Equine Reproduction - Prostaglandins PDF

P1: SFK/UKS P2: SFK Color: 1C c187 BLBK232-McKinnon February 1, 2011 16:2 Trim: 279mm X 216mm Printer Name: Yet to Come Chapter 187 Prostaglandins...

P1: SFK/UKS P2: SFK Color: 1C c187 BLBK232-McKinnon February 1, 2011 16:2 Trim: 279mm X 216mm Printer Name: Yet to Come Chapter 187 Prostaglandins Simon A. Staempfli Prostaglandin F2α (PGF2α ) does not possess the characteristic intertwining of the uterine vein and the ovarian artery in the ovarian In 1972, PGF2α was first reported to cause lysis of pedicle, as occurs in the ewe and other farm animals.3 the corpus luteum, thereby allowing cycling mares Therefore, when endometrial PGF2α is released dur- to return to estrus 3 to 4 days after treatment.1 ing Days 14 to 17 in the cycling mare, it can only Since then, PGF2α has been extensively used in reach the ovary by passing through the cardiovascu- equine stud medicine to manipulate normal and ab- lar system, where it is subject to rapid metabolism normal estrous cycles, terminate early pregnancies, in the lungs and other organs. The lack of a local synchronize horses for breeding or embryo trans- utero-ovarian transport system in the mare results in fer programs, induce gonadotropin secretion, and lower peripheral prostaglandin levels during luteol- treat uterine infections. The most frequently admin- ysis, requiring an increased sensitivity of the corpus istered prostaglandins in equine practice are the luteum to prostaglandin compared to other species, naturally occurring PGF2α , dinoprost tromethamine on a dose per bodyweight basis.4 Additionally, it is (Lutalyse
R ; Pfizer Animal Health, New York), suspected that there is a slower peripheral degrada- and the synthetic prostaglandin analog for cattle, tion of PGF2α in the tissues of the horse, especially in cloprostenol (Estrumate
R ; Schering-Plough Animal the lungs, which permits systemic distribution and Health Corp., Union, NJ). Luprostiol (Prosolvin
R ; recirculation.5 Virbac Animal Health, Bury St Edmunds, UK) and Administration of PGF2α is equally effective by the recently developed PGF2α analog d-cloprostenol intramuscular, intravenous, intrauterine, subcuta- (Genestran
R ; Dr E. Graeub AG, Bern, Switzerland) neous, and intraluteal routes,4, 6, 7 although the in- are two additional products registered in Europe tramuscular route is usually preferred for simplic- for use in horses. Other PGF2α analogs such as ity and reduced side-effects. However, even when fluprostenol, fenprostalene, or alfaprostenol have prostaglandins are given at recommended doses by been used in some studies cited in this chapter, but intramuscular injection, about 10% of horses expe- are no longer commercially available. rience some side-effects including sweating, diar- rhea, or abdominal discomfort for up to 20 minutes.5 Early studies using varying doses of PGF2α showed Dosage and administration dose-dependent side-effects, particularly in the fre- The standard recommended dose for a 500-kg quency and severity of sweating and hyperthermia.6 horse is 5–10 mg for dinoprost tromethamine (10– Very large doses of PGF2α (i.e., up to 500 µg/kg 20 µg/kg), 7.5 mg for luprostiol (15 µg/kg), 250 µg PGF2α given intramuscularly or subcutaneously) pro- for cloprostenol (0.55 µg/kg), and 22.5–37.5 µg for duced ataxia, increased gastrointestinal motility, pro- d-cloprostenol (0.045–0.075 µg/kg). Early studies tracted diarrhea, hyperthermia, labored respiration, showed that the minimal effective luteolytic dose rate and changes in blood chemistry and hematological of a single injection of free acid PGF2α in ponies parameters. Nevertheless, no mares have reportedly was 6 µg/kg and in horses 9 µg/kg in comparison died or appeared to sustain adverse effects following to 144 µg/kg for the ewe or the cow.2 The reason for recovery.6 Although even very large doses are not life the higher susceptibility of the equine corpus luteum threatening, and the standard clinical dose appears to prostaglandin is because of the unique vascular to cause only transient discomfort in some horses, anatomy of the mare’s reproductive tract. The mare several recent studies have aimed to minimize the Equine Reproduction, Second Edition. Edited by Angus O. McKinnon, Edward L. Squires, Wendy E. Vaala and Dickson D. Varner
c 2011 Blackwell Publishing Ltd. P1: SFK/UKS P2: SFK Color: 1C c187 BLBK232-McKinnon February 1, 2011 16:2 Trim: 279mm X 216mm Printer Name: Yet to Come 1798 Mare: Pharmacological Manipulation of Reproduction unpleasant side-effects by reducing the dose admin- induced luteolysis. A recent study using 22 Martina istered. Research published in 2001 showed that a Franca jennies showed that PGF2α administered on microdose of 25 µg cloprostenol administered intra- day 3 post-ovulation caused complete structural and muscularly was as effective at lysing the equine cor- functional regression of the corpus luteum, with in- pus luteum as the larger standard dose (250 µg).8 duction of estrus within 4 days in 95.5% of treated Two other studies induced luteolysis with a single animals.16 administration of 1.25 mg dinoprost,9, 10 and one pub- Timing of prostaglandin administration during lication reported successful luteolysis with admin- diestrus affects outcome. On average, mares return istration of two injections of 0.5 mg of dinoprost to estrus 3 to 4 days after administration, and ovula- 24 hours apart.11 In all four studies, the investiga- tion occurs 8 to 10 days after treatment.17 It is impor- tors did not observe the occurrence of PGF2α -induced tant to remember that elevated blood progesterone side-effects in treated mares. concentrations do not suppress increases in blood concentration of follicle-stimulating hormone (FSH) in the horse; therefore follicular development con- Indications tinues during diestrus. Consequently, there are often follicles of various sizes and stages of growth dur- Termination of a normal luteal phase ing the luteal phase. The size of the follicle at the In a high proportion of normally cycling mares, the time of prostaglandin administration is very impor- corpus luteum is not responsive to one single injec- tant for predicting the interval to estrus and ovu- tion of PGF2α until day 5 after ovulation.12 How- lation. When only small follicles are present at the ever, recent studies have discovered that treatment time of prostaglandin administration, a new follicular during early diestrus is capable of inducing func- wave has to emerge and ovulation of a dominant fol- tional regression of the corpus luteum. Bergfelt and licle occurs 7 to 12 days after treatment.18 Mares with co-workers investigated the response of the corpus medium-sized follicles (2.5 to 3.5 cm) will show signs luteum in the mare after injection of a single dose of of estrus soon after PGF2α administration and ovu- PGF2α (10 mg/mare) at 3 days after ovulation.13 They late within 3 to 6 days. When there is a large growing observed a functional and structural regression of the follicle present (3.5 to 5 cm) at the time of treatment, corpus luteum, based on an abrupt decrease in circu- ovulation is possible within 24 to 72 hours after treat- lating concentrations of progesterone, a decrease in ment, without overt signs of estrus (standing heat, luteal gland diameter, and