Allergic diseases in dogs and cats, particularly inhalant allergies (atopy) and flea-bite hypersensitivity, are clinically important and becoming more prevalent. Diagnosis is based on patient history, physical findings, evaluation of differential diagnoses, and appropriate allergy testing that correlates with patient history.

Atopy is a common disease of genetic predilection and affects an estimated 10-15% of dogs, as well as cats. It usually starts at a young age and most cases are lifelong. About 70% of affected dogs manifest clinical signs between ages 1-3 years, first as a seasonal problem which progresses to year-round. Pruritus is the predominant sign, resulting in self-trauma especially of the face, feet and abdomen. The central pathophysiologic mechanism is the inherited tendency to produce IgE antibodies in response to an antigenic stimulus delivered typically by respiratory or percutaneous routes. Production of allergen-specific IgE alone is not sufficient to cause clinical signs, unless the IgE first binds to the high-affinity Fc epsilon receptor (Fc* RI) on mast cells (or basophils) in the canine epidermis and dermis. Cross-linking of the mast cell-bound IgE by the offending antigen(s) results in degranulation and rapid release of the pre-formed pharmacologically active mediators that cause pruritus, erythema and inflammation of the skin (e.g. histamine, heparin, proteolytic enzymes).

Recent advances in in vitro serologic testing for allergies in veterinary medicine have facilitated diagnosis as they offer the advantages of simplicity, lowered costs, and less discomfort for the pet. However, questions remain about the specificity of these ELISA-based or RAST tests in comparison to the "gold standard" in vivo intradermal skin test (IDST). Another concern is their accuracy in predicting IgE-mediated allergic disease in patients receiving corticosteroids. Research has shown that most anti-IgE monoclonal and polyclonal detection antibodies used to identify IgE also cross-react with IgG. Since IgG is present in serum in concentrations 103-105 times higher than IgE, even a small degree of cross-reactivity can lead to false positive results. Use of the Fc*RI to identify allergen-specific IgE in the serum of dogs and cats therefore offers significantly improved specificity for IgE detection of allergic disease.

To respond to the need for improved diagnostics, Antech Diagnostics has recently introduced the Heska™ Allercept™ Detection System for in vitro allergy testing. This system uses the recombinant alpha chain of Fc* RI to detect allergen-specific IgE.

Results using the Heska Corp. assay system to date, show that:

• Testing of 300 atopic and 66 normal dogs evaluated by IDST and Heska Fc*RI-based ELISA, revealed 90% agreement for 14 allergens. Sensitivity (probability of obtaining a positive ELISA result when the IDST result was positive) was 86%, and specificity (probability of obtaining a negative ELISA result when the IDST was negative) was 92%. Results in 25 normal intradermal skin tested cats showed 89% specificity when IDST and ELISA results were compared. Use of the Fc*RI*-based ELISA in a group of 16 flea sensitized cats when compared to IDST results revealed 87% sensitivity, 71% specificity and 82% accuracy.
  
• Use of Fc*RI*-based ELISA and purified flea salivary antigens to characterize levels of flea saliva-specific IgE in the serum of normal dogs and dogs rendered hypersensitive to flea bites as determined by IDST, revealed 89% agreement between the IDST and ELISA results in a group of 66 dogs.
  
• In a population of 74 client-owned and experimental asymptomatic and flea bite allergic dogs, comparison of Fc*RI*-based ELISA results to IDST revealed 88% agreement.
  
• When Fc*RI*-based ELISA results to flea salivary antigens were compared to IDST reactions in 16 flea-sensitized cats, the ability of the assay to predict a positive IDST result (sensitivity) was 87%. Overall agreement was 82% for both positive and negative results.