Hormone Therapy Table Topic (AAEP 2011)

Topics discussed included induction of ovulation, estrus behavior, and suppression in the performance horse.

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At the 2011 American Association of Equine Practitioners (AAEP) Annual Meeting, held Nov. 18-22 in San Antonio, Texas, approximately 75 equine practitioners from around the country as well as abroad actively debated a list of issues related to hormone use in mares. The discussion centered on topics selected by the audience: Induction of ovulation, estrus behavior suppression in the performance horse, and management of the anestrus and transitional mare.

Although practitioners have been using hormones to induce ovulation in horses for several decades, primarily using human chorionic gonadotropin (hCG) and most recently different forms of the potent GnRH agonist Deslorelin, there are still questions concerning the variability of individual response. Practitioners debated some of these issues and the possible effect of breed and treatment timing in relationship to follicular growth. They particularly stressed the need to evaluate uterine edema (fluid swelling) and cervical relaxation before inducing ovulation. Based on the discussion, failure of ovulation and development of anovulatory hemorrhagic follicles appear to be common problems under some situations and merit more detailed studies. Other factors that might be linked to failure of ovulation were also discussed. Some practitioners reported possible induction of ovulation using Prostaglandin F 2 alpha or analogues in some mares, but this also merits more controlled studies.

Estrous behavior continues to be a common owner complaint about performance mares. Altrenogest is the drug of choice for eliminating this behavior. Veterinarians will often adjust dosage used depending on the individual cases. Some practitioners have been using oxytocin injections between Day 7 and Day 14 of the cycle to inhibit luteolysis (a shutdown or destruction of the progesterone-producing corpus luteum). Practitioners using this technique did not report any adverse effects in treated mares. The discussion centered on the importance of educating clients to actually work with the veterinarian in trying to link the behavior to ovarian activity through a series of examinations. This will allow the veterinarian to better define the problem and determine if hormonal treatment is appropriate. It is important to point out that some behaviors that might be perceived as estrus-related might be caused by other conditions such as vaginitis (vaginal inflammation) or cystitis (bladder inflammation). Using altrenogest to reduce aggressive behavior in stallions was also discussed briefly. However, long-term effects on young stallions have not been fully investigated.

Managing anestrus and the transitional mares remains a big challenge in practice. The best method for advancement of the breeding season remains manipulation of the photoperiod (hours of exposure to daylight). However, more and more research is being conducted using D2-Dopamine antagonists alone or in combination with estrogen and shows great promise. Most of these treatments have not yet been fully implemented in practice, and many of these drugs are only available through compounding pharmacies. An interesting approach presented at the convention is the use of recombinant equine follicle stimulating hormone (FSH), which was shown to be capable of inducing ovulation in anestrus mares. However, the majority of mares return to anestrus after the induced cycle. Attendees also discussed using multiple injections of GnRH to induce ovulation and raise the question of temperature’s effect on follicular dynamics. Managing transitional mares was discussed primarily in regards to using hormonal techniques to reduce the variability in the number of follicular waves before the first ovulation. Several treatments used for inducing cyclicity (ovulation) in anestrus mares are also used for the transitional mares. Altrenogest and progesterone/estradiol are still commonly used to reduce the length of transition period

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