an increase in luteal tis- relaxation of the cervix, endometrial edema).19, 20 sue echogenicity. Three days after treatment, func- Conversely, if the follicle has reached its maxi- tional resurgence of the corpus luteum was detected mal diameter during the progesterone-dominated in 75% of the mares (12 of 16), as indicated by a tran- luteal phase, this follicle is destined for atresia and sient major (six mares) and minor (six mares) increase will inevitably regress and a new cohort of folli- in progesterone. Despite the functional resurgence cles will have to be recruited, therefore estrus and in progesterone, the mean length of the interovula- ovulation will be delayed.21 If a large growing fol- tory interval in all mares treated on day 3 after ovu- licle is present, administration of microdoses of lation was significantly shortened (13.2 ± 0.9 days), prostaglandin (1.25 mg dinoprost) several times daily compared to mares injected on Day 10 after ovula- until regression of the corpus luteum, has been suc- tion (19.2 ± 0.7 days).13 When PGF2α (2.5 mg/mare) cessful in bringing mares slowly into estrus, and was administered once daily on Days 2, 3, and 4 thereby preventing the dominant follicle from ovu- after ovulation, 6 out of 10 mares underwent com- lating too rapidly (A.O. McKinnon, personal com- plete luteolysis.14 The six mares that underwent com- munication, 2008). The degree of endometrial edema plete luteolysis ovulated 9.4 ± 1.36 days after treat- can be used to predict the response to treatment, as ment. The remaining four mares underwent partial mares with a large, growing follicle can have substan- luteolysis, followed by resurgence in circulating pro- tial endometrial edema within 24 hours after treat- gesterone concentrations. Interestingly, three of the ment. However, mares with large atretic follicles do mares undergoing partial luteolysis ovulated prema- not exhibit endometrial edema until 3 to 5 days after turely with high concentrations of circulating proges- treatment.22 terone at days 9, 15, and 16 after treatment.14 When 10 mg of PGF2α was administered twice daily on days Termination of a persistent luteal state 0, 1, and 2 post-ovulation, the mean plasma proges- terone concentration on day 4 remained < 1.0 ng/mL, The corpus that forms after ovulation is usually and four of six mares ovulated 7.0 ± 1.8 days after functional for 14 to 15 days in the non-pregnant treatment.15 Unlike the horse, the donkey corpus lu- mare,23 after which time luteolysis occurs as a result teum seems to be susceptible earlier to prostaglandin- of prostaglandin release from the endometrium.24 P1: SFK/UKS P2: SFK Color: 1C c187 BLBK232-McKinnon February 1, 2011 16:2 Trim: 279mm X 216mm Printer Name: Yet to Come Prostaglandins 1799 Corpora lutea that fail to regress at the normal time especially when ovulation induction is included in post-ovulation are considered to be pathologically the protocol. persistent.25 The most common causes of a persis- tent luteal phase are ovulations late in diestrus, re- Induction of gonadotropin secretion sulting in immature corpora lutea (< 5 days old) at the time of prostaglandin release,25 embryonic loss Experiments in farm animals have shown that PGF2α after the time of maternal recognition of pregnancy, can increase blood luteinizing hormone (LH) concen- chronic uterine infections with destruction of the en- tration. Treatment with PGF2α increased LH concen- dometrium and therefore diminished prostaglandin tration in bulls,30, 31 diestrous mares,32, 33 and tran- release, and formation of a luteinized anovulatory sitional anestrous mares.34 In transitional anestrous follicle.26 The persistent luteal stage may persist for mares with low circulating progesterone concentra- 2 to 3 months, during which time the mare will not tions, the PGF2α analog luprostiol increased LH con- express behavioral estrus. Regardless of the cause of centration in the pituitary venous blood. The initial the persistent luteal state, administration of a sin- rise in LH concentration was attributed to a direct gle intramuscular dose of dinoprost tromethamine action on the pituitary gland. Following the rise in (5–10 mg) or cloprostenol (250 µg) will result in lu- LH, there was an increase in gonadotropin-releasing teolysis and subsequent estrus in more than 80% of hormone (GnRH), which in turn sustained the eleva- cases.5 tion of LH for 2 hours post-treatment.34 Similarly, in early diestrus intravenous cloprostenol caused an im- mediate marked increase in FSH and LH secretion, which lasted 1 to 2 hours and was not associated Estrous synchronization with any increase in GnRH concentration.5 Unlike in Synchronization of estrus and ovulation may allow anestrus or diestrus, PGF2α and its analogs failed to mares to be bred or inseminated at a predetermined induce an increase in LH and FSH concentration dur- time, thus simplifying breeding management and ing estrus.35, 36 It is suspected that hypothalamic or providing more efficient use of heavily booked stal- pituitary prostaglandin synthetic capabilities and re- lions over a predetermined time period. Additionally, ceptor numbers are markedly different in diestrous estrous synchronization is an integral part of equine and transitional anestrous mares compared to estrous embryo transfer programs, where a close synchrony mares. between donor and recipient mare is crucial for sur- Despite these findings, one study reported that vival of the transferred embryo. the PGF2α analog, fenprostalene, induced ovulation Synchronization of estrus has been used suc- in 81% of treated mares within 48 hours following cessfully as a management tool in the cattle in- treatment compared to 31% of control mares.37 The dustry, where the administration of two doses of exact mechanism by which fenprostalene was able prostaglandin 11 to 12 days apart results in more than to hasten ovulation remains unknown. The authors 80% of the cows exhibiting estrus within 2 to 5 days suggested that the site of action was either along the after the second injection.27 Although administration hypothalamic–pituitary axis or at the ovary. of two doses of prostaglandin 14 days apart has been described as a protocol for estrus synchronization in Induction of abortion mares, it is not a very reliable method of ensuring that a recipient mare will be synchronized with a Prostaglandins are the preferred method of inducing donor mare.19 After two injections of prostaglandin, abortion of the early equine pregnancy. As described ovulation occurs on average 7 to 10 days after the previously, the corpus luteum is not particularly second injection, but ovulation can potentially occur responsive during the first 5 days after ovulation. anywhere from 0 to 17 days.28 Therefore it was con- Thereafter administration of a single injection of cluded that one would need to treat at least 10 recipi- dinoprost (5–10 mg) or cloprostenol (250 µg) will ent mares to have an 80% chance that at least one re- result in luteolysis, with subsequent loss of the cipient would ovulate within 24 hours of the donor.29 conceptus from day 5 to day 30 of gestation,38 al- Due to the large variation in time of ovulation from though in some cases after maternal recognition of administration of prostaglandin, a double injection pregnancy