The Wide Applications of Estradiol for Horses

Estradiol is a medication and naturally-occurring steroid hormone. It is an estrogen and is used mainly in hormone therapy. Estrogen therapy is currently being used widely by veterinarians treating intermittent upward patellar fixation (IUPF) in horses. Intermittent upward fixation of the patella occurs when the medial patellar ligament remains hooked over the medial trochlear ridge of the femur and locks the reciprocal apparatus with the limb in extension. A horse with upward fixation of the patella stands with the hindlimb fixed in extension with the fetlock flexed.²

The basis for the development of the estrogen therapy regimen lies in the belief estrogen (being a hormone) increases the tension of all of the ligaments in the horse (collateral ligaments, cruciate ligaments, distal patellar ligaments, etc.).¹ By increasing the tone of the distal patellar ligaments, it is thought that the patella can be repositioned to an area where it is less likely to inadvertently interfere with stifle function.

Treatment of Joint Disorders in Horses

While some veterinarians express mixed results regarding the physiologic ability of estrogen to tighten a horse’s ligaments, many continue to utilize estrogen as a treatment strategy for IUPF, particularly when there is a considerable behavioral component to the problem.¹ IUPF can be very frustrating to the horses; depending on individual temperament, it can result in frequent bucking, bolting, spinning, kicking and other undesirable behaviors.

Since estrogen tends to be very effective in the area of behavior modification, many practitioners recommend it for horses that exhibit behavioral problems such as anxiety, or in cases where IUPF or proximal patellar hesitation (PPH) are present. In these cases, estrogen tends to help horses relax and work through the problem more effectively and safely.

Estradiol cypionate (ECP) is used to induce estrus in horses, and is frequently the hormone therapy of choice among veterinarians treating horses for IUPF and PPH. The administration of estradiol cypionate is designed to allow handlers to implement fitness training effectively and safely. This medication is effective at helping horses focus on work and mitigating behavioral resistance. It is also effective at reducing anxiety and promoting relaxation during show season.

The preliminary goal is to encourage a 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.¹

Moderation of Estrus Behavior

Estradiol cypionate functions like a naturally occurring estrogen, performing a variety of functions, including:

• Reproductive Effects
• Normal development of the reproductive tract and mammary glands
• Expression of normal estrus behavior
• Enhance uterine resistance to infection
• Gestational support during pregnancy
• Uterine contractions and cervical relaxations during pregnancy
• Skeletal Effects – Causes calcium deposition, epiphyseal closure at the growth plates and increased bone formation
• Inhibitory Effects – can inhibit ovulation and lactation via the pituitary gland.

Estradiol cypionate is a long acting pharmacological ester of estrogen, with reported durations of action of 14-28 days.³

Estrus behavior can be undesirable in performance and working horses and can be suppressed in mares by administration of progestagens. Behavioral estrus may be induced in anestrous or ovariectomized mares by administration of estradiol or estradiol cypionate.

Estradiol Cypionate for Treatment of Bacterial Placentitis

In 2017, researchers from the University of Illinois Urbana-Champaign and Brazil collaborated on a study in which they evaluated the efficacy of various combinations of estradiol cypionate and a long-acting form of altrenogest (a progestin related to progesterone) to treat ascending bacterial placentitis (placental inflammation).

In the study, “horses consisted of one control group of healthy pregnant mares and five groups of pregnant mares in which researchers induced bacterial placentitis by injecting Streptococcus equi subspecies zooepidemicus into the uterus. Group 1 received a traditional placentitis treatment of the antimicrobial trimethoprim-sulfamethoxazole (TMS) and the anti-inflammatory drug flunixin meglumine (FM). Group 2 received TMS, FM, and altrenogest, while Group 3 received TMS, FM, altrenogest, and estradiol cypionate; Group 4 received TMS, FM, and estradiol cypionate without altrenogest; and the fifth group was untreated.”⁴

The untreated group had a significant increase in dystocias (difficult births), preterm deliveries, and foal loss. All treatment groups benefited from their respective treatments and their foal survival rates were similar to those among the normal control mares.

However, the group treated with TMS, FM, and estradiol was the only one with no compromised foals and similar foal birth weights as controls. This suggests that the addition of estradiol may benefit mares with placentitis, and that the addition of altrenogest did not appear to improve the outcome.⁴