USE OF THE GNRH ANALOG, DESLORELIN ACETATE, IN A SLOW-RELEASE IMPLANTTO ACCELERATE OVULATION IN ESTROUS MARES

In two separate controlled clinical trials, the efficacy and safety of 2.2 mg of the GnRH analogue deslorelin, administered subcutaneously a s a short-term implant to normally cycling mares in oestrus with a dom inant ovarian follicle more than 30 mm in diameter, were evaluated, us ing a placebo as a negative control. The oestrous cycle of each mare w as followed by teasing, palpation per rectum and transrectal ultrasono graphy. Follicles were monitored every 24 hours by ultrasonography unt il ovulation occurred. The mares were either mated naturally or insemi nated artificially. In trial 1, 174 mares were treated at six location s in Canada, and in trial 2, 98 mares were treated at three locations in the USA. In trial 1, the treatment with deslorelin reduced the mean (sd) time to ovulation from 84.2 (48.4) hours to 50.2 (19.6) hours (P <0.001) and in trial 2 it reduced it from 88.8 (40.3) hours to 54.1 (2 6.5) hours (P<0.001). In trial 1, the percentage of mares ovulating wi thin 48 hours increased from 37.7 per cent in control mares to 86.1 pe r cent in treated mares (P<0.001) and in trial 2 the percentage increa sed from 26.5 to 80.9 per cent (P<0.001). In trial 2, the duration of oestrus in the deslorelin-treated mares was reduced from 6.1 days to 4 .3 days and the number of matings or artificial inseminations was redu ced from 2.5 to 1.7 (P<0.001). In trial 1, days 12 to 20 pregnancy rat es for matings at the treatment oestrus were not different for deslore lin-treated (75.6 per cent) and placebo-treated (66.1 per cent) mares. In trial 2, days 12 to 20 pregnancy rates from matings at the treatme nt oestrus were lower for deslorelin-treated (58.7 per cent) than for placebo-treated (83.3 per cent) mares (P<0.05), although pregnancy rat es were similar for deslorelin-treated (97.1 per cent) and placebo-tre ated (95.0 per cent) mares after mating at the second oestrus. In both trials, pregnancy losses due to early or late abortions were within t he normally expected range and similar for deslorelin-treated (3.6 and 3.7 per cent, respectively) and placebo-treated (13.4 and 7.5 per cen t) mares. The treatments did not cause systemic side effects and local reactions at the implantation sites were slight and of short duration .