Why some mares respond to hormones and others don't (Proceedings)

The use of ovulatory inducing agents is a key tool in the breeding management of mares. In order to maximize the efficiency of ovulatory inducing hormones it is imperative to have a clear understanding of the physiology and the dynamics of follicular growth during the mare's estrous cycle. The estrus period is characterized by receptivity to the stallion, cervical relaxation, presence of a large or dominant follicle and endometrial edema. The duration of behavioral heat is on the average 5 to 7 days but can vary widely between mares. The diestrus period is characterized by lack of stallion receptivity, presence of a corpus luteum, a tight cervix and lack of endometrial edema (circulating progesterone levels > 2 ng/ml). Although the normal mare diestrus lasts approximately 15 days, mares can, and often do, develop large preovulatory size follicles during diestrus.

Monitoring of the estrous cycle

Ultrasonographic examination of the reproductive tract of the mare has become standard practice. Monitoring and determination of the stage of the estrous cycle, early pregnancy diagnosis, diagnosis of pathological reproductive conditions and monitoring responses to uterine therapies are some of the uses of ultrasonographic technology in equine reproduction.

In order to implement therapies to hasten equine ovulation, the presence, detection and interpretation of endometrial edema is critical. Normal mares under the influence of estrogen will have edematous hyperplasia of the endometrial folds. The endometrial edema increases progressively during the first few days of estrus and decrease as the mare approaches ovulation (Samper et al. 1997). Alterations of this normal pattern such as the presence of endometrial edema after ovulation or during diestrus could be an indication of endometrial inflammation or an underlying problem.

Maximal pregnancy rates are obtained when mares are bred within 48 hours prior to ovulation by natural mating, 12-24 hours prior with cooled shipped semen and less than 12 hours with frozen semen. In order to breed mares at these "ideal" times, it is imperative that estrus be detected and ovulation be predicted accurately.

Induction of estrus

In the normal cycling mare, estrus is induced exclusively by terminating the luteal phase with an injection of prostaglandin. The authors use an intramuscular injection of 7.5 mg of prostaglandin F 2 alpha for this purpose as early as 5 days post ovulation. Onset of estrus and ovulation after a prostaglandin treatment usually occurs in 3-4 and 8-10 days respectively. If there is a large size follicle on the ovaries at the time of treatment, ovulation is possible as early as 72 hours after treatment without overt signs of heat (Samper et al 1993).

Induction of ovulation

The main advantages of having ovulation at a predictable time include:

1) Breeding of mares to a highly booked stallions only one service per cycle; 2) Facilitating appointment servicing for mares transported to stallions; 3) Insuring a single insemination in attempt to reduce uterine contamination (particularly in mares with delayed uterine clearance or susceptibility to uterine infections); 4) Increasing the likelihood of inseminating close to ovulation when using cooled transported or frozen semen or stallions with low fertility; 5) Aiding spacing the time between natural breedings in mares, presented in estrus at the same time for the same stallion; 6) Helping ovulation synchronization between donor and recipient mares in embryo transfer programs and 7) Reducing labor and veterinary costs.

Due to the advantages of having ovulations at a predictable time as well as the increase in pregnancy rates when mares are bred close to ovulation, many veterinarians are relying on pharmacological agents to induce ovulation. Maximal effectiveness of these agents occur in mares with obvious endometrial edema, a relaxed cervix and a follicle of at least 35 mm diameter. Presently there are three available products to induce ovulation in mares: A) Human Chorionic Gonadotrophin (hCG) and B) The GnRH analogue, Deslorelin. and C) Recombinant equine LH (rLH).

Table 1- Ovulatory-inducing agents commonly used in brood mare practice

A) HCG (Chorulon) is a large protein hormone with LH-activity. The recommended dose of hCG varies among practitioners but ranges between 1,500 and 3,300 international units (IU) given intravenously or intramuscularly. In a recent report by Barbaccini et. al. 75% of hCG-induced ovulations occurred between 24 and 48 hours after treatment. However a significant number of mares ovulated within 12 hours of treatment while others ovulated more than 48 hours post-treatment. In the authors experience hCG effectiveness is reduced when used multiple times over the breeding season in the same mare. The author's recommended dose is 2,500 IU administered intravenously given 12-24 hours prior to the breeding when mares are going to be bred by artificial insemination. In natural breeding situations mares can be treated at the time of breeding.

