Peter Green MRCVS shares important news that may explain why some horses have been testing positive for aminorex
Aminorex is an amphetamine-like stimulant drug banned in racing and other equestrian competitions because horses given clinical doses would be hyperactive and may perform better.
Doping agencies in the USA and Europe have been finding aminorex in the urine of racehorses and sports horses since the early 2000s. Trainers and owners have been heavily fined and banned as a result, despite pleading their innocence.
A number of these cases were eventually traced to the cattle wormer levamisole, which is sometimes used as an immune stimulant for horses with chronic lethargy or infections. As a result, trainers and riders and owners were warned to avoid the use of levamisole in competition horses.
But occasional cases of aminorex in post-competition urine samples have continued to crop up on both sides of the Atlantic. Affected trainers, riders and owners have been adamant that they have avoided the substance. So where is the aminorex coming from?
Equine vets in Kentucky, USA, have carefully scrutinised some of the affected urine samples in an attempt to answer this question. With thorough analysis, they found unidentified compounds in the urine that suggested a plant origin.
After consulting the botanical library, they discovered that plants in the Brassica family are known to contain a chemical called barbarin – which is similar to aminorex.
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Barbarin is named after the plant from which it was originally isolated, a weed called Barbarea vulgaris, called yellow rocket in the US and know in the UK as common wintercress.
To test whether this weed is implicated in the aminorex problem, the vets picked yellow rocket plants, then cleaned, dried and fed them to horses at the University of Kentucky.
Within eight hours of eating the plants, aminorex was detected in the horses’ urine. This shows the active compound barbarin is metabolised by the horse into aminorex, the amphetamine.
It seems likely that other plants in the brassica family may contain barbarin, and because this is the cabbage family, they could be palatable – even attractive – to horses.
The key question now is whether the very low levels of aminorex in these horses have an impact on their performance.
