Efficacy of a Y‐Design Silastic Elastomer Intrauterine Device as a Horse Contraceptive

Abstract

To successfully manage population growth rates of free-roaming horses (Equus ferus caballus), an effective, long-acting contraceptive could be beneficial. We evaluated the efficacy and safety of an intrauterine device (IUD) in a breeding trial using seasonal pasture assemblages of 2 males individually pastured in large enclosures (~81 ha) each with 10 females per male. The study took place at a large animal research facility at Oklahoma State University in Stillwater, Oklahoma, USA. Specific objectives of the study were to evaluate the effectiveness of IUDs in preventing pregnancy over 20.5 months with return to fertility assessed over the subsequent 6 months and to evaluate potential effects of this IUD on ovarian function and uterine health (e.g., endometrial inflammation, infection, fibrosis). We developed a Y-design IUD, made of silastic elastomer, and set the desired retention rate at 75%. After procurement of endometrial biopsies, we inserted IUDs into the uteri of 20 females and examined them via transrectal ultrasound every 2 weeks for detection of the device and assessment of uterine health. After 20.5 months we removed the remaining 15 IUDs, obtained a second endometrial biopsy for comparision, and returned the females to their respective males for continued breeding in efforts to assess uterine return to health as indicated by pregnancy rates. The Y-design IUD met the 75% retention rate goal, with 100% contraception in females that retained the device; 12 of 20 treated females became pregnant (i.e. returned to fertility) after removal of the IUD. Based on these results, we conclude that the Y-design IUD is a safe and effective device to control fertility in free-roaming horses.

PopEquus: A predictive modeling tool to support management decisions for free‐roaming horse populations

“…More recently, IUDs have been developed as another fertility control method. IUDs can be implanted in nonpregnant females and are 100% effective at preventing pregnancy so long as they remain in the uterus (Gradil et al, 2021; Holyoak et al, 2021). However, most types of IUDs tested over longer time periods (>1 year) are not retained perfectly, and preliminary studies suggest that annual retention rates are between 80% and 90% (Daels & Hughes, 1995; Gradil et al, 2019; Holyoak et al, 2021).…”

Suppression of reproductive behaviour and gonadal function in female horses—An update

“…Already in 1995, there was a report on effective, safe and practical contraception in mares kept together with stallions in paddocks by intrauterine introduction of O-ring-shaped silastic devices (Daels & Hughes, 1995). Similar success was achieved when iUpods (Gradil et al, 2019;Hoopes et al, 2021) or Y-shaped silastic devices (Holyoak et al, 2021) were used. In this study, there was a remarkably lower retention rate when O-ring shaped silastic devices were used (Lyman et al, 2021).…”

“…This hypothesis was recently supported by the finding of a modified protein composition of the Intrauterine devices are also an interesting approach for population control in free-roaming horses. They are likely to fulfil the requirements of a long-lasting contraceptive that does not require the animal being recaptured frequently (Holyoak et al, 2021). In this context, the sexual behaviour of horses living under wildlife conditions has to be considered.…”

“…When mares were kept with a stallion, a considerable percentage of mares showed oestrous behaviour (Lyman et al., 2021 ) and developed signs of endometritis (Daels & Hughes, 1995 ; Hoopes et al., 2021 ). Nevertheless, fertility was not considerably affected when intrauterine devices were removed and mares allowed mating (Daels & Hughes, 1995 ; Holyoak et al., 2021 ; Hoopes et al., 2021 ). The contraceptive effects of devices that do not break or hurt the endometrium are thus at least in part reversible.…”

The Wildlife Profession's Duty in Achieving Science‐Based Sustainable Management of Free‐Roaming Equids

“…In this special section, authors present a broad range of research on feral equids, including estimating space use and population size (Gedir et al 2021, King et al 2021, Schoenecker et al 2021), population control in the absence of co‐evolved predators (Andreasen et al 2021, Boyce and McLoughlin 2021, Hinchcliffe et al 2021), effects of feral equids on wildlife habitat and sympatric species (Burdick et al 2021, Clancy et al 2021, Coates et al 2021, Hennig et al 2021, Scorolli 2021, Stoner et al 2021), and a promising new equid fertility control method (Holyoak et al 2021). Although these papers provide techniques or approaches to manage feral equids, their application will vary by population, political jurisdiction, and country.…”