Aggression can be a serious problem in some stallions, particularly mature ones. Unruly or violent behaviour may endanger a stallion’s rider and handler and limit a stallion’s ability to perform to its potential.
The good news is that these aggressive traits may be easy to eliminate in the future with a behaviour-controlling vaccine.
When training fails to rein in aggressive behaviour, horse owners turn to castration or female hormone therapy to gain control of the boisterous stallions. Both treatments reduce the production of testosterone – the male hormone that triggers undesirable aggressive behaviours.
Castration is considered by many to be the treatment of choice for aggressive stallions, but it is not always ideal. Some stallions remain aggressive after castration because their behaviour has become ingrained and no longer relies on the influence of testosterone.
Another concern with castration is permanent loss of breeding potential. If a castrated male excels at its sport later in life, it will be unable to pass its superior genes on to future generations. This is a common problem in three-day eventing horses in which many top ranked competitors are geldings.
Finally, castration is more risky in a mature stallion than a young one because the testicular blood vessels are larger. Extensive bleeding is more likely during surgical removal of the testicles.
Female hormone therapy, though easier on a stallion than surgery, provides less predictable results than castration. Some stallions become more tranquil, but others retain their aggressiveness. Hormones can also have negative long-term effects on fertility.
Researchers have now developed a vaccine against Gonadotrophin Releasing Hormone.
GnRH is a hormone naturally released by an area of the brain that controls the production of all reproductive hormones. For this reason, it is often called the master sex hormone.
The GnRH vaccine stimulates the horse’s immune system to produce antibodies against its own natural GnRH. GnRH is destroyed, so the amount in the blood drops to a negligible level. This is the start of a domino effect. The sex hormone – luteinizing hormone – is only produced when stimulated by GnRH. When the amount of GnRH in the blood is low, so is the level of LH.
Because the testicles depend on LH to produce the male hormone testosterone, testosterone production is halted when LH is lacking. The result is a lower level of testosterone.
Vaccine trials have confirmed that the administration of the GnRH vaccine to stallions reduces testosterone production enough to decrease testicular size and sperm production, lower the animal’s sex drive and lessen aggressive behaviour.
Fortunately, the negative effects on semen quality are reversible so a stallion can be vaccinated and then be used for breeding later in life. Its sperm numbers will return to normal once the vaccinations are stopped.
The optimum time for revaccination to maintain the positive effects has yet to be determined because antibody levels against GnRH are variable between individual horses. At this time, it looks like the vaccine will need to be administered every three to four months.
The beneficial effect of the GnRH vaccine is not limited to stallions. Because it reduces sex hormones in both sexes, it also has potential in mares. Some mares express sexual behaviours during heat cycles that reduce their competitive edge. One way to remove these hormonal effects is to breed the mare. Unfortunately, pregnancy is not a suitable solution for a working mare. The GnRH vaccine could offer a viable solution. The vaccine would stop the mare’s cyclic behaviour, thereby controlling unwanted and sometimes dangerous behaviour.
Jeff Grognet is a veterinarian and writer practising in Qualicum Beach, B.C.