compounds – histamines, leukotrienes, prostaglandins, etc.
  

Classic clinical signs:  biting, chewing, scratching, licking

• Feet, face, axillae, inguinal regions are predominant
• Chronic ear infections / inflammation
• Chronic irritation from pruritus (itching) may lead to lichenification of the skin (thickening & pigmentation)
• Flea allergy often affects the tail and rump region
• Clinically you cannot tell the difference between food allergy and inhalant / percutaneous allergy (NOTE: there is no valid blood test for food allergy from any laboratory. You must do a food elimination diet trial).

Allergy Testing

1.  “in vitro” on blood (serum / plasma)

… RAST (radioallergosorbant test): uses radioactive tag

… ELISA (enzyme linked immunosorbant assay): uses an enzyme tag

2.  “in vivo”  (intradermal skin testing)

• The pet is sedated and an area is shaved on the side of the chest
• Anywhere from 50 to 80 allergens are then injected intradermally (into the skin)
• The reactions are then measured against a positive and negative control
• IDST is considered the “gold standard” of allergy testing

Preparation for intradermal skin testing

Certain medications may interfere with the test results and should be withdrawn prior to the test date (see the chart below for guidelines).

Medication                                                Approximate withdrawal time prior to testing

Steroids (oral, otic, ophthalmic, topical) ....................................... 3 - 4 weeks

Steroids (injectable-DepoMedrol) ................................................. 6 - 8 weeks

Antihistamines ........................................................................... 2 weeks

Phenobarbital ............................................................................. > 3 weeks

Acepromazine ............................................................................ > 1 week

Contributed by: Karen Helton-Rhodes, DVM, Diplomate ACVD

Illustration reprinted with permission by the copyright owner, Hill’s Pet Nutrition, Inc. 

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