IN-DEPTH: REPRODUCTIVE ENDOCRINOLOGY

Prolonging Function of the to Suppress Estrus in Mares

Dirk K. Vanderwall, DVM, PhD, Diplomate ACT

The orally active synthetic progestin altrenogest is widely considered to be the “gold-standard” method of inhibiting estrous behavior in mares. However, its expense, need for long-term daily administration, safety concerns for personnel during handling, and increased public scrutiny regard- ing the use of exogenous steroids in performance horses have collectively prompted interest in the development of practical methods of prolonging corpus luteum (CL) function, which allows continued secretion of endogenous to keep mares out of heat naturally. Placement of an intra- uterine glass ball has been the most widely used method of prolonging CL function, but variable efficacy and the potential for deleterious consequences associated with them (if not eventually removed), have stimulated interest in alternative methods of prolonging CL function. Currently, treatment appears to be the most practical and efficacious alternative method of prolonging CL function. Additional methods include inducing a late-diestrus ovulation, intrauterine infusion of plant oils, and establishment of followed by manual reduction of the conceptus after day 16 of gestation. Author’s address: Department of Animal, Dairy and Veterinary Sciences, School of Veterinary Medicine, Utah State University, Logan, UT 84322; e-mail: [email protected]. © 2013 AAEP.

1. Introduction necessarily associated with estrus. In addition, it A common complaint of horse owners, trainers, and is important to note that some behaviors displayed veterinarians is variable performance of mares that by mares that are thought to be associated with is related to the .1,2 In a survey of estrus are in fact not estrous behaviors. They are more than 750 veterinarians, approximately 90% instead signs of 1) submissive behavior, 2) urogen- 3,4 had the clinical impression that the estrous cycle ital discomfort, or 3) stallion-like behavior. Of affected the performance of mares, and the most these, submissive behavior may be most easily con- frequently reported clinical sign associated with an fused with estrous behavior. Submissive behavior effect of the reproductive cycle on performance was includes leaning away from perceived threats, attitude change, whereas other signs included tail- swishing/ringing the tail, and actively squirting swishing, difficulty to train, squealing, “horsing,” urine, which collectively can give the impression of excess urination, kicking, and a decrease in perfor- estrus, whereas true estrous behavior is manifested mance.1 As the preceding list of objectionable be- by leaning toward the stallion (or other stimulus), a haviors suggests, not all problematic behaviors are relaxed, lifting motion of the tail, stationary/squat-

NOTES

342 2013 ր Vol. 59 ր AAEP PROCEEDINGS IN-DEPTH: REPRODUCTIVE ENDOCRINOLOGY ting stance, and passive urination (full stream or ated with hormone secretion from the adrenal cor- small amounts in dribbles).4 Urogenital discomfort tex.11,12 The intensity of this type of may be the result of cystitis or renal disease, “unseasonable” estrous behavior can be equivalent whereas stallion-like behavior may be associated to the behavior intact cycling mares display during with neoplasia of the or administration of the initial and terminal days of estrus but is gener- exogenous anabolic steroids. ally less intense than the behavior displayed near Because numerous factors (both reproductive and ovulation.11 It has been postulated that behavioral nonreproductive) can adversely affect performance, receptivity to a stallion outside the breeding/ovula- when evaluating an owner/trainer complaint of an tory season that is independent of ovarian estrogen estrous cycle-related behavior/performance problem secretion may have developed as a means of main- in a mare, the first step is to determine if the prob- taining social bonds between a harem stallion and lematic behavior is or is not related to a specific his mares.11,13 This phenomenon has important phase of the estrous cycle. To thoroughly evaluate implications for the clinical management of estrous the mare, additional expertise may be needed in the behavior in mares, since simply suppressing