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:
INDICATIONS FOR SUPPRESSING ESTRUS
Pain/Colic During Estrus
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
METHODS OF SUPPRESSING ESTRUS
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.
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
