Determination of threshold concentrations of multiple allergenic extracts for equine intradermal testing using normal horses in three seasons.

The study focuses on determining the threshold concentrations (TC) of allergens in normal horses using intradermal testing (IDT), and the potential variations in reactivity due to seasonal changes. The research suggests suitable testing concentrations for various allergens and illustrates that reactions occurring within 1-4 hours of testing may be minor.

Study Methodology

• 41 healthy horses were assessed for their reactivity to aqueous allergens administered intradermally. This was to identify the TC, which is potentially significant for equine IDT.
• Each horse was tested thrice over a duration of one year to observe the possible variation in reactivity due to different seasons.
• The testing involved injecting three to five serial dilutions of 27 allergens on each occasion.
• The injection sites were evaluated four times post the injection: after 15 minutes, 1 hour, 4 hours, and 24 hours.
• The TC was defined as the maximum allergen concentration at which less than 10% of horses exhibited positive reactivity. Positive reactivity was subjectively scored, on a scale of 0 to 4, with a score of 2 or more deemed positive.

Study Results

• The TC was determined for several allergens, including nine types of pollen, four kinds of moulds, seven insects, and three out of four storage mites. However, due to excessive reactivity at the lowest concentrations tested, the TC was not determined for dust mites.
• There were minor seasonal variations in the TC for certain allergens.
• A trend towards progressive sensitization was observed with repeated testing for grain mill dust mix.
• At the determined TC, more than 10% of horses exhibited positive reactivity at 1 hour and 4 hours for nine out of 19 allergens, including pollens, mosquito, and storage mites. However, positive reactivity was infrequently noted at 24 hours.

Findings and Implications

• The research implies that appropriate testing concentrations of allergens for equine IDT in atopic horses could be equal to or more than 1000 PNU mL(-1) for pollens and moulds, 60 to 250 PNU mL(-1) for most insects, and less than 1 : 12,000 w v(-1) for dust mites.
• The findings also hint that the reactions observed at 1-4 hours after testing may not be significant.