Suspected aspirin resistance in individual healthy adult warmblood horses.

This study experimentally investigates the dosage regime of aspirin in horses and its differential effect on platelet function, suggesting that some horses may be resistant to aspirin’s antiplatelet effects.

Objective of the Study

• The main purpose of this research was to establish a practical dosing scheme for administering aspirin to healthy adult warmblood horses, with a focus on observing the inhibitory effects on platelet function.

Methodology and Participants

• The study was conducted prospectively on ten horses, administered with an initial loading dose of aspirin (between 4.7 to 5 mg/kg), followed by daily maintenance doses (1 to 1.3 mg/kg) for four days. The drug was given orally every morning.

Assessment Measures

• The researchers utilized whole blood impedance aggregometry, a tool used for measuring platelet function, which applies three different tests – ASPItest, COLtest, and ADPtest. Alongside these, they also measured the serum salicylic acid concentration, which is the main metabolite of aspirin.

Key Findings

• ASPItest results showed significant differences in platelet inhibition at 24, 48 (p < .05) and 96 hours (p < .01) post-aspirin administration.
• No significant change was detected for COLtest and ADPtest.
• There was a significant increase in serum salicylic acid concentrations at 6 and 12 hours (p < .01).
• Interestingly, despite the increase in serum salicylic acid concentration, three out of ten horses failed to show any inhibitory effect on platelet function, leading researchers to suspect aspirin resistance.
• In the case of the remaining seven horses, reduced platelet aggregation, as measured by ASPItest, was seen. The reduction ranged between 37% and 100% from baseline at 6 and 12 hours and between 0% and 98% over the next four days.
• However, no correlation was found between serum concentration of salicylic acid and aggregometry results.

Implications of the Study

• The study sheds light on the possibility that equine platelets may not be as susceptible to the effects of aspirin as expected.
• This potential resistance to aspirin could impact its anticoagulant efficacy in treating and preventing diseases linked to increased platelet activation, such as endotoxemia or laminitis in horses.