Comparison of the efficacy of Lecirelin and hCG for ovulation induction in mares

Abstract

Ovulating agents are very useful drugs in equine breeding, hCG being the first intention molecule, whereas GnRH agonists are especially interesting for any mare immunized against hCG, like repeat breeders and embryo donor mares. In Europe, no long acting GnRH agonist is available in veterinary practice. Saline solutions of buserelin or triptorelin, which is only available for human treatments in Europe are commonly used for equine ovulation induction (Dordas-Perpinya et al. Reprod. Dom. Anim. 2020; 55:374-383). An alternative, Lecirelin which is licensed in cattle and rabbits, has been evaluated in donkeys (Carluccio et al. Therio. 2007;68:914-919). The aim of our study was to compare the efficacy of lecirelin with hCG in horses. Forty mares underwent ovulation induction on four consecutive cycles, two cycles with hCG and two cycles with lecirelin, including one spontaneous estrus and one following chemical luteolysis. Mares were examined on a daily basis to check ovarian follicles diameter, uterine edema, and cervical softening. Once they presented a preovulatory follicle over 35 mm, and uterine edema (>=2) (Mc Kinnon. Equine diagnostic ultrasonography. 1998; 79-102) and a soft cervix, they were randomly injected with saline solution containing 200 µg of Lecirelin or 1500 IU of hCG intravenously. After induction, they were scanned every 12 hours until ovulation. Mares were inseminatedwith a mix of fresh semen of two fertile stallions 20 hours after induction on the spontaneous cycles. Pregnancy diagnosis was performed at day 12 and day 14. As each mare is her own control, statistical analysis was performed by χ², Fisher's test and Student's test for matched data. Follicle size at injection did not differ significantly between the two induction treatments (p>0,05), with or without distinction of spontaneous and induced cycles. Rates of ovulation in the 24-48 hours post-injection did not significantly differ between lecirelin (71%) and hCG (83%) on estrus following chemical luteolysis (p>0,05) or spontaneous estrus (43% for lecirelin and 68% for hCG) (p>0,05). We observed significantly more spontaneous ovulations occurring within the first 24 hours after induction with hCG (27% on spontaneous estrus, and 15% after chemical luteolysis) than with lecirelin (5% on spontaneous estrus, 7% after chemical luteolysis) (p < 0,05). Pregnancy rates were not significantly different following induction by lecirelin (67%) or hCG (57%) (p>0,05). These results highlight the usefulness of saline solution of lecirelin licensed in Europe for bovine and rabbit for induction and synchronization of ovulation for mares with control cycles. Lecirelin may be particularly suitable for embryo donor mares with a reduced hCG response.