BHA Notices Section
Administration of Levamisole/Tetramisole to horses in training
Levamisole and Tetramisole are sold as wormers for farm livestock, and are almost identical substances sharing the same molecular make-up but differing in their three-dimensional orientation They are not licensed for use in horses but the British Horseracing Authority is aware that they are being given to horses in training with and without veterinary advice, primarily as 'immune stimulants' or 'pick-me-ups'. Since these have been shown to have an effect on the immune system (in other species), Levamisole and Tetramisole are prohibited substances. Since 1999 these drugs have been found in horses tested on racedays, resulting in disqualifications and fines.
In addition, the Authority is, in some of the more recent cases, also finding the substance Pemoline. This is a drug that stimulates the brain, has historically been used for treatment of serious psychological disorders in people and has been shown to have a specific performance enhancing effect in horses (similar to that seen with Amphetamine). Pemoline was withdrawn from the market as a human drug due to it being linked with liver toxicity.
Finding Levamisole/Tetramisole in racehorses is itself of concern, but finding such a significant performance enhancing drug as Pemoline in racehorses is of great concern. The Authority has investigated the reason for these coincidental findings of Pemoline with Levamisole and Tetramisole. Studies at HFL, have demonstrated a mechanism by which Pemoline is produced in horse liver cells following exposure of these cells to a metabolite of Levamisole and Tetramisole. We therefore believe that the probable source of Pemoline found coincidentally with Levamisole and Tetramisole is from the metabolism in the horse of these two latter drugs.
In view of the association of Pemoline with the administration of Levamisole/Tetramisole we have reviewed the available scientific literature for evidence for the rationale use of Levamisole and Tetramisole in horses. Although Levamisole and Tetramisole have been shown to improve immune function in the laboratory, and under certain conditions in other species, we have not found any material evidence base for the rationale use of Levamisole and Tetramisole in horses for immune modulation (we are of course happy to be advised to the contrary). We would add that there are alternative licensed wormers available for use in the horse and that the drug safety of Levamisole, Tetramisole and Pemoline is not well characterised in horses.
The Rules of Racing state that every treatment must be fully justifiable by the medical condition of the horse receiving the treatment. Given this requirement, the lack of evidence for the rationale use of Levamisole/Tetramisole in horses for immune stimulation, and the likely association between use of Levamisole/Tetramisole in horses and the presence of Pemoline, we wish to issue clear advice to Trainers and their veterinary surgeons.
The Authority's position on this matter is therefore that as there is no material evidence base for the rationale use of Levamisole and Tetramisole in horses for immune modulation, and use of these drugs is associated with the finding of Pemoline, which is known to enhance performance, the Authority strongly advises against the use of Levamisole/Tetramisole in horses in training. Any such use is advised to be under veterinary supervision, following a relevant diagnosis, and should include an extensive withdrawal period. If Pemoline is found in horses, whether related to the use of Levamisole/Tetramisole or not, the Authority will regard this as a serious finding because Pemoline has been shown to have clear performance enhancing effects.