GLOBAL USE OF ETONOGESTREL IMPLANTS IN ZOO-HOUSED ANIMALS

Hormonal contraception is often used by zoos and aquaria to manage genetic diversity and the size of their populations. However, the contraceptive products used have typically not been designed for use in the target species. The Association of Zoos and Aquariums' Reproductive Management Center (AZA RMC) and the European Association of Zoos and Aquaria's Reproductive Management Group (EAZA RMG) collect data on contraceptive use in global zoos and aquaria to monitor trends and generate contraceptive recommendations. The human 68 mg etonogestrel contraceptive implant (Implanon^®/Nexplanon^®/Implanon NXT^®), a synthetic progestin, has three contraceptive mechanisms: preventing luteinizing hormone release and therefore ovulation, thickening of the cervical mucus, limiting the entry of sperm, and modification of the uterine lining, inhibiting implantation. Here, we review over 30 years of data from the Reproductive Management Center-Reproductive Management Group Contraception Database (CD; N = 3,510 records). Etonogestrel implant use has only been reported in mammals in the CD, including nonhuman primates (91.34% of records), chiropterans (5.78%), carnivores (1.05%), and other mammals (1.82%). The implants are highly effective when used as contraceptives (98.14% effective), and two-thirds of implant failures are attributable to implant loss or to the incorrect application of the product, rather than to true product failures. Etonogestrel implants are generally safe, although long-term use in carnivores is discouraged due to risks of developing reproductive pathology resulting from long-term progestin exposure. Reported noncontraceptive effects included weight gain and a reduction in sexual behavior, which are known noncontraceptive effects in humans. Etonogestrel implants are generally reversible as 63.09% of individuals given the opportunity to breed produced offspring. Etonogestrel implants are safe and effective contraceptives in most female mammals. Further research on noncontraceptive effects and reversibility is required to finetune management recommendations.