Review of Fertility Control Research

1. Introduction

A considerable amount of research and expertise has been directed toward fertility control in domestic and wildlife animals, including free-ranging horses, over the past 30 years. The purpose of this review is to assess current fertility control methods for free ranging feral horses. While methods for fertility control serve as an attractive alternative for reducing large-scale population growth rates for freeranging or semi-captive feral horses throughout the United States, it also serves as an alternative means of maintaining limited equid population growth in smaller-scale scenarios. Because of the large number of published reports on contraception in freeranging horses and its related issues, this review will attempt to focus on published reports salient to how we have arrived and where we are today with available fertility control agents. Any failure to cite other relevant reports is not intended. A more detailed and comprehensive review of the literature can be found in the National Research Council (NRC), 2013, “Using Science to Improve the BLM Wild Horse and Burro Program: A Way Forward.”¹ 

Despite accomplishments in the development of fertility control agents for horses, an optimum contraception strategy has yet to be achieved for wildlife suitable for management of large population considerations. A primary difficulty in the development of the optimal fertility control agent for large scale population reduction may be due to the failure of current methods to meet suitable management strategies on a large-scale basis and to meet the requirements set forth by regulatory agencies responsible for the horses and their stakeholders. Historically, the requirements set forth to limit populations of free-ranging horses suggest that agents used for fertility control must adhere to the following criteria: 1) be reversible and temporary, 2) the strategies used for contraception must be noninvasive procedures, 3) have no adverse effects (long-term or short-term) on targeted animals, 4) contraception agents capable of passing along the food-chain (predatory/prey) are not acceptable, 5) behavior must not be altered, 6) social structure and band integrity must not be interrupted, 7) a single injection achieving long-term contraception is essential for management considerations, and 8) it must be practical and economically suitable for ongoing management strategies of large-scale populations of free-ranging horses. These requirements justifiably were designed to ensure the overall management of reducing large populations of wildlife where necessary and ensure the humane treatment of the targeted animal while maintaining its natural state as much as possible. Recently, however, the overwhelming need to limit population growth rates has forced management agencies to rethink some of the limitations previously prescribed for fertility control agents. Longer lasting and permanent contraceptive agents are now of interest to managers of wild and feral animals. While protection of the food chain, the environment, and the welfare of the animals are still primary concerns, some agencies have come to accept that some side effects on social organization, behavior, and even the animals themselves (such as injection site reactions) may be unavoidable. As long as these effects do not have an adverse impact on animal welfare, they are now regarded by some as unfortunate but necessary for the application of effective fertility control on a large scale.

Several agents designed for use as fertility control agents in free-ranging horses are identical to those used for domestic horses to suppress estrus behavior. Therefore, it is worthy of clarifying that the formulations of the agents used to suppress estrous behavior and those used as fertility control agents are strikingly different, and its intended use and the desirable length of effectiveness varies considerably.

For horses, there are several chemical agents suitable and capable of inhibiting fertility. However, the use of these agents is dependent upon the reason(s) for their use and to which population of horses they are intended. While the majority of agents used in domestic horses are targeted primarily toward suppression of estrus behavior, the agents do disrupt cyclicity and, thus, could be considered a form of indirectly inhibiting fertility. For example, exogenous progesterone/progestins as well as gonadotropin-releasing hormone (GnRH) analogs are commonly used to suppress short-term estrus behavior in domestic mares. The agents have also been investigated for long-term contraception use, albeit in different formulations, in free-ranging horses and will be discussed later.

Agents used in domestic horses to suppress estrus behavior may also indirectly serve as agents for inhibiting fertility by prolonging the corpus luteum. Examples of agents used for extending the corpus luteum include administering 1500 to 3000 IU of hCG when a 30 mm follicle develops during the luteal phase of the estrous cycle, thus, inducing a luteal phase ovulation and extending the luteal phase of the estrous cycle.² The use of 60 units of oxytocin daily before day 10 after ovulation can also extend the luteal phase of the estrous cycle for at least 30 days.³ Another method used to enhance prolonged endogenous secretion of progesterone is the insertion of a sterile 35 mm glass marble ball⁴ or 20 mm water-filled propylene ball into the uterine lumen to mimic pregnancy in the mare.⁵ In these two studies, the persistence of corpus luteum lasted for 60 to 90 days if the devices remained in the uterus of the mare (approximately 50% were spontaneously expelled) with no reported evidence of adverse effects on the uterus and mare.⁴ Although these methods have primarily been investigated for use solely for estrus suppression in domestic horses and can also be considered an indirect form of inhibition of fertility, the present format would not be suitable for contraception use in large scale wild horse populations due to 1) the extensive monitoring required to ensure the correct timing of the estrous cycle when administering the agent, 2) the requirement for multiple injections or applications, and 3) the relatively short period of the effects.

For longer-term contraception targeted for large scale population use, several chemical agents have been investigated in stallions and mares.

2. Fertility Control in the Male

In earlier studies, testosterone propionate was investigated for use in free-ranging horses. Several studies using various doses of testosterone propionate (200 µg to 1.3 g) reported successful attainment of oligospermia and reduction of foal crops in treated harems.⁶ However, multiple doses were required in the majority of the studies, and the treatment was recognized as impractical for large-scale management purposes. GnRH vaccines have also been tested and reported in the stallion.^7-10 These reports demonstrate the effectiveness of GnRH vaccines to significantly reduce sperm quality while changes in behavior varied. However, multiple doses are also required during treatment, and in some studies a complete removal of sperm production was not evident.⁹ Use of GnRH agonists in stallions has also been reported in earlier studies.^11-14 It appears that only transient effects were found following its use, and it has been postulated that perhaps larger doses may be required for a desired effect.

Vasectomy 

Surgical vasectomy has been performed and reported in free-ranging horses.^15,16 Dominant stallions were used in these studies, and the results were successful in reducing foal production, particularly in bands where subordinate stallions were not present. However, foal production was eminent in bands where subordinate and bachelor stallions were present. While surgical vasectomy of stallions may not be a practical or economically feasible method for reducing large-scale populations of free-ranging horses, clearly, vasectomy of stallions, if performed in feral horse herds, should be inclusive of dominant, subordinate, and bachelor stallions, and a very large number of stallions being treated are a must in order to have any impact on foal production. Scientific scrutiny must provide evidence of its long-term effectiveness prior to its acceptance for large-scale use. Potentially, vasectomy, whether used chemically or surgically in a large number of stallions within select herd management areas, may have a deleterious effect on the genetic diversity of the herds. Surgical and chemical vasectomy has been investigated on the Sheldon Wildlife Refuge in the Western states. [...]