two or more consecutive injections may scheme is not very useful when used alone in the be necessary to lyse the primary corpus luteum.39 synchronization of the estrous cycles in mares. How- Abortion can be expected 2 to 5 days after treat- ever, if examination via transrectal ultrasonography ment.40 The endometrial cups, a band of specialized is used to eliminate those mares that will not respond trophoblastic cells invading the endometrium, are es- appropriately to the administration of prostaglandin, tablished during days 28 to 35 of pregnancy.3 The cup tightening of the date of ovulation can be achieved, cells produce equine chorionic gonadotropin (eCG), P1: SFK/UKS P2: SFK Color: 1C c187 BLBK232-McKinnon February 1, 2011 16:2 Trim: 279mm X 216mm Printer Name: Yet to Come 1800 Mare: Pharmacological Manipulation of Reproduction which stimulates the primary corpus luteum and the induced uterine contractions may assist in lymphatic formation of secondary or accessory corpora lutea flow as lymph drainage relies on myometrial con- (jointly known as supplemental corpora lutea).39 tractions.55 Cloprostenol produces a more consistent Therefore, multiple injections of prostaglandin are response than PGF2α and fenprostalene.54 The sug- necessary for successful termination of pregnancy gested dose of cloprostenol is 250 µg given at 12 and after establishment of the endometrial cups. Two 24 hours after ovulation. Cloprostenol needs to be ad- studies showed that all mares given 2.5 to 5.0 mg ministered with caution after ovulation because ad- PGF2α or 250 µg of fluprostenol, once or twice a day, ministration after Day 2 from ovulation is associated aborted 3 to 5 days after starting treatment.41, 42 Al- with decreased plasma progesterone concentrations though the pregnancy is terminated, the endometrial on Days 3 through 7 of the estrous cycle. Although cups remain functional until they are removed by the plasma progesterone concentrations rebounded by maternal immune system approximately 60–80 days Days 7 to 9 of the estrous cycle, two reports showed after establishment.43 As long as the cups are func- decreased pregnancy rates when cloprostenol was tional and eCG is produced, the mare will not return given at 500 µg until Day 2 after ovulation.56, 57 When to normal estrous cycles after pregnancy termination. 250 µg was given every 24 hours from 4 hours after Elective termination of pregnancy beyond the breeding until Day 2 post-ovulation, pregnancy rates first trimester may be complicated by dystocia, re- were similar to those of mares treated post-ovulation tained fetal membranes, or trauma to the genital with oxytocin.58 tract. Although multiple injections of PGF2α have resulted in abortions in mares between 100 and 245 days of gestation,44 two injections of fluprostenol Prostaglandin E2 (PGE2 ) or prostaglandin administered between 160 and Dinoprostone, a naturally occurring PGE2 , has been 320 days of gestation failed to induce abortion.45 widely used in human gynecology as a vaginal sup- Therefore PGF2α is not the preferred method of in- pository for induction of abortion, labor, and initi- ducing abortion after the first trimester of pregnancy, ation of cervical ripening near term.59–61 Although and other methods such as large-volume saline infu- dinoprostone is not approved for use in horses, the sions should be considered.46 effects of PGE2 on the reproductive tract in the mare and other species have been extensively investigated Induction of parturition in research studies over the last 20 years. As in hu- man medicine, it is thought that PGE2 induces cervi- Induction of parturition with PGF2α is possible,47, 48 cal relaxation in the mare near term.62 Additionally, but no longer recommended because of an increased PGE2 appears to play a role in gamete and embryo incidence of premature placental separation and de- transport through the oviduct.63 One study showed creased foal viability associated with its use. Detailed the potential role of PGE2 as a stimulator of uterine information on induction of parturition is presented contractions during the embryo mobility phase of the in Chapter 232. first two weeks of pregnancy.64 Treatment of uterine infections Indications Progesterone predisposes the uterus to infection by Accelerated spermatozoal transport constricting the cervix and inhibiting uterine con- through the reproductive tract tractions, whereas estradiol has the opposite effect on the uterus and also stimulates the local immune PGE2 may play an important role in gamete and system.5 Administration of PGF2α to diestrous mares embryo transport through the oviduct, by increasing allows them to come into heat, thereby increasing es- oviductal smooth muscle contraction. Electromyo- trogens and relaxing the cervix. More importantly, graphic activity of the equine oviduct is stimulated PGF2α has a direct effect on the myometrium.49–51 by intramuscular administration of 10 mg PGE2 ,53 Prostaglandins have been demonstrated to be re- and locally applied PGE2 induces relaxation of circu- leased very early in mares with endometritis, and lar smooth muscle of the oviductal isthmus.65 Smooth this has subsequently been shown to play a useful muscle contractions at the utero-tubular junction role in increasing myometrial activity and assisting (UTJ) may play a role in propulsion of spermatozoa in uterine clearance.52 While oxytocin induces strong through the UTJ into the isthmus in the pig.66 Addi- uterine contractions for 30 to 50 minutes, PGF2α pro- tion of PGE2 to ram semen increased pregnancy rates duces low-amplitude contractions that persist for after artificial insemination in ewes.67 Although PGE2 4 to 5 hours.53 Oxytocin clears radiocolloid more has been shown to promote Ca2+ influx and acro- rapidly than prostaglandins,54 but prostaglandin- some reaction in human spermatozoa,68 it remains P1: SFK/UKS P2: SFK Color: 1C c187 BLBK232-McKinnon February 1, 2011 16:2 Trim: 279mm X 216mm Printer Name: Yet to Come Prostaglandins 1801 unknown whether increased pregnancy rates after foal vigor in treated mares.62 PGE2 has also been addition of PGE2 to the inseminate in some studies combined with estrogens (estradiol cypionate, 6 mg, were caused by accelerated spermatozoal transport given 24 hours before induction) to dilate the cervix through the reproductive tract or by direct effects in a protocol for oxytocin-induced parturition.74 of PGE2 on spermatozoa. The intrauterine infusion of To the author’s knowledge there are no pub- 0.25 mg PGE2 2 hours prior to the insemination of lished controlled studies on the use of PGE2 or PGE1 good-quality semen improved pregnancy rates in for cervical dilation in mares with impaired cervi- a study of 34 light-breed mares; however, no im- cal drainage and subsequent uterine fluid accumula- provement in pregnancy rates was observed when a tion during estrus. Topical application of PGE1 4 to stallion with poor semen quality was used.69 Further 6 hours before natural mating or artificial insemina- research is needed to determine the effects of locally tion, was reported to relax the cervix for up to 8 hours applied PGE2 and pregnancy rates achieved by after treatment.75 In some cases multiple applications subfertile stallions. may be necessary (A.O. McKinnon, personal