CHAPTER 4Estrous Suppression
Horses are a seasonally polyestrous species. Known as long-day breeders, mares will experience cyclic reproductive activity during the spring and summer months. During multiple estrous cycles experienced by a mare each season, gonadal steroids, estrogen and progesterone, alternate dominance. Each estrous cycle is roughly 21 to 22 days in length. In general, behavioral signs of estrus are typical of a period (usually 5 to 7 days) during a cycle when estrogen dominates and progesterone is at basal levels, whereas the opposite is true during diestrus. Owners, trainers, managers, and veterinarians may all from time to time feel it necessary to suppress behavioral signs of estrus in their mares. Behavioral signs of estrus are suppressed in mares by suspending cyclicity. The primary indications for suspending cyclicity include the following:
Some indications are specific and well defined with treatments that are efficacious and have measurable outcomes. In contrast, other indications are nonspecific and poorly defined with treatments that can be questionable with immeasurable outcomes.
INDICATIONS FOR SUPPRESSING ESTRUS
Estrous Synchronization for Breeding Management
Estrous synchronization is discussed in depth in the preceding section; therefore specific protocols for synchrony will not be covered here. Synchronizing estrus in mares is necessary or at least highly advantageous in a variety of breeding management circumstances, including shipped cool or frozen semen breedings, scheduling around stallion availability, delaying the first postpartum ovulation, and embryo or gamete transfers, to name a few. Protocols typically involve suspending cyclicity in a mare by administering a progestin (with or without estrogen) for a defined/finite period. Treatment suppresses hypothalamic-pituitary-gonadal activity with resultant suspension of cyclic activity and suppression of behavioral signs of estrus. When treatment is removed, the hormonal axis again becomes active and the cycle resumes as the mare enters estrus. In this case the indication is specific, the treatment highly efficacious, and the outcome measurable.
Pain/Colic During Estrus
Some mares experience pain related to the preovulatory follicle or postovulatory luteinizing structure. This condition appears variable and rarely occurs during every estrous cycle in an afflicted mare. Evaluating the reproductive tract when discomfort is observed allows a veterinarian to document an association with ovarian structures. Some mares may benefit from use of ovulatory agents to reduce the interval that a large preovulatory follicle is present on the ovary. The outcome in these mares is measurable in that upon ovulation the discomfort rapidly dissipates. However, a response in others experiencing discomfort associated with a postovulatory structure is less dramatic. In some cases, nonsteroidal antiinflammatory agents may alleviate some of the discomfort. Suspending cyclic activity may benefit the rare mare in which discomfort is irregularly associated with multiple estrous periods each season.
Alternating hormonal dominance during the estrous cycle, between estrogen and progesterone, may also affect the physical condition of a mare.1 Muscles relax during periods of estrogen dominance, providing less support than when progesterone dominates.1 As a result, a minor lameness may become more uncomfortable for a mare during estrus and therefore more conspicuous to the owner.1 Tolerance of discomfort may also vary with hormone levels.1 This condition is nonspecific and difficult to accurately define. A response to treatment may also be difficult to objectively measure. Nevertheless, mares afflicted with discomfort associated with the cyclic changes in gonadal steroids may benefit from suspension of cyclic activity.
Cycle-Related Behavior/Performance Problems
When mares behave or perform in ways contrary to owner desire or expectation in the absence of obvious pathologic conditions, the reason is often felt to be estrous cycle–related. Common names used for this behavior are “Painful,” “Moody,” “Feisty,” or “Pissy Mare Syndrome” or simply “Mare Madness.” In fact, the behavior is most often not associated with estrus or the estrous cycle.1 Many behaviors displayed by mares can be misinterpreted as estrous behaviors. An explanation for problematic behaviors that are not estrous behaviors include the following1:
When evaluating an owner complaint of a cycle-related behavior/performance problem in a mare, a veterinarian should rule out these other conditions as part of a complete workup. In order to thoroughly evaluate the mare, additional expertise may be needed in the form of consults with or referrals to behavior and/or reproduction experts. A team approach to evaluating and solving the problem may be beneficial for everyone involved.
Once other potential causes of an undesirable behavior/performance problem have been ruled out, we should consider the estrous cycle. There are behavior problems that are truly estrous cycle–related.1 Some mares simply display intense behavioral signs when in a normal estrus. This can create an unruly situation during training and performance. In other mares the condition may be much more subtle. Owners and trainers often report that the mare is less cooperative or attentive during estrus. Changes in estrogen and progesterone dominance during the estrous cycle may indeed affect physical condition.1 Supportive muscles relax under estrogen influence and tone in response to progesterone. A minor lameness may become more pronounced during estrus. The ability to tolerate discomfort may also vary with hormonal dominance during the cycle.1
Unfortunately, today our understanding of the reasons for cycle-related behavior/performance problems is incomplete. Once the problem has been thoroughly evaluated and all other causes have been ruled out, we might consider treating the mare to suppress estrus. Depending on the nature of the behavior/performance problem, complete suspension of cyclicity may be necessary. However, some mares apparently respond to progestin treatment at levels that do not suppress ovarian activity.2
There is very little published information on the use, response, and safety of many of the progestin products labeled for use in other species. Much of the information on treatment, response, side effects, and safety is from anecdotal information. Careful consideration is suggested before using any of the methods discussed below for suppressing estrus.
METHODS OF SUPPRESSING ESTRUS
There are a number of methods used to suppress behavioral signs of estrus. Some methods employ approved products with well-documented efficacy, whereas others are not approved and use is based entirely on anecdotal information. Every method, whether efficacious or not, has advantages and disadvantages. The primary methods that have been used include the following:
Hormonal Methods
Altrenogest
Altrenogest (Intervet, Millsboro, DE) is a synthetic progestin approved for use in horses for the purpose of suppressing estrus. Administered orally as an oil solution, the labeled dose of altrenogest is 0.044 mg/kg (1 ml per 110 lb of body weight). It is highly effective for suppressing estrus when administered daily in mares and has been demonstrated to be safe when administered long-term.3–6 However, it has a labeled contraindication for use in mares with uterine inflammation.
It may take several days to effectively suspend cycling and fully suppress estrus. Therefore administration should be planned in accordance with the need for estrous suppression. In general, daily dosing is required to maintain adequate suppression of estrus. The duration of treatment is often tailored to individual management needs. Surveyed veterinarians indicated an overwhelming preference by practitioners for altrenogest when recommending a method of estrous suppression to their clients.
Progesterone
Progesterone in oil, administered intramuscularly, will effectively suppress signs of estrus in mares.7 Daily doses of 100 mg or greater are sufficient to maintain circulating progesterone concentrations comparable to production from a functional corpus luteum.8,9 Progesterone in oil is available from several sources, including compounding pharmacies. The duration of estrous suppression achieved with injectable progesterone appears variable in mares, because surveyed veterinarians report dosing once every 1 to 4 days. Though effective, a significant disadvantage of the form of progesterone is muscle soreness at the injection site. Side effects associated with short-term use have not been reported; however, as with altrenogest, progesterone administration is contraindicated in mares with uterine inflammation. The effects of long-term administration have not been evaluated.10
Full access? Get Clinical Tree
