The FEI’s threshold for the common nonsteroidal anti-inflammatory painkilling drug aspirin in horses appears to be too generous and leaves the door open for misuse, German researchers suggest.
Although aspirin – more formally known as acetylsalicylic acid – is not frequently used as a therapeutic agent in horses, its metabolite salicylic acid is of special interest in equestrianism since it is a natural component of many plants eaten by horses, such as alfalfa hay or willow bark.
For this reason, thresholds for salicylic acid in urine and blood plasma were set by horse sport organizations rather than an outright ban.
Manfred Kietzmann and his colleagues set out to investigate plasma and urine concentrations of salicylic acid after oral administration of three different single dosages − 12.5 mg/kg, 25 mg/kg and 50 mg/kg of acetylsalicylic acid − to eight horses in a cross-over designed study.
The study team, reporting in the open-access peer-reviewed journal BMC Veterinary Research, noted that, in the 12.5 mg/kg group, salicylic acid concentrations in urine peaked at 2675 μg/mL two hours after receiving the dose. Plasma concentrations peaked after 1.5 hours, at 17 μg/mL.
In the 25 mg/kg group, maximum concentrations were detected after 2 hours in urine (2785 μg/mL) and 1.5 hours in plasma (23 μg/mL).
In the 50 mg/kg group, maximum concentrations were detected after 5 hours in urine (3915 μg/mL) and 1.5 hours in plasma (45 μg/mL).
The plasma half-life for salicylic acid was calculated to vary between 5 and 5.7 hours.
The urine concentration fell below the threshold of 750 μg/mL set by the FEI and most of the horseracing authorities between 7 and 26 hours after administration of the 12.5 and 25 mg/kg doses of the drug, and between 24 and 36 hours after administration of the 50 mg/kg dose.
“The established salicylic acid thresholds of 750 μg//mL urine and 6.5 μg/mL plasma appear too generous and are leaving space for misuse of the anti-inflammatory and analgetic compound acetylsalicylic acid in horses,” the study team concluded. This was especially so for countries where horse feed was not naturally rich in salicylates, they added.
The authors noted that concentrations higher than the 750 μg/mL threshold were generally detected for only 24 hours after the 50 mg/kg dose and for 12 hours after administration of the 25 or 12.5 mg/kg dose. After administration of 12.5 mg/kg one horse did not exceed the threshold at all.
Salicylic acid is one of the oldest nonsteroidal anti-inflammatory drugs and has been used as a fever-reducing agent and in acute rheumatic events in humans for a long time.
In veterinary medicine, it has been given in cases of joint disease in dogs and horses.
Acetylsalicylic acid – what most of us call aspirin – was synthesized to have the same effects as salicylic acid but with an improved gastric and systemic tolerability. After being swallowed, the drug is rapidly converted to salicylic acid.
The research was centered at the University of Veterinary Medicine Hannover, Foundation.
The study team comprised Kietzmann, Kathrin Buntenkötter, Maren Osmers, Ina Schenk, Wilhelm Schänzer, Marc Machnik and Michael Düe.
Pharmacokinetics and in vitro efficacy of salicylic acid after oral administration of acetylsalicylic acid in horses
Kathrin Buntenkötter, Maren Osmers, Ina Schenk, Wilhelm Schänzer, Marc Machnik, Michael Düe and Manfred Kietzmann
BMC Veterinary Research2017 13:28 DOI: 10.1186/s12917-017-0955-1