commu- nication, 2008), although repeated use has been as- sociated with inflammation of the cervical and vagi- Re-establishment of oviductal patency nal mucosa.75 However, success of PGE2 treatment in In 2006 Allen and co-workers reported laparoscopic older maiden mares with a tight cervix remains vari- application of PGE2 to re-establish oviductal patency able, due to a decrease in elastic tissue and an increase in infertile mares.70 The study was performed on in the amount of fibrotic tissue in those mares.74 15 infertile mares, with no identifiable pathology of the reproductive tract, that consistently failed to conceive when bred to fertile stallions during one Summary to four breeding seasons. The authors hypothesized Despite a wide range of possible applications of that the oviducts of these mares were temporarily prostaglandins in equine stud medicine, the major in- obstructed by naturally occurring debris (oviductal dication for PGF2α treatment remains luteolysis. Re- masses), thus preventing oviductal transport of the cent studies suggest that doses smaller than those embryo to the uterus. After application of 0.2 mg of recommended by the manufacturer are effective in PGE2 onto the external surface of the oviduct, 93% inducing complete luteolysis by 5 days after ovula- (14/15) of the mares conceived within the same or tion. The subsequent induced estrus is characterized the subsequent breeding season. Although these re- by normal ovarian dynamics and conception rates. sults are very promising, this study is limited by a lack of controls and no direct evidence that oviductal blockage was the cause of infertility. References 1. Douglas RH, Ginther OJ. Effect of prostaglandin F2α on Cervical ripening prior to induction length of diestrus in mares. Prostaglandins 1972;2:265–8. of parturition 2. Oxender WD, Noden PA, Bolenbaugh DL, Hafs HD. Con- trol of estrus with prostaglandin F2α in mares: minimal Intracervical administration of PGE2 in women effective dose and stage of estrous cycle. Am J Vet Res results in cervical ripening, a term used in human 1975;36:1145–7. medicine to describe softening and dilation of the 3. Ginther OJ. Equine pregnancy: physical interactions be- tween the uterus and conceptus. P Annu Conv Am Assoc cervix.61 It is believed that PGE2 leads to a relaxation Equine 1998;44:73–104. of the cervical smooth muscle fibers, resulting in 4. Douglas RH, Ginther OJ. Route of prostaglandin F2α a dilated cervix, which allows passage of the fetus injection and luteolysis in mares. P Soc Exp Biol Med through the birth canal.60 In women, intracervical 1975;148:263–9. deposition of PGE2 has been associated with a 5. Irvine CH. Prostaglandins. In: McKinnon AO, Voss J (eds) shorter induction-to-delivery interval, a lower num- Equine Reproduction. London: Lea & Febiger, 1993; pp. 319– 24 ber of required caesarean sections, and a decreased 6. Lauderdale J, Miller P. Regulation of reproduction in maximum dosage of oxytocin required to induce mares with prostaglandins. P Annu Conv Am Assoc Equine parturition.71, 72 In cattle, intravaginal application of 1975;21:235–44. 2 mg dinoprostone increased cervical mucus secre- 7. Weber JA, Causey RC, Emmans EE. Induction of luteolysis tion, and significantly relaxed the cervix within 6 to in mares by ultrasound-guided intraluteal treatment with 10 hours after application.73 In a controlled equine PGF2α. Theriogenology 2001;55:1769–76. 8. Nie GJ, Goodin AN, Braden TD, Wenzel JGW. Luteal study, Rigby and co-workers deposited 2.0 to 2.5 mg and clinical response following administration of dinoprost PGE2 intracervically 6 hours prior to induction of tromethamine or cloprostenol at standard intramuscular parturition at term, and observed shorter delivery sites or at the lumbosacral acupuncture point in mares. Am periods, facilitated fetal delivery, and increased J Vet Res 2001;62:1285–9. P1: SFK/UKS P2: SFK Color: 1C c187 BLBK232-McKinnon February 1, 2011 16:2 Trim: 279mm X 216mm Printer Name: Yet to Come 1802 Mare: Pharmacological Manipulation of Reproduction 9. Wuestenhagen A, Handler J, Kindahl H, Aurich C. Luteal 28. Bradecamp EA. Estrous synchronization. In: Samper JC, function and estrous cycle characteristics in mares treated Pycock JF, McKinnon AO (eds) Current Therapy in Equine with subtherapeutic doses of prostaglandin F2α. Theri- Reproduction. St Louis: Saunders Elsevier, 2007; pp. 22–5. ogenology 2002;58:537–9. 29. Irvine CHG. 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J Reprod Fertil Suppl sponse time following prostaglandin. Unpublished work 1991;44:686–8. cited in Asbury AC. P Annu Conv Am Assoc Equine, 1988, 37. Savage NC, Liptrap RM. Induction of ovulation in cyclic pp. 191–6. mares by administration of a synthetic prostaglandin, 19. Bristol F. Control of the mare’s reproductive cycle. In: fenprostalene, during oestrus. J Reprod Fertil Suppl Proceedings of the Annual Meeting of the Society of The- 1987;35:239–43. riogenology, 1987, pp. 309–21. 38. Penzhorn BL, Bertschinger HJ, Coubrough RI. Reconcep- 20. Asbury AC. The use of prostaglandins in broodmare prac- tion of mares following termination of pregnancy with tice. In: Proceedings of the Annual Convention of the Amer- prostaglandin F2α before and after day 35 of pregnancy. ican Association of Equine Practitioners 1988, pp. 191–196. Equine Vet J 1986;18:215–17. 21. Samper JC. Induction of estrus and ovulation: Why 39. Ginther OJ. Reproductive Biology of the Mare: Basic and Ap- some mares respond and others do not. Theriogenology plied Aspects, 2nd edn. Cross Plains, WI: Equiservices, 1992. 2008;70:445–7. 40. Baucus KL, Squires EL, Morris R, McKinnon AO. The ef- 22. Samper JC, Geertsema H, Hearn P. Rate of luteolysis, fol- fect of stage of gestation and frequency of prostaglandin liculogenesis and interval to ovulation of mares treated injection on induction of abortion in mares. In: Proceedings with a prostaglandin analogue on d 6 or 10 of the es- of the 10th Equine Nutrition and Physiology Symposium, trous cycle. In: Proceedings of the Annual Convention of 1987, 255–8. the American Association of Equine Practitioners, 1993, 41. Squires EL, Hillman RB, Pickett BW, Nett TM. Induction pp. 169–71. of abortion in mares with equimate: effect on secretion of 23. Ginther OJ, Pierson RA. Regular and irregular characteris- progesterone, PMSG and reproductive performance. J Anim tics of ovulation and the interovulatory interval in mares. Sci 1980;50:490–5. J Equine Vet Sci 1989;9:4–12. 42. Rathwell AC, Asbury AC, Hansen PJ, Archbald LF. Re- 24. Neely DP, Kindahl H, Stabenfeldt GH. Prostaglandin re- productive function of mares given daily injections of lease patterns in the mare: Physiological, pathophysio- prostaglandin F2α beginning at day 42 of pregnancy. The- logical, and therapeutic responses. J Reprod Fertil Suppl riogenology 1987;27:621–30. 1979;27:181–9. 43. Allen WR. Immunological aspects of the endometrial cup 25. Stabenfeldt GH, Hughes JP, Evans JW, Neely DP. Sponta- reaction and the effect of xenogenic pregnancy in horses neous prolongation of luteal activity in the mare. Equine Vet and donkeys. J Reprod Fertil Suppl 1982;31:57–94. J 1974;6:158–63. 44. Madej A, Kindahl H, Nydahl C, Edqvist LE, Stewart DR. 26. McCue PM. Review of ovarian abnormalities in the mare. Hormonal changes associated with induced late abortions P Annu Conv Am Assoc Equine 1998;44:125–33. in the mare. J Reprod Fertil Suppl 1987;35:479–84. 