Determination of threshold concentrations of plant pollens in intradermal testing using fluorescein in clinically healthy nonallergic cats

Background: Currently the same allergen concentrations for canine intradermal testing (IDT) are recommended for feline IDT. Feline skin reactions are subtle and more difficult to read than canine reactions. This difference may be due to suboptimal allergen concentrations used for IDT in cats. Hypothesis/Objectives: To determine the irritant threshold concentration (ITC) of 16 pollen allergens using serial dilutions of allergen and intravenous fluorescein. The hypothesis tested was that feline IDT currently is performed at suboptimal allergen concentrations for pollens. Animals: Twenty privately owned healthy clinically nonallergic cats. Methods: IDT was performed in duplicate using 16 pollen allergens (weeds, grasses and trees) at a dilution of 8000 PNU/mL. Two blinded investigators graded the test reactions independently using subjective and objective criteria. Intravenous fluorescein was then administered and the test reactions were re-evaluated. IDT was repeated for any allergen that was positive, using serial dilutions of allergen at a concentration of 6000 and 4000 PNU/mL. Results: The ITC for 2 of 16 of the allergens was determined. The ITC of Cynodon dactylon (Bermuda grass) and Schinus spp. (Peppercorn) was determined to be between 6000 and 8000 PNU/mL. The ITC of all other allergens tested in this study was >8000 PNU/mL. Conclusions and clinical importance: This study confirms that suboptimal allergen concentrations currently are used for feline IDT as the ITC is >8000 PNU/mL for 14 of 16 of the grass, weed and tree pollens evaluated. The ITC of Cynodon dactylon and Schinus spp. was determined to be between 6000 and 8000 PNU/m