Insect Bite Hypersensitivity in Horses is Associated with Airway Hyperreactivity

Background: Genetic and epidemiologic evidence suggests that in horses, as in other species, different manifestations of hypersensitivity may occur together. Hypothesis: Horses affected with insect bite hypersensitivity (IBH) show airway hyperreactivity (AH) to inhaled histamine, even in the absence of overt clinical signs of equine asthma (EA). Animals: Twenty-two healthy controls (group C), 24 horses suffering from IBH alone (group IBH), and 23 horses suffering from IBH and EA (group IBH/EA). Methods: The clinical histories were assessed using 2 standardized questionnaires, the Horse Owner Assessed Respiratory Signs Index (HOARSI), and IBH scoring. Horses were classified as EA-affected if their HOARSI was >1 and as IBH-affected if IBH score was >0. Confounding disorders were excluded by clinical examination. The arterial partial pressure of oxygen (PaO2) was measured and flowmetric plethysmography used to assess airway reactivity to increasing doses of inhaled histamine. Results: The median histamine provocation concentration (PC) when Delta(flow) values increased by 35% (PC35) was significantly higher in group C (5.94 [1.11-26.33] mg/mL) compared to group IBH (2.95 [0.23-10.13] mg/mL) and group IBH/EA (2.03 [0.43-10.94] mg/mL; P < 0.01). The PC50 and PC75 showed very similar differences between groups. Furthermore, PaO2 was significantly lower in group IBH (84 +/- 8 mmHg) and group IBH/EA (78 +/- 11 mmHg) compared to group C (89 +/- 6 mmHg; P < 0.01). Conclusions and Clinical Importance: IBH is associated with AH and decreased PaO2, even in the absence of overt respiratory clinical signs.