27. Wright PJ, Malmo J. Pharmacologic manipulation of fertil- 45. Leadon DP, Rossdale PD, Jeffcott LB, Allen WR. A com- ity. Vet Clin N Am-Food A 1992;8:57–89. parison of agents for inducing parturition in mares in the P1: SFK/UKS P2: SFK Color: 1C c187 BLBK232-McKinnon February 1, 2011 16:2 Trim: 279mm X 216mm Printer Name: Yet to Come Prostaglandins 1803 pre-viable and premature periods of gestation. J Reprod Fer- 61. Trofatter KF, Bowers D, Gall SA, Killam AP. Preinduction til Suppl 1982;32:597–602. cervical ripening with prostaglandin E2 gel. Am J Obstet Gy- 46. Paccamonti DL. Elective termination of pregnancy in necol 1985;153:268–71. mares. J Am Vet Med Assoc 1991;198:683–9. 62. Rigby S, Love C, Carpenter K, Varner D, Blanchard T. Use 47. Jeffcott LB, Rossdale PD. A critical review of current meth- of prostaglandin E2 to ripen the cervix of the mare prior ods for induction of parturition in the mare. Equine Vet J to induction of parturition. Theriogenology 1998;50:897– 1977;9:208–15. 904. 48. Ley WB, Hoffman JL, Crisman MV, Meacham TN, Kiracofe 63. Weber JA, Freeman DA, Vanderwall DK, Woods GL. RL, Sullivan TL. Daytime foaling management of the mare Prostaglandin E2 hastens oviductal transport of equine em- 2: Induction of parturition. J Equine Vet Sci 1989;9:95–9. bryos. Biol Reprod 1991;45:544–6. 49. Goddard PJ, Allen WE. Genital tract pressures in mares II. 64. Gastal MO, Gastal EL, Torres CAA, Ginther OJ. Effect of Changes induced by oxytocin and prostaglandin F2α. The- PGE2 on uterine contractility and tone in mares. Theri- riogenology 1985;24:35–44. ogenology 1998;50:989–99. 50. Cross DT, Ginther OJ. The effect of estrogen, proges- 65. Weber JA, Woods GL, Lichtenwalner AB. Relaxatory ef- terone and prostaglandin F2α on uterine contractions in fect of prostaglandin E2 on circular smooth muscle iso- seasonally anovulatory mares. Domest Anim Endocrinol lated from the equine oviductal isthmus. Biol Reprod Monogr 1987;4:271–8. 1995;1:125–30. 51. Cross DT, Threlfall WR, Rogers RC, Kline RC. Uterine elec- 66. Persson E, Rodriguez-Martinez H. The ligamentum tromyographic activity in horse mares as measured by ra- infundibulo-cornuale in the pig: morphological and phys- diotelemetry. Theriogenology 1991;35:591–601. iological studies on the smooth muscle component. Acta 52. Cadario ME, Merritt AM, Archbald LF, Thatcher WW, Anat 1990;138:111–20. LeBlanc MM. Changes in intrauterine pressure after oxy- 67. Dimov V, Georgiev F. Ram semen prostaglandin con- tocin administration in reproductively normal mares and centration and its effect on fertility. J Anim Sci 1977;44: in those with a delay in uterine clearance. Theriogenology 1050–4. 1999;51:1017–25. 68. Shaefer M, Hofmann T, Schultz G, Gudermann T. A 53. Troedsson MHT, Liu IKM, Ing M, Pascoe JR. Smooth mus- new prostaglandin E receptor mediates calcium influx cle electrical activity in the oviduct, and the effect of oxy- and acrosome reaction in human spermatozoa. Cell Biol tocin, prostaglandin F2α, prostaglandin E2 on the my- 1998;95:3008–13. ometrium and the oviduct of the cycling mare. Biol Reprod 69. Woods J, Rigby S, Brinsko S, Stephens R, Varner D, Blan- Monogr 1995;1:475–88. chard T. Effect of intrauterine treatment with prostaglandin 54. Combs GB, LeBlanc MM, Neuwirth L, Tran TQ. Effects of E2 prior to insemination of mares in the uterine horn or prostaglandin F2α, cloprostenol and fenprostalene on uter- body. Theriogenology 2000;53:1827–36. ine clearance of radiocolloid in the mare. Theriogenology 70. Allen WR, Wilsher S, Morris L, Crowhurst JS, Hillyer MH, 1995;45:1449–55. Neal HN. Laparoscopic application of PGE2 to re-establish 55. LeBlanc MM. Persistent mating induced endometritis in oviductal patency and fertility in infertile mares: a prelimi- the mare: pathogenesis, diagnosis and treatment. Elec- nary study. Equine Vet J 2006;38:454–9. tronic citation. International Veterinary Information Service 71. O’Herlihy C, MacDonald HN. Influence of preinduction (www.ivis.org) 2003. prostaglandin E2 vaginal gel on cervical ripening and la- 56. Troedsson MHT, Ababneh MM, Ohlgren AF, Madill bor. Obstet Gynecol 1979;54:708–10. S, Vetscher N, Gregas M. Effect of periovulatory 72. Ferguson JE, Ueland FR, Stevenson DK, Ueland K. prostaglandin F2α on pregnancy rates and luteal function Oxytocin-induced labor characteristics and uterine activity in the mare. Theriogenology 2001;55:1891–9. after preinduction cervical priming with prostaglandin 57. Brendemuehl JP. Effect of oxytocin and cloprostenol on E2 intracervical gel. Obstet Gynecol 1988;72:739– luteal formation, function and pregnancy rates in mares. 45. Theriogenology 2002;58:623–6. 73. Duchens M, Fredriksson G, Kindahl H, Aiumlamai S. Ef- 58. Nie GJ, Johnson KE, Wenzel JGW, Braden TD. Effect of ad- fect of intracervical administration of a prostaglandin E2 ministering oxytocin or cloprostenol in the periovulatory gel in pregnant and non-pregnant heifers. Vet Rec 1993;133: period on pregnancy outcome and luteal function in mares. 546–9. Theriogenology 2003;60:1111–18. 74. Estrada AJ, Samper JC. Cervical failure to dilate. In: Sam- 59. Thiery M. Induction of labor with prostaglandins. In: Keirse per JC, Pycock JF, McKinnon AO (eds) Current Therapy MJNC, Anderson ABM, Gravenhorst JB (eds) Human Partu- in Equine Reproduction. St Louis: Saunders, Elsevier, 2007; rition. Boston. Martinus Nijhoff, 1979; pp. 155–64. pp. 134–6 60. Bryman I, Lindblom B, Norstrom A. Extreme sensitivity 75. LeBlanc MM. Reproduction deduction – part 2. In: Proceed- of cervical musculature to prostaglandin E2 in early preg- ings of the North American Veterinary Conference 20, 2006, nancy. Lancet 1982;ii:1471. pp. 139–42. P1: SFK/UKS P2: SFK Color: 1C c188 BLBK232-McKinnon February 1, 2011 16:4 Trim: 279mm X 216mm Printer Name: Yet to Come Chapter 188 Human Chorionic Gonadotropin John R. Newcombe Introduction intended that the powder be resuspended imme- diately prior to administration and discarded after The use of human chorionic gonadotropin (hCG) 24 hours. to advance ovulation in mares was first suggested The manufacturers of Chorulon (Intervet UK Ltd) by Fred Day, a practitioner, in 1939. It has been suggest doses of 1500 or 3000 IU regardless of body- widely used in broodmare practice since then and, weight. They warn of rare cases of anaphylactoid although various gonadotropin-releasing hormone reaction. (GnRH) agonists are now available, it is probably still the drug of choice for most practitioners. HCG is a protein consisting of two peptide chains containing galactose and hexosamine. Its molecular Use weight is 30,000 and it has a half-life of between 8 Although it is unlikely that hCG will promote the and 12 hours.1 It is formed in the chorionic villi of ovulation of follicles that would not otherwise ovu- the human placenta and is detectable from only a few late spontaneously, it allows for a more predictable days after conception, peaking at about 35–50 days. timing of ovulation and may even reduce the interval In primates it is an essential luteotrophin to maintain between ovulations of mares destined to have mul- and stimulate luteal function until the placenta is suf- tiple ovulations. Therefore, it