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ESTROGEN Therapy in Horses

In some cases, we elect to treat horses exhibiting clinical signs of intermittent upward patellar fixation (IUPF) or Proximal Patellar Hesitation (PPH) with estrogen. As you know, estrogen is a naturally-occurring hormone that is present in all horses (both male and female).

Estrogen is available in two primary forms:

1) Estrone Sulfate:

  • ADVANTAGES: This is a water-based suspension that is easily administered by the client and easily absorbed in the horse's muscle.

  • DISADVANTAGES: Its water-soluble nature results in a shorter duration of effect (since it is absorbed and utilized relatively quickly). Also, this preparation is usually considerably more expensive than it's oil-based counterpart (Estradiol Cypionate). In our experience, estrone sulfate is less effective at modifying behavior in horses over prolonged periods of time.

2) Estradiol Cypionate:

  • ADVANTAGES: This is an oil-based product that is slowly resorbed into the horse's system. Consequently, it has a long-lasting effect in the horse. In our experience, estradiol is far more effective at modifying behavior in horses over prolonged periods of time. Furthermore, it is generally considerably less expensive than estrone sulfate.

  • DISADVANTAGES: Due to it's oil-based nature, this product that is quite "thick" and therefore more difficult to administer to horses. Since it resides in the tissue for longer periods of time, there is greater propensity for secondary complication(s) such as infection. Therefore, we recommend that the injection site be cleaned and disinfected appropriately and that good technique be used during intramuscular (IM) administration.

Estrogen therapy is currently being used widely by veterinarians treating IUPF based on the idea that estrogen (as a hormone) increases the tension of all of the ligaments in the horse (collateral ligaments, cruciate ligaments, distal patellar ligaments, etc.). By increasing the tone of the distal patellar ligaments, we can presumably reposition the
patella (kneecap) to an area where it is less likely to inadvertently interfere with stifle function.

At The Atlanta Equine Clinic, we have not been very impressed with the physiologic ability of estrogen to "tighten" the ligaments. In our experience, most horses treated for IUPF with little or no behavioral component exhibit only marginal response to estrogen therapy.

However, we do often utilize estrogen as a treatment strategy for IUPF, particularly when there is a considerable behavioral component to the problem.

IUPF is generally very frustrating to the horses, represented by frequent bucking, bolting, spinning, kicking, etc. Since estrogen is also a very competent behavior-modificator, we recommend it for horses that exhibit behavioral problems (anxiety, nervousness) or resistance (such as with IUPF or PPH). It seems to help the horses to "relax" and work through the problem more efficiently and safely.

In short, if your horse is exhibiting behavioral resistance we would recommend estrogen administration. If not, then we might consider alternate forms of therapy.

Administration of estrogen is designed to allow us to implement fitness training on our horse effectively and safely. Both medications are effective at helping horses to focus on work and curtailing behavioral resistance. They are also effective at reducing anxiety and promoting relaxation during show season. They are "non-testable", and can therefore be used indiscriminately during this period.

Our preliminary goal is to encourage your horse to "get over the hump" during fitness training. Once a horse is fit, their stifles work better and their training becomes easier. It is at this point that we can consider discontinuing hormonal (estrogen) therapy.

Following is the recommended administration protocol for Estradiol Cypionate or Estrone Sulfate:

Administer 1 dose (5cc) in the muscle twice weekly (e.g. every Monday and Wednesday) for 5 consecutive weeks, then as desired.

If you are using Estradiol Cypionate, remember to clean the injection site thoroughly and scrub with a bacteriocidal soap to avoid any complication associated with injection. The medication is oil-based, and therefore administration is more likely to cause complication when compared to water-based products.

Please click HERE to view our recommended intramuscular injection sites.

Due to the considerable beneficial effects of estrogen on behavior, many clients elect to maintain their horses on some level of the product indefinitely. Clients continue to administer estrogen at rates ranging from once weekly to once monthly. On average, clients maintain their horses on twice monthly doses. In some cases we recommend administering one (5cc) dose each Monday and Wednesday during show season to maximize its effect during the weekend events.

There are no known deleterious side effects to long-term administration of Estrogen in horses.

Estradiol Cypionate is supplied in a 50cc bottle, which contains 10 doses (5cc per dose) and is enough for a 5-week period when administered twice weekly. The cost for a 50cc bottle of Estradiol Cypionate is $98.75 (plus tax for Georgia residents).

Estrone Sulfate is supplied in a 30cc bottle, which contains 6 doses (5cc per dose) and is enough for a 3-week period when administered twice weekly. The cost for a 30cc bottle of Estrone Sulfate is $135.00 (plus tax for Georgia residents).


If you have any questions regarding Estrogen Therapy for your horse please call our office at (678) 867-2577. We look forward to serving you!
THE ATLANTA EQUINE CLINIC: 1665 Ward Road, Hoschton, Georgia 30548 - ph. 678-867-2577

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