ovarian form of consultation with or referrals to behavior follicular activity or removing the ovaries may not and/or reproduction experts, because a systematic ensure complete elimination of estrous behavior. team approach to evaluating and solving the prob- lem may be most successful.5 2. Exogenous Progesterone/Progestins If problematic behavior is consistently associated Historically, the use of exogenous progesterone/pro- with estrus, it is necessary to determine if the un- gestins has been the most widely used method of derlying cause is physical or behavioral. Some suppressing estrous behavior in mares. It was first mares that exhibit performance problems during es- demonstrated in the 1960s that daily intramuscular trus may be having pain in the periovulatory period administration of 0.2 mg/kg progesterone in oil ef- that can vary from sensitivity to weight and/or ma- fectively suppressed signs of estrus in mares.14 nipulation of the back (caused by tack and/or rider) Although progesterone in oil is available from com- to overt colic-like symptoms.6–9 Mares that have pounding pharmacies, the need for daily administra- back pain and/or colic-like symptoms in the peri- tion and the potential for soreness at the site of ovulatory period may benefit from the use of an injection are limitations to its use. An alternative ovulatory agent to reduce the time a large pre-ovu- to daily administration of progesterone in oil is a latory is present or complete sup- compounded long-acting injectable formulation of pression of cyclical reproductive activity. In progesterone containing a total dose of 1.5 g proges- contrast to mares with an underlying physical cause terone that will maintain blood levels of progester- of performance issues during estrus, some mares one above 1.0 ng/mL for approximately 10 days,15 display such profound signs of estrus that the be- which is a sufficient level of progesterone to block havior itself impairs performance. For example, estrous behavior14,16; however, the potential for even under saddle, some mares may “break down” soreness at the injection site is a limitation to its use and show estrus in response to being around other in performance horses. horses and/or other stimuli.4 For these mares, sup- In contrast to native progesterone, the orally ac- pression of estrus is clearly warranted. In other tive synthetic progestin altrenogesta,b is approved mares, the condition may be much more subtle, for suppressing estrus in mares and is widely con- causing owners and trainers to report that the mare sidered the “gold-standard” method of inhibiting es- is less cooperative or attentive during estrus, trous behavior. Daily oral administration of prompting them to pursue estrus suppression.2 altrenogest at a dose of 0.044 mg/kg is very effica- It is also common to suppress estrus when the signs of cious for suppressing estrus in mares17,18; however, estrous behavior are simply perceived to be associated its expense, need for long-term daily administration, with performance problems or to preemptively block and safety concerns for personnel during handling the behavior to preclude the possibility of an adverse and administration19 are drawbacks to its use. effect of estrus on performance. When suppression As an alternative to oral administration, it was re- of estrous behavior is desired, the following general cently reported that intramuscular administration methods have been used in mares: 1) administra- of a compounded preparation containing 225 mg or tion of exogenous progesterone or synthetic proges- 450 mg of altrenogest in a sustained-release vehicle tins, 2) extending the duration of corpus luteum blocked estrous behavior for approximately 12 and (CL) function, 3) downregulating ovarian follicular 15 days, respectively, and administration of 500 mg activity, and 4) ovariectomy (for complete review, altrenogest in microparticles suppressed estrous be- see Vanderwall and Nie10). havior for approximately 30 days.20 Veterinarians When discussing methods of suppressing estrus, it should use FDA approved products for this indica- is important to note that mares are unique among tion. Certain compounded products can vary sig- domestic animals, because in addition to ovarian- nificantly in potency and stability. Consequently derived estrogen-induced signs of estrous behavior, results may vary substantially. There can also be many seasonally anovulatory and ovariectomized legal and ethical issues with using a compounded mares exhibit paradoxical estrous behavior associ- product when there are FDA approved products