is possible to delay the ficiently developed to maintain the pregnancy, which mating or insemination of mares until relatively close is at least the seventh week in women.2 In the hu- to ovulation without the need for further insemina- man it is not only luteotrophic but possibly it is also tions. When demand for semen is high and when a luteoprotective against endogenous prostaglandin stallion may have 100 or more mares to mate in a sea- release, binding to the luteinizing hormone (LH) son, it is essential during the busiest part of the breed- receptors. It has LH-like activity in most species and ing season that no mare should need to be mated promotes ovulation and development of the corpus more than once per estrous. luteum (CL).3 A recent development in broodmare practice When injected into mares in mid- to late estrus, is ‘appointment mating’. The mare resides at the it accelerates follicle maturation and hence ovulation owner’s farm or locally but is given an appointment occurs sooner, while estrus is also shortened. Its role (time and date) to visit the stallion, which may be a in the development of the CL is not clear since, rou- few, but is often several hundred, miles away. The tinely, its activity will have waned before the time of mare is transported to the stallion farm, mated, and ovulation. returned home directly. Although this system saves money in keep/agistment charges, transport to meet an appointment several hundred miles away can be Preparation and presentation very expensive. It is therefore highly desirable that, HCG is available commercially as a freeze-dried by the next examination after mating, the mare is preparation of 1500, 2000, 5000, or 10 000 inter- found to have ovulated and does not have to visit the national units (IU) supplied with a solvent. It is stallion again in that same estrous period. Equine Reproduction, Second Edition. Edited by Angus O. McKinnon, Edward L. Squires, Wendy E. Vaala and Dickson D. Varner
c 2011 Blackwell Publishing Ltd. P1: SFK/UKS P2: SFK Color: 1C c188 BLBK232-McKinnon February 1, 2011 16:4 Trim: 279mm X 216mm Printer Name: Yet to Come Human Chorionic Gonadotropin 1805 When fresh cooled semen has to be ordered in ad- in normally cyclic mares.12–21 One report13 suggested vance and then transported overnight across conti- that, although hCG mares may fail to conceive at the nents, it is important that it arrives within 24 hours, first estrus, it enhances their conception rate at subse- or preferably even nearer, of ovulation, since the quent cycles without further treatment. longevity of the semen prior to some stallions can Given at the correct time, when a developing ovu- be seriously impaired by cooling. Similarly frozen/ latory follicle is about 2–4 days away from ovu- thawed semen needs to be inseminated within lation, ovulation can normally be assured in the 6–12 hours either side of ovulation for optimum re- period 36–48 hours after treatment. However, most sults. The use of hCG greatly assists these timings. reports suggest that, in practice, only about 85% of follicles ovulate within 48 hours. Brugmans22 re- ported 91% ovulating between 24 and 48 hours when Timing and follicle diameter follicles of 35–40 mm were treated with 1500 IU Developing follicles are treated to accelerate and de- intravenously. Barbacini et al.23 reported that only fine the time of ovulation. HCG will not induce ovu- 76% of mares ovulated in the period between 25 and lation of atretic follicles. These are most commonly 48 hours when mares with 35 mm or larger follicles found in the spring transition but may also be found were given 2000 IU intravenously. Another 16% ovu- in cyclic mares in early to mid-estrus. The latter are lated within the first 24 hours, whereas 9% took more follicles which have developed to near pre-ovulatory than 48 hours. It is noteworthy that in this report only diameters during diestrus but become atretic at, or 42% ovulated within the period 37–48 hours. Some of just after, luteolysis. At examination of a mare for the these observations may have varied owing to breed first time in early estrus it will not be known whether (see below) and experience of the authors. Other such follicles are atretic or whether they have devel- studies clearly show at least 90% of mares ovulating oped since luteolysis without overt estrus. Knowl- in 36 ± 4 hours under controlled conditions.24, 25 edge of the interval since the previous ovulation is The ovulation of follicles which are already des- helpful in recognizing such follicles. However, large tined to ovulate within 0–36 hours from the time follicles still developing at the time of luteolysis may of treatment may not be accelerated. When hCG is ovulate rapidly, resulting in a short follicular phase, given, it must be remembered that ovulation can a shortened interovulatory interval, and a very short occur before 36 hours. It should be noted that, in the period of estrus. Atretic follicles may or may not report by Barbacini et al.,23 50% of mares ovulated show echogenic particles floating in the antral fluid before 36 hours. However, experienced practitioners but some also show palpable softening. If it were will be aware of this possibility, either from previous known when a follicle would ovulate spontaneously experience with a particular mare or from the vari- then hCG could be given 3–4 days ahead. Unfortu- ous other palpable and ultrasonic parameters, apart nately, this is not known and therefore the time of from diameter, that are used to predict the time of ovulation has to be predicted on the basis of clinical ovulation. judgment and experience. Although it is considered routine to treat with hCG when a follicle has grown to 35 mm or larger, some mares will ovulate from diameters consid- Diameter at ovulation and after hCG erably smaller than this, especially in summer Evans et al.4 showed that the immediate pre- and when multiple follicles are developing. Also ovulatory diameter was smaller after hCG treatment there are individual mares and breeds of horses (38.7 mm) than controls (44.9 mm). In fact, the folli- which regularly ovulate from diameters of 50 mm cles of hCG-treated mares grew only about 3 mm in or greater. Draught and Friesian horses frequently 42 hours (1.7 mm/day), whereas in the controls they ovulate from follicles > 50 mm. Larger than av- continued to grow at about 2.25 mm/day for 4 days. erage diameters have been noted in Welsh cobs This observation that, after hCG, follicle growth (J. Newcombe, unpublished observation), whereas slows was confirmed.5 A recent study suggested Thoroughbreds only infrequently reach such diam- there was little or no growth following hCG in fol- eters. In a study of over 700 ovulatory follicles in licles which subsequently ovulated within 48 hours.6 Standardbred mares, 41% of spontaneous ovulations were from follicles < 40 mm while 9.8% ovulated from those > 50 mm (J. Newcombe, unpublished ob- Efficacy servations). In the latter mares, a 35-mm follicle There is no evidence that ovulations induced by hCG could be 5–7 days from ovulation and still be too im- (or any GnRH preparation) are any less fertile than mature to ‘respond’ to hCG. Treatment must there- spontaneous ovulations. Some reports suggest en- fore be delayed until the follicle