Evaluation of an in vitro sulphidoleukotriene release test for diagnosis of insect bite hypersensitivity in horses.
Authors: Baselgia S, Doherr M G, Mellor P, Torsteinsdottir S, Jermann T, Zurbriggen A, Jungi T, Marti E
Journal: Equine veterinary journal
Summary
# Insect Bite Hypersensitivity: Evaluating a Cellular Diagnostic Test Insect bite hypersensitivity (IBH) is an IgE-mediated allergic response to *Culicoides* and *Simulium* bites affecting many horses, yet diagnosis has traditionally relied on clinical observation and exclusion, creating a need for objective laboratory confirmation. Baselgia and colleagues evaluated the cellular antigen stimulation test (CAST)—which measures sulphidoleukotriene release from activated basophils—in 314 horses, including longitudinal testing over 12 months, to assess its diagnostic accuracy and stability for IBH. Using adult *Culicoides nubeculosus* extracts, the CAST achieved 78% sensitivity and 97% specificity, with positive and negative predictive values exceeding 80% across the likely prevalence range of IBH (12–52%). Notably, test positivity fluctuated seasonally (90% in November to 68% in March) and males showed stronger responses than females, whilst clinical severity and age did not affect results; additionally, some horses demonstrated cross-reactivity to *Simulium* allergens alongside *Culicoides* sensitisation. For practitioners, this means CAST offers a reliable, objective method for confirming IBH diagnosis—particularly valuable for identifying subclinical sensitisation—though seasonal variation and sex-related differences warrant consideration when interpreting results and counselling clients on management timing.
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Practical Takeaways
- •The CAST test can reliably identify horses with IBH using Culicoides extracts, making it a useful diagnostic tool for managing insect bite allergies in your practice
- •Seasonal variation in test results should be considered when interpreting results; positive results are more common in autumn/winter months
- •The test can identify asymptomatic but sensitised horses, allowing early intervention and management strategies before clinical signs appear
Key Findings
- •CAST with adult C. nubeculosus extracts achieved 78% sensitivity and 97% specificity for IBH diagnosis
- •Horses with IBH showed significantly higher sulphidoleukotriene (sLT) release compared to controls
- •Most IBH-affected horses are sensitised to Culicoides allergens, with some showing additional sensitisation to Simulium allergens
- •Test positivity varied seasonally (90% in November to 68% in March) and was significantly higher in affected males than females