Comparison of human chorionic gonadotropin (hCG), deslorelin, deslorelin combined with hCG, and histrelin to induce ovulation in the mare.

1. Introduction: Administering hCG induces ovulation at a median of 36 hours [1] in 82-89% of mares [2]. No difference was found between buserelin and deslorelin [3], nor histrelin to hCG in the interval to ovulation [4]. Deslorelin (liquid vehicle) has equal efficacy to hCG and pelleted deslorelin in inducing ovulation [5, 6]. An hCG/deslorelin combination is used in clinical practice, with one report finding no difference in ovulation rate between hCG; deslorelin; and the combination of hCG/deslorelin [7]. This study compared hCG/deslorelin combination to hCG, deslorelin and histrelin and a placebo. The null hypotheses tested were: 1) There is no difference in days to ovulation between “ovulating inducing drugs” and a placebo; and 2) There is no difference in days to ovulation between the hCG/deslorelin combination and hCG, deslorelin and histrelin. 2. Materials and Methods: Mares were Quarter Horses and Australian Stock Horses. All had ovulated that season. Mares were recruited when in oestrus based on transrectal palpation and ultrasonography. Treatments were administered when the mares were in oestrus (>35mm follicle; uterine oedema>2/5[8]; relaxed cervix). Thereafter mares were examined daily until ovulation. Treatments were randomised by the compounder and numbered sequentially, and treatments were administered as mares entered the study (double blinded). Five treatment groups were: 1) 2500IU hCG in 1mL; 2) 2.25mg deslorelin in 1 mL; 3) 2500IU hCG/2mg deslorelin in 3mL; 4) 0.5mg histrelin in 1mL and; 5) Saline in 1mL or 3mL (10 each). Twenty mare cycles in each group, a total of 100 treatments allowing for repeated administration. Treatments comprising 1mL were administered intramuscularly and 3ml treatments were intravenous. The deslorelin formulation contained a hybrid water and polyethylene glycol base. The hCG/deslorelin combination was dissolved in a hybrid water and polyethylene glycol base. The statistical platform R and packages "nlme" and "epiR" were used [9, 10]. Comparisons of time to ovulation were made between treatment groups and controls, then treatment groups without the control group using linear mixed models. Days to ovulation was the dependant variable and treatment groups were included as a fixed effect variable. Mare identity was included as a random effect to account for repeated observations in some mares. Significance was set at P <0.05. 3. Results: Sixty-eight mares, aged 4 to 26 years were included (25 maiden, 6 lactating, 37 barren). In Group 1 one mare was lost to follow up. In Group 2 one mare was inadvertently treated twice, and follicular regression occurred in another mare; both mares were removed. In Group 3 one mare developed a haemorrhagic anovulatory follicle and was removed. In Group 4 follicular regression occurred in one mare and haemorrhagic anovulatory follicles occurred in two mares; both were removed. In Group 5 all 20 treatments were analysed. Days to ovulations are presented as boxplots (Figure 1); outliers (5 or more days to ovulation in the treatment groups) were observed in deslorelin and histrelin groups, these were included in the analysis. The placebo group had significantly longer time to ovulation (4.05 ± 1.61 days) than the treatment groups (histrelin 1.88 ±0.49, hCG 2.21 ±0.71, deslorelin 2.36 ± 1.5, hCG-deslorelin combination 1.89 ±0.31 days, Table 1). Regression analysis of time to ovulation predicted by treatment demonstrated a significantly decreased number of days to ovulation between treatment groups and the control group (P < 0.05). There was no significant difference between treatments (P > 0.1). 4. Discussion: The hCG/deslorelin combination is a viable alternative to other ovulatory drugs. However, consistent with another study, we found no advantage to using the combination [7]. One perceived advantage of the combination might be greater consistency in ovulation timing, but this was not demonstrated in this study. Although the addition of GnRH might cause ovulation in the mares that fail to ovulate from the hCG administration, larger studies with more power will be required to assess whether the combination might increase the predictable ovulation rate above 85% as reported for drugs used individually [11]. Follicular regression and haemorrhagic anovulatory follicles occurred several times during the study. In the control group no follicles regressed or became haemorrhagic, but in the treatment groups that contained GnRH analogues there were follicles that regressed (5% in deslorelin group, 5% in histrelin group) or became haemorrhagic (5% in hCG/deslorelin group and 5% in histrelin group). Inappropriate timing of ovulation drugs could interrupt the normal endocrinological control of a maturing follicle. Haemorrhagic anovulatory follicles are more common in spring and autumn transition [12] and they are probably related to a decoupling of hypothalamic, pituitary and ovarian events [13]. The timing of administering ovulation inducing drugs could cause a haemorrhagic anovulatory follicle. In conclusion, the first null hypothesis was not supported; there was a significant difference in mean days to ovulation between “ovulating inducing drugs” and a placebo. The second null hypothesis was supported; there was no significant difference in efficacy between the use of hCG, deslorelin, hCG/deslorelin and histrelin. Acknowledgements: Drugs were supplied by Bova Compounding, Caringbah, NSW, Australia. This study was made possible with the assistance of Tamarang Farm, NSW, Australia. References: [1] McKinnon A, Perriam W, Lescun T, Walker J, Vasey J, Trigg T. Effect of a GnRH analogue (Ovuplant), hCG and dexamethasone on time to ovulation in cycling mares. 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