AAEP PROCEEDINGS ր Vol. 59 ր 2013 343 IN-DEPTH: REPRODUCTIVE ENDOCRINOLOGY available in the appropriate dosage form and for the normal duration after placement of the glass ball appropriate indication. before CL function was prolonged. Therefore, on a Although there have been anecdotal reports on “per-cycle” basis, the incidence of prolonged CL func- the use of synthetic progestins other than altreno- tion was only 11% (7/62 cycles) in the mares that gest for estrus suppression in mares, none have been received a glass ball compared with 8% (4/50 cycles) found to be efficacious when rigorously tested.10 in the nontreated control mares, which was not sig- Because of the ineffectiveness of other synthetic pro- nificantly different between groups. gestins, altrenogest remains the primary exogenous More recently, Rivera del Alamo et al23 examined hormone used for suppression of estrus; however, the effect of intrauterine placement of a 20-mm, because it is a steroid hormone, it is facing increased water-filled polyproplylene ball on the duration of scrutiny because of public concern about the use of CL function in mares with the specific objective of steroids in performance horses. Therefore, finding investigating two potential mechanisms by which effective alternatives to the use of exogenous steroid CL function is extended: 1) the intrauterine device hormones is particularly prudent at this time. induces mild endometrial inflammation that com- pletely blocks or markedly attenuates PGF2␣ secre- 3. Prolonging Function of the Corpus Luteum tion (ie, prevents high-magnitude luteolytic pulses) An alternative method of suppressing estrus that or 2) the physical presence of the device (movement does not require administration of exogenous pro- and/or contact with the ) directly mim- gesterone/progestins is prolonging function of the ics the inhibitory effect of a conceptus on PGF2␣ CL, which allows continued secretion of endogenous secretion. Corpus luteum function was extended in progesterone to suppress estrus naturally. In non- nine of 12 mares (75%) that received the intrauter- pregnant mares, the CL secretes progesterone for ine device with an average duration of 57 days com- approximately 2 weeks after ovulation (ie, the dura- pared with none of 12 control mares in which the tion of diestrus) and then stops functioning when average duration of CL function was 16 days. In the endometrium secretes prostaglandin F2␣ ␣ 21 six of the nine mares with extended CL function, (PGF2 ) during luteolysis. As a result of luteoly- small accumulations of intrauterine fluid (Յ10 sis, progesterone levels fall below 1.0 ng/mL and the mm ϫ 20 mm) were identified during the luteal mare returns to estrus. Development of therapeu- phase, but no neutrophils or bacteria were recovered tic strategies for prolonging function of the CL (ie, on uterine swabs when they were examined during maintaining progesterone Ͼ1.0 ng/mL) as a means the subsequent estrus. In addition, changes in of long-term suppression of estrus has gained in- uterine biopsy scores for inflammation and glandu- creasing interest because it obviates the need for lar dilation before and after treatment were similar administration of exogenous progesterone/proges- tins to mares. The remainder of this review will for control and uterine device mares (with or with- discuss various methods of prolonging CL function out extended CL function); therefore, there was no for suppression of estrus in mares. evidence that the intrauterine device induced an inflammatory response in the uterus. On the basis 4. Intrauterine Glass Ball of intensive blood sampling and measurement of ␣ The most common method of prolonging CL function PGF2 metabolite (PGFM) levels in the systemic in mares has been intrauterine insertion of a glass circulation, on days 11 to 16 after ovulation in four ␣ ball (ie, marble). Nie et al22 reported that place- control and eight uterine device mares, PGF2 se- ment of a 25- or 35-mm sterile glass ball into the cretion was attenuated