Efficacy of a silicone Y design intrauterine device as horse contraceptive in captive breeding

Due to continued population growth on western rangelands in US, there are now ~ 70,000 feral horses administered to by Bureau for Land Management (BLM) in an area where range ecologists have established 27,000 as desired population size. Therefore, need for an effective contraceptive for feral horses on western range remains a BLM priority. Goal was to find a means of reducing unwanted pregnancies within US feral horse population, while maintaining reversibility so that genetic diversity can be maintained within population. Multiple O Ring designs were deemed unsuccessful due to their inability to resist uterine expulsatory forces in face of immediate and subsequent breeding behavior. We hypothesized that coadministration of long acting progesterone at IUD insertion would increase IUD retention by preventing these immediate aftereffects. Furthermore, discovering that a fundus seeking design in human IUDs ameliorates normal uterine expulsion forces, we formulated an IUD ‘Y’ design to test for a desired minimum > 75% retention rate. This current study compared 50 and 60 durometer (an indicator of material hardness) ‘Y’ design IUDs, using 2 breeding pods of 10 mares and 1 stallion in each group. In each pod, 5 mares received a 50 durometer IUD and 5 mares received a 60 durometer IUD. Every other mare received an IM injection of saline or long acting P4 . After early data indication that 60 durometer ‘Y’ shaped IUDs had a higher retention rate, 50 durometer IUDs were replaced with 60 durometer IUDs. Over next 2 years, all mares in both breeding pods were then monitored by transrectal ultrasonographic examination every 2 weeks to determine IUD retention and pregnancy rates for those mares that lost their IUDs. Endometrial biopsies were obtained, and a Kenney Biopsy grade assigned, before and after research period in efforts to assess IUD impact on endometrial health. Five of 20 mares expelled their IUD (75% retention rate) with each of these 5 mares becoming pregnant. Seven of 20 had no change in Kenney biopsy grade, 4 mares worsened by 1 grade, 5 mares worsened by more than one grade, and 4 mares improved by 1 grade. After 4 cycles following removal of IUDs, 12 mares became pregnant. Based on histopathologic and pregnancy results, mares that failed to become pregnant not due to a worsening endometrial grade but possibly due to involvement of other intrinsic factors. Y design IUD met intended parameters and achieved study goals.

Keywords: Intrauterine device, feral horse, Y design, endometrial grade, fertility

This manuscript was originally published in the journal Clinical Theriogenology Vol 12(3) Sept 2020.  Clinical Theriogenology is the official journal of the Society for Theriogenology (SFT) and the American College of Theriogenologists (ACT).  This content has been reproduced on the IVIS website with the explicit permission of the SFT/ACT.