B) Deslorelin (Ovuplant) is a biodegradable short-term implant containing 2.2 mgs of the GnRH analogue Deslorelin (McKinnon et al. 1993). The implant comes as a preloaded syringe with an attached needle to facilitate placing the inert matrix subcutaneously. Recent studies indicate that Ovuplant induces ovulation predictably between 38 and 42 hours after treatment (Samper et al 2002). Some mares, if not pregnant, may have a day delay in returning to the next natural estrus (McCue et al. 2002). A small percentage of mares induced to ovulate with Ovuplant have a delay of several days or weeks in returning to estrus, seemingly exacerbated when prostaglandin is used on day 5 or 6.(McCue et al 2002) There is no way to predict which mares will experience this delay but it may be avoided by removing the implant shortly after ovulation. Easy access for Ovuplant removal is facilitated by submucosal placement in the vulva instead of the neck, which is the manufacturer's recommended site. The removal of the pellet results in no deleterious effect on the subsequent cycle when compared to control mares or those treated with hCG (McCue et al 2002).

Compounded Deslorelin is also available from multiple pharmacies in a liquid form. The injection of 1.5 mgs IM induces ovulation in a reliable form 40-46 hrs after injection. There is no evidence of a delay in return to estrus in mares treated with injectable deslorelin.

Although hCG and Ovuplant are the only approved products for induction of ovulation in horses, anecdotal evidence by practitioners would suggest that gonadotropin releasing hormone (GnRH) is effective. GnRH has to mimic the natural pulsatile secretion from the hypothalamus and thus must be administered at doses of 20 to 25 micrograms IM or SQ two or three times daily for 2 to 3 days. This regime makes GnRH impractical for induction of ovulation in mares. Mare's ovulating after a single injection of GnRH were committed to ovulating regardless of the therapy. However GnRH can be useful in inducing heat and ovulation in mares with unexplained ovarian inactivity during the normal ovulatory season. In most cases twice a day injections for 2 to 3 weeks will be necessary to stimulate follicular development.

Prostaglandin analogues have been used to induce ovulation. Further studies have not been able to reproduce the results of those preliminary studies and therefore prostaglandin F 2 alpha or any of its analogues are not recommended for ovulation induction. However veterinarians must be cautious about the fast ovulations in response to prostaglandin when mares have a mature corpus luteum and a large diestral follicle.

References

1. Barbaccini S., Zavaglia G., Gulden P., Marchi V., Necchi D., 2000. Retrospective study on the efficacy of hCG in an equine artificial insemination programme using frozen semen. Eq. Vet. Ed. 2: 404-408.

2. McCue P.M.,Farquhar V.J.,Carnevale E.M. and Squires E.L.Removal of deslorelin (Ovuplant ™) implant 48 hours after administration results in normal interovulatory intervals in mares. Theriogenology. 58. 865 – 870. 2002.

3. McKinnon A.O.,Nobelius A.M.,Tarrida del Marmol Figueroa S.,Skidmore J.,Vasey J.R. and Trigg T.E. Predictable ovulation in mares treated with an implant of the GnRH analogue deslorelin. Equine Vet J 25. (4) 321 – 323. 1993.

4. Samper JC, Geertsema H and Hearn P. (1993) Rate of luteolysis, folliculogenesis and interval to ovulation of mares treated with a prostaglandin analogue on day 6 or 10 of the estrous cycle. Proc. Am. Assoc. of Equine Pract. San Antonio, TX pp. 169-171.

5. Samper J.C., Hearn P.H., Ganheim A., 1994. Pregnancy rates and effect of extender and motility and acrosome status of frozen-thawed stallion spermatozoa. Procc. 40th Ann. Convention Amer. Assoc. Eq. Pract. p.p.41-43.

6. Samper JC, (1997) Ultrasonographic appearance and the pattern of uterine edema to time ovulation in mares. Procc. Am. Ass. Equine Pract. Phoenix pp.189-191

7. Samper JC, Jensen S, Sargent J and Ruth L(2002)Timed ovulation in mares using the Deslorelin Implant Ovuplant. J. Equine Vet. Sc. 65:121-124

8. Woods J, Bergfelt DR, Ginther OJ (1990) Effects of time of insemination relative to ovulation on pregnancy rate and embryonicloss rate in mares. Equine Vet J. 22:410-415.