Intradermal skin tests in equine dermatology: a study of 83 horses.
Authors: Lebis C, Bourdeau P, Marzin-Keller F
Journal: Equine veterinary journal
Summary
# Intradermal Skin Testing in Equine Dermatology: Establishing Diagnostic Parameters Allergic skin and respiratory disease in horses frequently goes unconfirmed by objective testing, leaving practitioners reliant on clinical impression alone. Lebis and colleagues evaluated intradermal skin testing across 83 horses (14 with suspected allergic signs) using six allergens plus positive and negative controls, establishing optimal read times and reaction thresholds for clinical application. A positivity threshold of ≥15 mm diameter or ≥13 mm diameter matching the positive control response proved most discriminatory at 20 minutes; notably, Culex pipiens and Dermatophagoides farinae reactions differentiated allergic from healthy horses, though false positives occurred regularly in non-allergic animals, whilst Tabanus sp. showed poor specificity. Reading reactions at 1 hour produced no additional diagnostic value over 20 minutes, but 4-hour assessments became unreliable due to inconsistent negative control responses. Poor repeatability of results when testing was performed months apart, likely driven by seasonal allergen exposure variations, suggests intradermal testing has significant limitations for confirming specific allergies in clinical practice. Farriers and equine practitioners should recognise that whilst intradermal testing may support—rather than establish—a diagnosis of allergic disease, results require cautious interpretation and integration with clinical findings, and further validation work with expanded allergen panels remains essential before confidence in this diagnostic tool can be recommended.
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Practical Takeaways
- •Use 20-minute readings for intradermal skin testing in horses with a threshold of ≥15 mm diameter to improve clinical reliability; avoid 4-hour readings due to unpredictable reactions
- •Mite and insect allergens show promise for differentiating allergic from non-allergic horses, but recognize that some healthy horses will test positive
- •Plan testing carefully around seasons, as allergen sensitivity varies and affects test repeatability; this test is still developing and should not be the sole diagnostic tool
Key Findings
- •A positivity threshold of ≥15 mm diameter or ≥13 mm with skin thickness similar to positive control was proposed for intradermal skin test interpretation at 20 minutes
- •Culex pipiens and Dermatophagoides farinae showed marked differentiation between healthy and allergic groups, though positive reactions occurred in healthy horses
- •Reaction progression at 4 hours was highly variable with unreliable negative controls, making interpretation difficult
- •Repeatability of intradermal skin tests at 20 minutes was poor, likely influenced by seasonality of allergen sensitivity