has grown to hanced fertility,7–11 but others suggest no advantage 40 mm or even larger. Samper et al.,26 using 2500 IU P1: SFK/UKS P2: SFK Color: 1C c188 BLBK232-McKinnon February 1, 2011 16:4 Trim: 279mm X 216mm Printer Name: Yet to Come 1806 Mare: Pharmacological Manipulation of Reproduction intravenously, found that even by delaying treatment some cases the ‘response’ interval is a little longer until the follicle was over 40 mm only 83% ovulated than 48 hours. Follicles destined to ovulate within within 48 hours, with 8% taking between 72 and 24 hours of treatment will of course have already 96 hours. Sauberli et al.27 found that the mean time to ovulated by the 24-hour examination. Frequent ex- ovulation was only 6 hours shorter when treatment aminations have determined the peak time of ovu- was delayed until follicles were 40–44 mm than when lation more closely. Evans et al.4 with 6-hourly ex- treatment was carried out at 35–39 mm. aminations found 96% and 95% of mares treated The most erratic responses are experienced during with 2500 and 1500 IU, respectively, had ovulated the spring transition when it is well known that fol- within 48 hours, with means of 43.5 and 44 hours, re- licles can grow to the normal pre-ovulatory diameter spectively. Ginther34 reported that 65% of 55 mares or even larger before regressing.28 It is doubtful that given 2000 units intramuscularly and examined ev- such steroidally incompetent follicles can be induced ery 6 hours had ovulated in the period 36–42 hours. to ovulate with hCG. However, Bour and colleagues29 Newcombe,35 however, examining mares every reported success giving multiple 200-IU doses in the 8 hours found that most mares ovulated between 34 transitional period. The most rapidly growing folli- and 45 hours when given 1500 IU subcutaneously. cles at this stage, especially when accompanied by a strong endometrial edema pattern, are the ones most likely to ovulate and therefore to ‘respond’ to hCG Effect of mare’s age treatment. Carnevale et al.30 induced ovulation in Erratic response to hCG seems to be more prevalent 17 out of 19 > 40-mm transitional follicles with 3300 in older mares.36, 37 Whether this is due to interference units given intravenously, whereas a control group from antibodies generated by previous hCG treat- took from 2 to 30 days to ovulate. Although mares can ment or to delayed follicular development has not be induced to develop pre-ovulatory size follicles us- been established. McCue and colleagues38 described ing various GnRH regimes, treatment with hCG can a clear relationship between age and delayed ovula- never guarantee ovulation. Mares treated with pro- tion, whereas Green et al.39 found none. gesterone (by injection or intravaginally) or progesta- gen (orally) if started at the right stage of the spring transition will ovulate most reliably. However, if pro- Effect of dose and route gesterone treatment is started too soon, although fol- licles of over 35 mm may develop, they may not re- Reports describe the successful use of hCG by either spond to hCG treatment.31 the intravenous or intramuscular route. This author has found the subcutaneous route to be equally suc- cessful. Effect of hCG on corpus luteum Dosage is empirical and successful reports vary development and progesterone from 1000 IU to 5000 IU. Most recent reports find concentrations 1500–2500 IU to be as effective as larger doses. Day40 found 80% of mares ovulated from 1500 IU One report32 showed that treatment with 1000 IU on in about 40 hours. There is little evidence that the days 3, 4, and 5 after estrus resulted in enhanced pro- larger doses act more successfully on slightly less gesterone (P4) concentrations from day 7 to day 14. mature follicles or that the interval from treatment Paccamonti et al.33 found that 1000 IU on days 0–2 to ovulation is any shorter. Grimmet and Perkins11 did not increase P4 until day 7. Carnevale et al.30 found no difference between 1500, 3000, and 6000 IU. found no effect of hCG treatment before ovulation Evans and colleagues4 studied LH levels after 500, on CL formation or function. However, Michel et al.9 1500, and 2500 IU given intravenously and found found that hCG-treated mares had an earlier rise no difference between 1500 and 2500. After 500 IU in P4 concentration 72 hours after treatment (i.e., however, LH was slower to rise and reached only about 30 hours after ovulation) than GnRH- or saline- 10 ng/mL at 40 hours compared with 14 ng/mL with treated mares. the other two dosages. The mean time to ovulation was similar to controls (83 hours) compared with 43.5 hours and 44 hours for 1500 and 2500 IU, Interval from treatment to ovulation respectively (6-hourly examinations). At ovulation Most investigators have found that, when mares are (83 hours and 98 hours), levels of LH had reached examined daily following hCG treatment, ovulation 13 and 14 ng/mL in the 500-IU and control groups, has occurred by the time of the 48-hour examina- respectively. tion. A small percentage of follicles are found very J. Newcombe (unpublished observations) found close to ovulation at this time, suggesting that in an equal response rate to follicles of 40–50 mm with P1: SFK/UKS P2: SFK Color: 1C c188 BLBK232-McKinnon February 1, 2011 16:4 Trim: 279mm X 216mm Printer Name: Yet to Come Human Chorionic Gonadotropin 1807 either 1250 IU or 2500 IU intravenously (82%) but since there is little value in administering hCG at this the larger dose was more effective at diameters of time, this finding, although curious, appears unim- less than 40 mm (76% vs 63%). In this study of portant. Aruda and Fleury47 reported an increase Standardbred mares, most spontaneous ovulations in plasma P4 after hCG on day 7, while Hendricks were from diameters of 45–60 mm. et al.48 found no effect on P4 concentration or on In a field study, Thoroughbred mares were exam- bloodflow in the uterine arteries when 3000 IU was ined every 48 hours and the decision when to advise given intravenously on day 7. mating was based on various parameters acquired by clinical experience and by other management fac- Synchronization of ovulations tors, but not necessarily on follicular diameter. Sec- (reduction of spread between ondary follicles that regressed were omitted from the data, but data of the smaller follicle in sets of twins multiple ovulations) which ovulated up to 144 hours after treatment were About half of all spontaneous multiple ovulations are included. Some 327 single follicles (mean diameter not synchronous but occur over a period of more 39.7 mm, range 27–62 mm) and 294 multiple follicles than 12 hours, often 2–3 days. hCG will induce fol- (mean diameter for larger follicles 38.6 mm, range licles that may otherwise have ovulated more than 27–50 mm; and for smaller 33.3 mm, range 21–45 mm) 12 hours apart to ovulate synchronously (within a were treated with either 3000 IU intravenously or few hours). When the longevity of the sperm may 1500 or 750 IU of Chorulon subcutaneously on the have been too short for sufficient viable sperm to be day of, or the day after, mating. Ovulation occurred still present in the fallopian tubes to fertilize a sec- within 48 hours in 93.3%, 96%, and 89.4% of the ond oocyte, then only a single embryo can be ex- three treatment groups, respectively. Mean