in mares with prolonged CL uterine body immediately after ovulation resulted in function, with the exception of two mares; one mare prolonged CL function in seven of 18 mares (39%) showed a single PGFM peak and another showed that retained the glass ball after insertion (six of 12 two isolated PGFM peaks. Because there was no mares expelled the 25-mm glass ball soon after in- evidence of inflammatory changes caused by the in- sertion). In mares with prolonged CL function af- trauterine device, the author concluded the physical ter placement of the glass ball, CL function was presence of the device in the uterine lumen somehow maintained for approximately 90 days, during which mimicked the effect of a conceptus by impairing time progesterone levels remained above 1.0 ng/mL endometrial secretion of PGF2␣. In a subsequent and estrous behavior was not exhibited. In non- brief report, the same group determined the “em- treated control mares, spontaneous prolongation of bryo-mimicking” effect of the intrauterine device CL function occurred in four of 32 mares (13%). prevents up-regulation of endometrial gene expres- Although placement of a glass ball appeared to be an sion of cyclooxygenase-2 (COX-2) in mares that de- efficacious means of blocking estrous behavior for an velop prolonged CL function, which is the key extended period of time, it should be noted that in regulatory enzyme in PGF2␣ synthesis/secretion.24 addition to the 11 mares that retained the glass ball In nonpregnant mares, endometrial COX-2 expres- and never developed extended CL function (i.e., con- sion is upregulated on days 14 to 15, coincident with tinued to cycle normally while the glass ball was in the onset of PGF2␣ secretion, whereas the expres- the uterine lumen), three of the seven mares that sion of COX-2 remains at basal levels in pregnant received a glass ball had one or two estrous cycles of mares.25–27

344 2013 ր Vol. 59 ր AAEP PROCEEDINGS IN-DEPTH: REPRODUCTIVE ENDOCRINOLOGY nous oxytocin to mares around the time of luteolysis (ie, days 11 to 15 after ovulation) stimulates an immediate onset of PGF2␣ secretion,31–33 when oxy- tocin is administered starting in the mid- prior to the expected time of luteolysis (ie, before day 10 after ovulation), it does not induce PGF2␣ secretion and often paradoxically disrupts luteolysis causing prolonged CL function.32 Experimentally, continuous infusion of oxytocin from day 8 to 20 after ovulation blocked luteolysis in four of five mares, whereas luteolysis occurred at the expected time in all four control mares that received saline infusion.34 Although it successfully induced prolonged CL function, continuous infusion of oxy- tocin would not be a practical method of long-term suppression of estrous behavior. As an alternative, in a “proof of principle” study, we showed that twice- Fig. 1. Two glass balls manually recovered from the uterine daily intramuscular administration of 60 units (3 lumen of a mare displaying signs of urogenital discomfort. The mL) of oxytocin on days 7 to 14 after ovulation was current owner was unaware these glass balls were present in the an efficacious method of disrupting luteolysis, be- mare’s uterus. It is not known whether both glass balls were cause it caused prolonged CL function through day inserted at the same time or if one ball was placed initially and 30 after ovulation in six treated mares, whereas six then at a later date the second ball was inserted without knowl- saline-treated control mares underwent luteolysis edge of the first one. Both glass balls had noticeable pitting on by day 16 after ovulation.35 In a subsequent study, their surfaces, indicating it was likely that particulate matter from the glass balls had been deposited in the uterine lu- we compared the use of the same 60-unit dose of men. Photo courtesy of Dr Candace Jacobson. oxytocin given intramuscularly twice daily versus once daily on days 7 to 14 after ovulation and found that CL function was maintained for 50 days after ovulation in five of seven mares (71%) treated twice Although placement of an intrauterine glass ball daily, five of eight mares (63%) treated once daily, has been the most widely used method of prolonging and one of seven (14%) untreated control mares.36 CL function in mares, there have been reports of There