Contraceptive effect of a gonadotropin-releasing hormone vaccine on captive male brown bears (Ursus arctos).

Abstract

Fertility control has traditionally been applied in zoos to control captive populations, and reversible contraception is important. However, contraceptive methods for male bears have not been reported. We aimed to establish a reversible contraceptive for male brown bears by investigating the effects of a gonadotropin-releasing hormone (GnRH) vaccine (Improvac®) that was developed for the immune castration of pigs. We vaccinated six bears with two sequential doses of 400 (n = 2) or 600 μg Improvac® (n = 4) with a 1-month interval during the pre-breeding season (February to April). We compared the reproductive parameters (testosterone levels and semen parameters) of the six vaccinated and four non-vaccinated (control) bears once during the breeding season (May or June). To investigate whether the reproductive performance could be restored in the following year of contraception, we also compared the reproductive parameters once during the breeding season in two bears between the year with GnRH vaccination and the following year without vaccination. Vaccination treatments suppressed reproductive parameters in 5 bears, although vaccination with 400 μg of Improvac® was not effective in one bear. Testosterone levels and the rate of progressive sperm motility were significantly lower, and total sperm count and testis size tended to be lower in vaccinated bears, compared with the controls. Blood biochemical findings and direct observations after Improvac® vaccination did not reveal side effects. Moreover, testosterone levels and spermatogenic scores of two bears were restored in the following year. We confirmed that the Improvac® vaccine elicited a reversible contraceptive effect in male brown bears.