The fourth cluster, September/October 2020 involved 18 or so identifications in French racing, reported by the French Laboratories des Courses Hippiques, (LCH), and in July 2021, a fifth cluster of 10 Zilpaterol identifications in South Africa. The third cluster, March 2019, involved 24 racehorses in Mauritius this cluster traced to South African feedstuffs, triggering an alert concerning possible Zilpaterol feed contamination in South African racing. The first cluster, April 2013, involved 48 racehorses in California the second cluster, July 2013, involved 15 to 80 racehorses in Hong Kong. Zilpaterol readily presents in horse urine, yielding clusters of feed related Zilpaterol identifications in racehorses. In horses, urinary steady-state concentrations are reached 25 days (5 half-lives) after exposure to contaminated feed. These “identifications” occur because Zilpaterol is highly bioavailable in horses, resistant to biotransformation and excreted as unchanged Zilpaterol in urine, where it has a 5 day or so terminal half-life. Trace amounts of Zilpaterol can transfer to horse feed, yielding equine urinary “identifications” of Zilpaterol. Zilpaterol is a β2-adrenergic agonist medication approved in certain countries as a cattle feed additive to improve carcass quality.
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