was no difference (P Ͼ 0.05) in the proportion deleterious consequences associated with their use, of mares with extended CL function between once- such as spontaneous fragmentation of the glass ball and twice-daily administration of oxytocin, whereas in the uterine lumen.28 Although that specific com- collectively oxytocin treatment increased (P Ͻ 0.05) plication can be avoided by the use of a device made the proportion of mares with extended CL function. of an alternative material such as plastic,23 it is Therefore, the oxytocin treatment protocol can be important to note that regardless of the composition simplified to once-daily administration on days 7 to 14. of the device, if it is not eventually removed (spon- In a third study, our objective was to monitor CL taneously or manually), some mares may retain the function and estrous behavior in mares for 90 days device for an extended period of time (ie, years), such after administration of 60 units of oxytocin once that its presence in the uterine lumen may not be daily on days 7 to 14 after ovulation.37 Two of nine known to individuals working with the mare (Fig. 1). control (22%) and six of nine oxytocin-treated (67%) This has led to situations in which mares have been mares had prolonged CL function (P ϭ 0.08). The bred when an intrauterine device was present, and mean duration of CL function in the two control there are anecdotal reports of mares becoming preg- mares with prolonged CL function was 78 days and nant despite the presence of an intrauterine device in the six oxytocin-treated mares was 69 days. and subsequently aborting because the device com- In both of the control mares and one of the six promised the pregnancy later in gestation. Be- oxytocin-treated mares with prolonged luteal func- cause of potential complications such as these, there tion, estrus was not observed while progesterone is a need for alternative methods of extending CL remained above 1.0 ng/mL. For the remaining five function that are practical, efficacious, and safe. oxytocin-treated mares with prolonged luteal func- tion, weak estrus was occasionally observed during 5. Oxytocin Treatment the period of elevated progesterone. Although oxy- In contrast to the use of an intrauterine glass ball, tocin treatment effectively prolonged CL function for administration of exogenous oxytocin during di- approximately 2 months in two thirds of the treated estrus is an alternative method of blocking luteoly- mares, enigmatically, weak estrus was occasionally sis to prolong CL function. Endogenous oxytocin is observed in some mares during the period of pro- involved in regulating PGF2␣ secretion from the longed CL function. endometrium during spontaneous luteolysis in the Other research groups have recently evaluated mare,29,30 and, although administration of exoge- modifications to the oxytocin protocol for prolonging

AAEP PROCEEDINGS ր Vol. 59 ր 2013 345 IN-DEPTH: REPRODUCTIVE ENDOCRINOLOGY CL function by altering the dose, route of adminis- lation, it prevents luteolysis by inhibiting the rise in tration, and/or duration of treatment. In 2012, Gee endometrial oxytocin receptor concentration that et al38 reported that five of six mares (83%) treated would otherwise permit endogenous oxytocin-in- with 10 units of oxytocin intravenously once daily on duced PGF2␣ secretion at the time of luteolysis; days 7 to 14 had prolonged CL function compared however, there was no difference in endometrial with only one of six (17%) and two of six (33%) mares oxytocin receptor concentrations between saline- treated with 10 units oxytocin or saline intramuscu- treated control mares and oxytocin-treated mares on larly, respectively. The apparent inability of 10 day 15, which does not support that hypothesis.36 units of oxytocin administered intramuscularly to More recently, it was demonstrated that oxytocin prolong CL function indicates the threshold intra- treatment suppresses PGF2␣ secretion by prevent- muscular dose of oxytocin needed to disrupt luteoly- ing upregulation of endometrial gene expression of sis and prolong CL function is between 10 and 60 COX-2,39 which, as noted previously, appears to be units. In all of the oxytocin-treated mares that de- the mechanism by which intrauterine placement of veloped prolonged CL function, estrous