diame- pected. Synchronization of ovulation increases the ters at treatment of follicles which failed to ovu- chance that at least one oocyte, if not both, will be late within 48 hours were: single, 36.9 mm (range fertilized and therefore increases the chance of a preg- 28–45 mm); larger twin follicles, 39.6 mm (34–42 mm); nancy. Multiple embryos must of course be efficiently and smaller twin follicles 29.8 mm (range 21–40 mm). reduced to ensure a singleton pregnancy. However, a The percentage of follicles which hemorrhaged with- recent study21 found that, although hCG tended to out collapse was 0.3%, 1.4%, and 2.0% for the three increase the synchronicity, there was no increase in groups (J. Newcombe, unpublished data). pregnancy or multiple pregnancy rate. In another field trial, mares with single follicles were mated on the same basis and Chorulon given Antibody formation on a random basis, either 750 IU (n = 283) or 1500 IU subcutaneously (n = 437), or 3000 IU intravenously That repeated dosage with hCG causes antibody for- (n = 112). Response rate (ovulated within 48 hours) mation is no longer disputed. There is, however, for mares treated at follicle diameters of 26–30 mm, much conflicting evidence of whether formation of 31–35 mm, 36–40 mm, and 41–45 mm for the three antibodies affects the efficacy of hCG or even delays doses were 72% (750 IU), 85.7% (1500 IU), and 62.5% ovulation in untreated estrous periods. Sullivan and (3000 IU) for 26–30 mm; 90.1%, 89.7%, and 86.5% for colleagues12 reported that, by the third treated es- 31–35 mm; 96.3%, 94.3%, and 95.3% for 36–40 mm; trus, ovulation was delayed by 1.2 days (based on and 98%, 96.6%, and 100% for 41–45 mm. Other 15 treated mares). Duchamp and colleagues49 found than the better response of 1500 IU on follicles of that none of 10 immunized mares ovulated within 26–30 mm, there was little difference between dose 72 hours. Roser et al.50 treated mares at each cycle rates regardless of follicular diameter (J. Newcombe, from March to October. Five out of 12 mares devel- unpublished data). oped significant antibody levels after two to five in- jections, and the half-life of antibodies ranged from 1 to several months. However, their production did not cause ovulatory refractiveness owing to absence HCG in diestrus of cross-reaction with endogenous LH. Evans et al.4 It has been clearly demonstrated in the cow that hCG found no effect on LH concentrations after three hCG is luteotrophic41–44 and induces accessory CL forma- treatments. Nor did Blanchard et al.51 find any loss of tion.45 The data sheet for Chorulon suggests that in efficacy in mares treated up to four times in the same cattle its use on day 12 both enhances the lifespan of season. the CL and increases P4 concentration. This use has Wilson and colleagues52 found antibodies, but no not been investigated in mares. effect on time to ovulation until the fifth treatment. Repeated doses of hCG in pregnant mares before However, in the next year, the same mares had mean day 38 may cause luteolysis and pregnancy loss,46 but time to ovulation intervals from 53 hours at the sixth P1: SFK/UKS P2: SFK Color: 1C c188 BLBK232-McKinnon February 1, 2011 16:4 Trim: 279mm X 216mm Printer Name: Yet to Come 1808 Mare: Pharmacological Manipulation of Reproduction treatment (including five in the previous year) to percentage of mares ovulating within 0–72 hours ac- 62 hours at the seventh treatment, and to 88 hours tually increased from 92% to 93% by the fifth cycle. in three mares that received 10 treatments. Studies A similar explanation could account for the results of of antibodies, however, showed a very diverse mare Green and colleagues,39 who started with 357 mares response and high antibody levels did not correlate and were down to 43 by the fourth treatment; surely, with longer time to ovulation. Green and colleagues39 43 older and ‘problem’ mares would not be expected found that 80% of 580 mares ovulated within 48 to respond as reliably as the 314. hours up to two treatments, but only 58% (n = 109) A further study by the author involved mares ex- after the third. However, 67% (n = 18) ovulated after amined every 8 hours so that the time of ovulation six to eight treatments. Loy and Hughes8 did not no- was known to within ± 4 hours.54 In that study, 23 tice any loss of efficiency following repeated doses. mares were treated four times in the same season at In a retrospective study of the records of Standard- mean follicle diameters of 39.6 ± 1.1 mm. The inter- bred mares over seven seasons,53 32 individual mares val to ovulation did not vary significantly with the were found which had been given 2500 IU intra- number of treatments, namely 45.1, 44.1, 43.1, and venously on 5–16 occasions, with a mean number of 47.8 hours for the first to fourth treatment respec- treatments of 8.3 times over 3.9 seasons. Mares were tively. The response in mares given repeated doses examined on Mondays, Wednesdays, and Fridays so over several seasons was analyzed. There was no ef- that a ‘response’ to hCG treatment was found after ei- fect of hCG dose number given successively in the ther 48 or 72 hours; 80.2% of follicles (mean diameter same season (P = 0.24) nor an accumulative effect of 42.1 mm) had ovulated within 48 or 72 hours. When season (P = 0.89), but a significant effect of follicle the data were divided into mares receiving either diameter at the time of treatment (P < 0.001) and of 5–8 (n = 16) treatments or 9–16 treatments (n = 16) the mare (P = 0.002). 48/72-hour response rate was 83.5% and 83.2%, re- While there is still some debate on this question, spectively, after follicles which regressed or hemor- practitioners will continue to make their own choice rhaged had been excluded. The mean pre-ovulatory of ovulatory agent and dose based on experience. follicular diameter in the group treated 9–16 times was 41.6 mm, only 0.9 mm smaller than the 5–8 treat- ments group, and the percentage of follicles which References took more than 48/72 hours to ovulate was also 1. McDonald LE. Hormones of the pituitary gland. In: Booth less, 10.2% compared with 12.7%. One mare treated NH, McDonald LE Veterinary Pharmacology and Therapeutics, 16 times over seven seasons had a 100% response rate 6th edn. Ames: Iowa State University Press, 1988; p. 590. from follicles of 39.2 mm mean diameter. 2. Stewart F, Allen WR. Comparative aspects of the evolution McCue and colleagues,38 in a retrospective analy- and function of the chorionic gonadotrophins. Reprod Dom sis of 1321 cycles treated with 2500 IU intravenously, Anim 1995;30:231–9. thought that there was clear evidence of reduced effi- 3. Patton PE, Stouffer RL. Current understanding of the cor- pus luteum of women and non-human primates. Clin Ob- cacy even by the time of the second treatment. When stet Gynecol 1991;34:127–43. follicles which did not ovulate were excluded from 4. Evans MJ, Gastal EL, Silva LA, Gastal MO, Kitson NE, the data, they found that the number of mares ovu- Alexander SL, Irvine CHG. Plasma LH concentrations af- lating within the 24- to 48-hour period fell from 71% ter administration of human chorionic gonadotrophin to (n = 839) after the first treatment to 55% by the third oestrous mares. 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