Sexual Misbehavior Vaccine?

I am looking into ways to settle down a colt that is showing full-blown sexual interest already at a 1 1/2 years of age. He's just too much for us to handle, but we are not ready to geld him. If you try to correct him when he's all excited, he rears and turns toward you and almost mounts you. I'm not sure if he's just playing with us like colts do with each other, but sometimes he comes right at you as if he is trying to mount you or wrestle you to the ground. Sometimes he has an erection and sometimes not. But whatever his goal, we need to cool his jets.

We heard that there is a vaccination against sexual behavior. How does that work? How well does it work? Does it make them sterile?     via e-mail

What you might have heard about is a fairly new equine vaccine that so far is marketed only in Australia. It is targeted against the reproductive hormone GnRH, and it was developed and is marketed for suppressing ovulation and estrus in mares. There has been discussion about its potential use in stallions, and you hear from time to time that people are trying it off label in stallions. Sometimes it is referred to as chemical castration.

The appeal is to have a pharmacologic and possibly reversible removal of sex drive rather than surgical castration for suppressing sexual and aggressive behavior and/or fertility. As far as I know, an anti-GnRH or other type of vaccine protocol is not yet on the market anywhere for use in stallions.

How Does It Work?

GnRH is a hormone from the brain that plays a role in the secretion of the reproductive hormones downstream in the pituitary gland, which in turn affects the production and release of hormones from the testicle. In this way, GnRH is a key regulator of reproductive physiology and sexual behavior, both in mares and stallions. The vaccine is based on the concept of disrupting the effectiveness of the animal's own GnRH by causing the animal to produce antibodies against GnRH.

The anti-GnRH vaccine is essentially a GnRH molecule that has been modified by attaching a foreign protein. When this modified molecule is administered to the animal, the foreign protein incites an antibody response to GnRH in general. Sometimes an additional substance, called an adjuvant, is included in the vaccine protocol to further promote the immune response to the foreign protein.

How Well Does It Work?

Most of the work on efficacy of anti-GnRH vaccines so far has been with mares, but theoretically it should work the same way in stallions. A group of researchers from Utrecht University in the Netherlands and their colleagues in France have been working on development of an anti-GnRH vaccine for use in stallions. At the recent International Stallion Symposium in Hannover, Germany, Dr. Tom Stout from Utrecht presented a summary of the work to date (T.A.E. Stout, "Modulating Reproductive Activity in Stallions, A Review." Animal Reproduction Science 89:93-103).

His report indicated that the immune response has been variable among individual stallions in terms of the antibody levels, and so efficacy would be expected to vary accordingly. In the research so far, the immune response was greater in stallions four years of age and younger than in mature stallions.

How Long Does It Work?

Each of the studies reported so far during the development of an anti-GnRH vaccine for stallions has followed different protocols, so it is difficult to compare or combine the results concerning how long a vaccination might be effective. The duration of suppression of steroid hormones from the testicle is related to the degree of immune response. Since there is a variable response among individual stallions, there is also expected to be considerable variability in how long the antibodies from each booster will effectively suppress GnRH.

In one study from the Netherlands, testicular hormones were suppressed for four or five months beginning after the second injection (first booster). So to maintain suppressed hormone levels, it would be expected that a stallion would need periodic booster vaccinations. But more work needs to be done before that is known.

Does It Make the Stallion Sterile?

Since the hormones that support sperm production and maturation are also affected by the blocking of GnRH, sperm production is also suppressed. In studies so far, both the number of sperm and the percentage of normal sperm have been reduced. Again, the individual stallion response has varied quite a bit, the suppression has been far from total, and it appears to have been temporary. In one of the Utrecht studies in which young pony stallions received two anti-GnRH vaccinations 11 weeks apart, there was measurable depression of total and normal sperm numbers. But within just a few months after the antibody levels dropped, the sperm numbers returned to levels equal to untreated control stallions. Sperm production capacity of the testicles appeared to recover within less than a year after a two-vaccination series.

Since no long-term studies have been undertaken, long-term effects (in years) have not been studied. One of the proposed applications of anti-GnRH vaccination is for birth control of wild populations, so there will likely be work to address protocols that induce temporary sterility.

Alternative Recommendations?

Since this anti-GnRH vaccination approach is not yet developed for stallions, what would we recommend for your current situation? You might get this colt to a trainer who is good at handling stallions and enjoys figuring out positive ways to overcome these problems. He or she could evaluate the behavior and make training suggestions, or if necessary, work with the colt to get him organized for you, then share tips on how to avoid that type of behavior. While the hormones certainly play a role in these behaviors, they are usually not the whole story with behavior such as you describe.

This might be play behavior rather than sexual behavior. Sometimes there are a few moves the handler makes unknowingly that provoke inter-male play behavior. One example that we commonly see is the type of halter and chain shank placement over the nose or through the mouth, that when inadvertently jerked, elicits rearing and wrestling in some young colts. In some cases, just a few minor changes in the method of restraint and handling style and/or mild and well-timed corrections of the colt's behavior will eliminate the problem peacefully.

About the Author

Sue McDonnell, PhD, Certified AAB

Sue M. McDonnell, PhD, is a certified applied animal behaviorist and the founding head of the equine behavior program at the University of Pennsylvania's School of Veterinary Medicine. She is also the author of numerous books and articles about horse behavior and management.

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