behavior a polypropylene ball inhibits luteolysis and prolongs was inhibited throughout the period of extended CL CL function.24 Also, it seems clear from the results function, demonstrating the clinical efficacy of oxy- of Bare et al40 that carbetocin (with use of the dose tocin treatment for estrus suppression. In a 2013 and treatment schedule in their study) does not study, Keith et al39 initiated intramuscular treat- have the same effect. ment with 60 units of oxytocin on day 8 and com- Collectively, the studies described above provide pared three different durations of treatment (2, 4, or convincing evidence that administration of exoge- 6 days). Administration of oxytocin on days 8 to 10, nous oxytocin on days 7 (or 8) to 14 after ovulation is 8 to 12, and 8 to 14 induced prolonged CL function in an effective method of disrupting luteolysis to pro- three of seven (43%), four of seven (57%) and six of long CL function that can be used as a means of seven (86%) mares, respectively, compared with suppressing estrous behavior in mares. An advan- none of seven (0%) control mares. The proportion tage of using oxytocin treatment to prolong CL func- of mares with prolonged CL function increased (P Ͻ tion is that it can be readily reversed by simply 0.01) as the number of days of oxytocin administra- administering a luteolytic dose of PGF2␣ to initiate tion increased, confirming the need to continue oxy- resumption of cyclical reproductive activity, in con- tocin treatment until the expected time of luteolysis trast to the need to physically remove an intrauter- (ie, day 14) for maximum effectiveness. Also in ine glass ball. Disadvantages of the oxytocin 2013, Bare et al40 compared the estrous cycle char- protocol include the need to determine the exact day acteristics of mares treated with oxytocin or the of ovulation before initiating treatment and poten- synthetic oxytocin analog carbetocin on days 7 to 14 tial difficulties associated with its use in mares with after ovulation. Carbetocin has a circulating half- a needle aversion (ie, “needle-shy”). Although the life after intravenous administration 2.5 times lon- dose of oxytocin is fairly high (60 units), no differ- ger than oxytocin (17.2 minutes versus 6.8 minutes, ence in body temperature, heart rate, or respiratory respectively).41 Oxytocin-treated mares received rate was noted before and after treatment; however, 60 units of oxytocin intramuscularly once daily, mild sweating, urticaria around the injection site, whereas carbetocin-treated mares received 1.19 mg and mild diarrhea were noted in some mares.40 carbetocin intramuscularly, which, on the basis of Notably, there was no evidence of abdominal cramp- its pharmacokinetics, is equivalent to 60 units of ing and/or transient signs of colic,40 which suggests oxytocin. Compared with nontreated control cy- the myometrium is much less responsive to oxytocin cles, administration of oxytocin increased the inter- during mid- to late-diestrus than it is during other estrus and inter-ovulatory intervals (P Ͻ 0.01), physiologic states (eg, immediately post-partum). whereas carbetocin shortened the inter-estrus and inter-ovulatory intervals (P Ͻ 0.01), essentially 6. Inducing a Late-Diestrus Ovulation “short-cycling” the mares. Therefore, with the use In 2006, Hedberg et al43 described the results of a of this dose and treatment schedule, carbetocin was preliminary study in which their objective was to not efficacious for prolonging CL function. prolong the luteal phase in mares with the use of In nonpregnant mares, the ability of the endome- human chorionic gonadotropin (hCG) to induce a trium to secrete PGF2␣ in response to oxytocin (en- late-diestrus ovulation to produce a new CL that dogenous or exogenous) increases markedly between would be too immature to respond to the luteolytic days 10 and 15 after ovulation concomitantly with effects of endogenous PGF2␣ secretion at the end of an increase in the concentration of oxytocin recep- diestrus (ie, day 14 to 15 after the initial “primary” tors33,42 and PGF2␣ synthetic enzymes25 in the en- ovulation). Mares were randomly assigned to con- dometrial cells. In contrast, before day 10, the trol (n ϭ 4) and experimental groups (n ϭ 5), and, concentration of endometrial oxytocin receptors33,42 beginning on approximately day 8 after ovulation (or and PGF2␣ synthetic enzymes25 are low, which ef- last signs of estrous in 3 mares), their ovaries were fectively blocks the ability of oxytocin to stimulate examined with transrectal ultrasonography every PGF2␣ secretion. We hypothesized that when oxy- other day to determine the size(s) of their diestrus tocin treatment is initiated before day 10 after ovu- follicles. When a diestrus follicle Ն30 mm was de-

