GnRH suppression therapy has been used for some time in research, attempted treatment for EVA and control of wild horses herds. The product being discussed actually causes the body to reduce production of GnRH which in turn in the stallion suppresses other hormone production that ultimately results in a suppression of Testosterone. A side-effect of this is a suppression of sperm production (through reduced FSH production).
The mode of effect in attempted treatment of EVA is that the EAV (the disease is EVA, the virus is EAV) is testosterone-dependent for maintenance in the body, which is why only stallions harbour the virus on an ongoing basis (mares and geldings are only infectious during the acute phase of up to about 28 days). In the stallion the virus may be harboured in the secondary sex glands and shed during breeding or semen collection. If the shedding stallion is castrated, he will cease to shed the virus not because his testicles are removed (which is not where the viruses is harboured), but because testosterone levels subsequently drop. It was therefore hypothesised that GnRH-suppression therapy - which in turn suppresses Testosterone production - may therefore result in elimination of the virus from the body. This was found to be the case in a percentage of animals, but was not absolute. In some animals when the testosterone levels again rose, the virus was found to still be present. This has led to changes in some International regulations for shipping semen - New Zealand for example requires a declaration that the stallion from whom the semen is being shipped has not undergone GnRH suppression therapy.
Use in wild horse herds has had an interesting - and unfortunate - side effect. It seemed logical that treating herd stallions with the suppression therapy would reduce the resulting foal crop numbers, but because the GnRH suppression results in Testosterone suppression it upset herd dynamics, as the dominant herd sire no longer had the levels of Testosterone required to remain dominant. A similar effect has been seen where herd sires were gelded and returned to the herd.
It was inevitable that given the negative impact on Testosterone production that unethical trainers and competitors would look at the use of GnRH suppression therapy to enhance performance by "stallions". Progestin therapy has also been used (notably Regumate) for the same effect. While a suppressive effect on stallion-like behaviour has been seen in some of these treated animals (in others - particularly older animals - they behave as a result of learned responses and thus the behaviour may not change), the long-term effect of treatments have not been analysed adequately to consider them safe for fertility effect. One also has to question the ethical - and possibly eventually legal - question of whether one is truly breeding to a "World Champion" stallion if he is treated with Testosterone suppressing therapies, or if one is actually breeding to a potentially raving lunatic on drugs. Is this a fraudulent presentation of the stallion's potentials - both performance and behaviour wise?
Another interesting article has been presented discussing the use of drugs in competition animals in a more widespread manner, which can be seen The New York Times site today. It's another interesting and thought provoking read and should hopefully result in readers - equestrian and otherwise - asking questions about the ethics and controls being seen in competition these days.