Transient suppression of ovulatory ovarian function in pony mares after treatment with slow-release deslorelin implants

Suppression of reproductive behaviour and gonadal function in female horses-An update

The behaviour of mares is often detrimental to their performance resulting in frequent demand for methods to suppress gonadal function. In addition, prevention of unintended reproduction especially in feral horse populations may require methods for suppression of gonadal function. Surgical ovariectomy is a safe method but not an acceptable approach in feral mares and undesired in mares where future breeding is considered. There are different approaches for artificial prolongation of the luteal phase resulting in transient inhibition of oestrus and ovulation. Among those, treatment with natural or synthetic progestogens is considered the most common and successful method. Whereas application of intrauterine devices may result in prolongation of luteal function in non-pregnant mares, intrauterine insertion of glass balls is no longer recommended because of complications in individual mares. There are several safer alternatives that may be of interest, especially for population control in free-roaming horses. Treatment with long-acting deslorelin implants inhibited ovulation and oestrus behaviour in mares for limited and variable time intervals in a dose-dependent manner. The effect of GnRH vaccines varies considerably among individual mares, is age dependent, and oestrus-like behaviour may still occur. Contraception via immunization against native porcine or recombinant zona pellucida antigen is successful, but immunocontraception is as much a result of ovarian inactivity as an antibody-based block to sperm-oocyte binding. In conclusion, several treatments for suppression of gonadal function in mares are available, but there are advantages and disadvantages associated that have to be considered. The treatment of choice will thus differ with regard to the demands.

Altrenogest treatment reduces the stress response of three-year-old warmblood mares during their initial equestrian training

Horse mares are frequently treated with the progestin altrenogest with the aim to suppress estrous behavior and its negative impact on equestrian performance. Progestogens, however, also have sedative effects in males, and females of different species. The aim of our study was therefore to investigate altrenogest-induced changes in the stress response of female horses during initial equestrian training. Three-yr-old Warmblood mares were randomly assigned to treatment with altrenogest (ALT; 0.044 mg/kg once daily; n = 6) or sunflower oil (CON; n = 5) for 12 wk during training. At predefined steps of the training program (free movement, lunging without and with side reins, lunging with saddle, mounting of a rider, free riding, riding by an unfamiliar rider) salivary cortisol concentration, and heart rate were determined from 60 min before to 120 min after training. The same procedures were performed during repeated gynecologic examinations and 2 novel object tests. Bodyweight and body condition scores (BCS) were assessed at 4-wk intervals. During all training units, salivary cortisol concentration and heart rate increased (P < 0.001), but the increase was smaller in group ALT mares (time x treatment P < 0.001). Gynecologic examinations and novel object tests induced a much smaller increase in cortisol and heart rate (P < 0.001) than equestrian training with no difference between groups ALT and CON. Initially, bodyweight, and BCS decreased during training. The subsequent increase was larger in group ALT vs CON (time x treatment P < 0.05). In conclusion, altrenogest reduced the stress response of 3-yr-old mares to equestrian training. The use of altrenogest during equestrian competitions should therefore be reconsidered.

Deslorelin Slow-Release Implants Delay Ovulation and Increase Plasma AMH Concentration and Small Antral Follicles in Haflinger Mares

Simple Summary

In horses, oocyte collection followed by intra-cytoplasmatic sperm injection is increasingly used. The yield of oocytes is a limiting factor and depends on the number of follicles present on the ovary during oocyte collection. Therefore, the aim of this study was to analyze the effect of slow-release implants containing the GnRH analogue deslorelin on the number of follicles and on hormones regulating follicular development. Six mares received a deslorelin implant and six mares served as controls. The interval to the first spontaneous ovulation was prolonged in treated mares. The treatment changed the release pattern of the gonadotrophins LH and FSH. Changes in the number of follicles 10 to 15 mm in diameter were detected in deslorelin-treated mares. These changes were also reflected by increasing plasma anti-Muellerian hormone concentrations, a hormone produced by growing follicles. In conclusion, deslorelin implants induce changes in ovarian follicle subpopulations and could be a promising tool for the preparation of mares for assisted reproductive procedures.

Abstract

There is an increasing interest in the manipulation of ovarian follicular populations in large domestic animals because this could prove beneficial for assisted reproductive techniques such as ovum pick-up (OPU). The aim of the present study was to evaluate the effects of deslorelin slow-release implants (SRI) on the interovulatory interval, antral follicle count (AFC), number of follicles of different size ranges and plasma anti-Muellerian hormone (AMH) concentration in mares. To synchronize their estrous cycles, Haflinger mares (n = 12) were treated twice with a PGF_2α analogue. One day after the second injection (day 0), mares received a 9.4 mg deslorelin SRI (group DES, n = 6) or 1.25 mg deslorelin in a short-acting formulation (CON; n = 6), respectively. Regular transrectal ultrasonography of the genital tract was performed and blood samples were collected for the analysis of progesterone, AMH and gonadotrophins. The interval from implant insertion to the first spontaneous ovulation was 23.8 ± 10.5 days in group DES compared to 17.0 ± 3.9 days in group CON (p < 0.05). For the concentrations of LH, FSH and AMH, interactions between time and treatment were detected (p < 0.05). The AFC and the mean number of follicles with 5 to 10, 10 to 15 and 15 to 20 mm in diameter changed over time (p < 0.05). A time x treatment interaction was demonstrated for follicles of 10 to 15 mm in diameter (p < 0.05). The changes in this follicular subpopulation were reflected by increased plasma AMH concentration in group DES. In conclusion, 9.4 mg deslorelin implants show minor effects with regard to estrus suppression in mares, whereas the changes in the subpopulation of small ovarian follicles could be a promising tool for preparation of mares for OPU.