346 2013 ր Vol. 59 ր AAEP PROCEEDINGS IN-DEPTH: REPRODUCTIVE ENDOCRINOLOGY tected, control mares were treated with saline and peanut oil (neither containing estradiol) on day 10 experimental mares were treated with 3000 IU hCG induced prolonged CL function in 92% of the treated intramuscularly. After treatment, the mares were mares in both groups. In contrast to infusion of monitored with transrectal ultrasonography for up plant oils, infusion of mineral oil on day 10 did not to 72 hours or until ovulation was detected, and then reliably prolong CL function, whether it was admin- once weekly for 3 weeks. If a mare did not have a istered alone or in combination with estradiol (17% diestrus follicle Ն30 mm during the first diestrus and 25% prolonged CL function, respectively). The period, she was monitored for a second, and if nec- authors postulated that the fatty acid milieus in essary, a third diestrus period. both plant oils modulated/attenuated synthesis Three of the nine mares had development of a and/or secretion of PGF2␣ at the expected time of follicle Ն30 mm during the first diestrus period, four luteolysis, resulting in prolonged function of the CL. mares during the second diestrus period, and one Given the high proportion of mares in which pro- mare in the third diestrus period. One mare never longed CL function occurred when fractionated co- had a diestrus follicle that was Ն30 mm during conut oil or peanut oil was infused on day 10 (92% in three diestrus periods and therefore could not be both groups), infusion of plant oil appears to be a treated with hCG. Overall, three of the four mares plausible method of prolonging CL function for es- (75%) treated with hCG ovulated within 72 hours trus suppression, although additional work will be after treatment with hCG, which resulted in luteal needed to fully develop a practical protocol for that phases that lasted for 58 to 82 days after treatment. purpose and to ensure there is no detrimental effect None of the control mares ovulated during the luteal on subsequent fertility. phase; however, one control mare had a spontane- ously prolonged luteal phase during both a non- 8. Pregnancy treated cycle in which she never developed a Pregnancy is another means of suspending cyclicity diestrus follicle Ն30 mm (CL function was termi- by taking advantage of the natural ability of the nated with exogenous PGF2␣) and during the sub- conceptus to block luteolysis and maintain CL func- sequent cycle in which she was treated with saline tion/progesterone secretion. Although efficacious, when she had a large diestrus follicle (that did not this method has obvious disadvantages that may ovulate). make it undesirable for many horse owners. In Although, on the basis of this study, the use of addition to the time and expense necessary to estab- hCG to induce a late-diestrus ovulation looks prom- lish pregnancy is the need to eventually terminate a ising for prolonging CL function, it is important to normally developing pregnancy (unless an offspring note that some mares (five of nine) required multiple is ultimately desired). Lefranc and Allen46 re- estrous cycles to develop a diestrus follicle Ն30 mm. ported that manual transrectal rupture of the con- In addition, one mare never developed a large di- ceptus between days 16 and 22 of gestation in 11 estrus follicle during the three cycles that were mon- mares resulted in continued CL function for at least itored, which precluded her from receiving the hCG 60 days in all of the mares, during which time they treatment. Therefore, in addition to the effort (and did not display estrous behavior. Although effica- expense) of monitoring mares to evaluate their suit- cious, as noted above, terminating a normal, healthy ability for treatment, the fact that some mares may pregnancy may be untenable to many horse owners. not develop a diestrus follicle large enough to war- rant treatment, the use of hCG to induce a late- 9. Conclusions diestrus ovulation does not appear to be a reliable, Because of drawbacks and concerns associated with “on-demand” method of blocking estrous behavior in the use of altrenogest for suppression of estrus in mares. It is interesting to note that although the mares, there is increasing interest in the develop- use of hCG was efficacious for inducing ovulation of ment of practical methods of prolonging CL function, diestrus follicles Ն30 mm in diameter in the study which allows continued secretion of endogenous pro- by Hedberg et al,43 previous work by Glazar et al44 gesterone to keep mares out of heat naturally. found that administration of the GnRH agonist Although placement of an intrauterine glass ball has deslorelin acetate failed to induce ovulation and/or been the most widely utilized method of prolonging luteinization of diestrus follicles Ͼ30 mm in CL function, their variable efficacy and potential for diameter. deleterious consequences (if not eventually re- moved), has prompted interest in the development of 7. Intrauterine Infusion of Plant Oils additional methods of prolonging CL function. Of In 2011, Wilsher and Allen45 reported that intra- the current alternatives to an intrauterine glass uterine infusion of 10 mg estradiol in 1 mL fraction- ball, oxytocin treatment appears to be the most prac- ated coconut oil on day 6, 8, 10, 12, or 14 after tical and efficacious method of prolonging CL func- ovulation resulted in prolonged CL function in 25%, tion. Administration of 60 units of oxytocin 75%, 92%, 83%, and 50% of treated mares, respec- intramuscularly once daily on days 7 (or 8) to 14 tively; however, they also demonstrated that estra- after ovulation induces prolonged CL function in diol was not needed to induce prolonged CL function, 60% to 70% of treated mares, which can suppress because infusion of 1 mL fractionated coconut oil or estrous behavior for approximately 2 months. Ad-

AAEP PROCEEDINGS ր Vol. 59 ր 2013 347 IN-DEPTH: REPRODUCTIVE ENDOCRINOLOGY ditional methods of prolonging CL function in mares 24. Rivera Del Alamo MM, Reilas T, Kindahl H, et al. The include inducing a late-diestrus ovulation, intra- luteostatic effect of intrauterine devices in mares is associ- ated with endometrial expression of COX-2. Anim Reprod uterine infusion of plant oils, and establishment of Sci 2010;121S:S227–S228. pregnancy followed by manual reduction of the con- 25. Boerboom D, Brown KA, Vaillancourt D, et al. Expression of ceptus after day 16 of gestation. key prostaglandin synthases in equine endometrium during late diestrus and early pregnancy. Biol Reprod 2004;70: 391–399. References and Footnotes 26. Ealy AD, Eroh ML, Sharp DC, Prostaglandin H synthase 1. Jorgensen JS, Vivrette S, Correa M, et al. Significance of Type 2 is differentially expressed in endometrium based on the estrous cycle on athletic performance in mares, in Pro- pregnancy status in pony mares and responds to oxytocin and ceedings. Am Assoc Equine Pract 1996;42:98–100. conceptus secretions in explant culture. Anim Reprod Sci 2. McDonnell SM. Estrus cycle-related performance problems. 2010;117:99–105. J Equine Vet Sci 1997;17:196. 27. Atli MO, Kurar E, Kayis SA, et al. Evaluation of genes 3. McDonnell SM. Evaluation and modification of mare behav- involved in prostaglandin action in equine endometrium dur- ior problems. In: Proc Ann Mtg Soc Therio 1993;185–189. ing estrous cycle and early pregnancy. Anim Reprod Sci 4. McDonnell S. Performance problems in mares. Horse 2000; 2010;122:124–132. April:61–70. 28. LeBlanc MM. Oxytocin. In: McKinnon AO, Squires EL, 5. McDonnell SM. Is it psychological, physical, or both?, in Vaala WE, Varner DD, editors. Equine Reproduction. 2nd Proceedings. Am Assoc Equine Pract 2005;51:231–238. edition. Oxford: Wiley-Blackwell; 2011:1830–1835. 6. Pryor P, Tibary A. Management of estrus in the perfor- 29. Vanderwall DK, Silvia WJ, Fitzgerald BP. Concentrations mance mare. Clin Tech Equine Pract 2005;4:197–209. of oxytocin in the intercavernous sinus of mares during lute- 7. Cox JH, DeBowes RM. Colic-like discomfort associated with olysis: temporal relationship with concentrations of 13,14-

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