Introduction Many progestins have been investigated in equine veterinary medicine, including medroxyprogesterone acetate, hydroxyproges- terone hexanoate, norgestomet, megesteral acetate, melengestrol acetate, and proligestone (Table). Several studies1-5 investigated the progestogenic efficacy of these progestins and determined only altrenogest to be effective. Altrenogest maintained pregnancy after a luteolytic dose of prostaglandin F2α (PGF2α), whereas medroxyprogesterone acetate (MPA), hydroxyprogesterone hexanoate, norgestomet, and megesteral acetate treatment were unable to prevent abortion.1 Furthermore, the ability of proligestone, MPA, and melengestrol acetate to bring mares out of seasonal transition was not consistent.2-4 In contrast, both progesterone in oil2 and altrenogest5 effectively hastened estrus and conception in both early and late seasonally transitional mares. Therefore, this review will focus on altrenogest, with minimal inferences to other progestins. Pharmacokinetics of altrenogest Altrenogest (17α-allyl-17β-hydroxy-estra-4, 9, 11-trien-3-one) is also known as allyltrenbolone. Although classified as a progestin, altrenogest structurally resembles androgens and binds to the androgen receptor with 75% affinity of dihydrotestosterone,6 although minimal anabolic effects have been noted, including no increase in body mass in either intact stallions or mares.7 However, mares treated with altrenogest in late pregnancy had fillies born with an enlarged clitoris (Figure 1).8 Trace concentra- tions of androgen and anabolic steroid trenbolone were noted in horses receiving altrenogest, leading to restrictions of its use in many equine organizations, including Irish Horseracing Regulatory Board, British Horseracing Association, Australian Progestin use in mares Carleigh Fedorka Department of Veterinary Science, University of Kentucky, Lexington, KY Abstract Progesterone is in constant fluctuation throughout the lifespan of the horse. As a key regulator of the hypothalamic-pituitary-gonadal axis, progesterone is involved in numerous aspects of reproduction. Synthetic analogs of progesterone, deemed progestins, are widely available to industry to act as progestogenic within the reproductive tract, although few are bioactive in the horse. Utilized to suppress estrous, delay ovulation, maintain pregnancy, and for behavioral modifications, progestins are one of the most common class of pharmaceuticals in veterinary medicine. In this review, we discuss the progestins available to equine industry, their efficacy, pharmacokinetics, and potential side effects following treatment. Keywords: Progestin, estrus synchronization, delay of ovulation, pregnancy maintenance Racing Board, and Federation Equestre Internationale. To predict withdrawal times for altrenogest, its pharmacokinetics were determined after oral, injectable, and rectal treatments. Figure 1. Anabolic effects (enlarged clitoris) of altrenogest treatment during late pregnancy Clinical Theriogenology 2021; 13: 247 Adult mares (n = 10) were treated with oral altrenogest (0.044 mg/kg).9 In most mares, maximum concentration (Cmax) was reached within 15 - 30 minutes. Mean Cmax was 35 ng/ml on day 1 and 31 ng/ml on day 5. In contrast, mean Cmax in urine was 1720 ng/ml on day 1 and 2107 ng/ml on day 5. Time for total clearance (with a limit of detection of 2 ng/ml) was 12 days. Due to risk of human interaction with oral altrenogest, injec- table forms have gained popularity within industry, and the pharmacokinetics of which have been investigated.10 After intramuscular treatment (0.3 mg/kg), mean circulating concen- trations were 33.52 ng/ml. Additionally, the area under the curve for intramuscular treatment was 5.6-fold higher than the oral product. It was hypothesized that intramuscular treatment formed a depot that slowed absorption, prolonging clearance compared to oral treatment. Several pregnant mares that are admitted to the hospital for medical or surgical events may be prohibited from oral medi- cations. Therefore, pharmacokinetics of intrarectal treatment of altrenogest have also been assessed. Altrenogest was detected in circulation as early as 15 minutes after oral and rectal treatment (0.088 mg/kg).11 However, the Cmax for per rectal altrenogest was considerably lower than that of oral treatment (2.54 versus 16.00 respectively), and clearance of altrenogest was more rapid in the rectal treatment group. Although rectal treatment was effective in delivering altrenogest, the bioavailability was only 5.47% compared to oral treatment. Despite the option that rectal treatment may be a viable alternative for hospitalized mares, a dosage of 0.088 mg/kg every 4 - 8 hours would be necessary to maintain therapeutic concentrations. Use of progestins Suppressing estrus Suppression of estrus in sport and race mares is desirable and progestins are commonly used to accomplish this goal. Progestin treatment suppresses estrus in cycling mares through its high affinity for progesterone receptor (PR) resulting in endometrial and myometrial environments similar to a pregnant mare. Progesterone in oil (100 mg) effectively suppressed estrus and mares returned to estrus within 3 - 4 days after cessation of treatment.2 Repeated treatment with progesterone (either intramuscular or repositol) led to its accumulation within the system that persisted for > 10 days after the last treatment.12 Daily oral treatments with 0.044 mg/kg altrenogest was also sufficient to bind to the PR to suppress estrus.13 Similarly, compounded long-acting injectable altrenogest formulations suppressed behavioral estrus when used as recommended by the manufacturers.10,14 Furthermore, long-term altrenogest treatment did not affect future fertility potential15 and high- dose and prolonged altrenogest treatment did not affect overall hematologic or biochemical properties of the mare.16 In contrast, Norgestomet, MPA, hydroxyprogesterone caproate, and megesterol acetate had no effect on estrus suppression17-20 To our knowledge, no other synthetic progestins have been criti- cally evaluated for their ability to suppress estrus-like behavior. Delaying ovulation Although select progestins successfully suppressed estrus in the mare, conflicting results exist for the ability to delay ovulation, and success may depend on the phase of the estrous cycle in which treatment is initiated. In one study, neither oral altrenogest nor a progesterone implant (controlled intravaginal releasing device) were effective in preventing ovulation when short-term treatment began in the preovulatory phase.21 In contrast, short- term treatment of 0.044 mg/kg altrenogest successfully delayed ovulation when treated at the detection of a 35 mm follicle.22 When a double dose of altrenogest (0.088 mg/kg) was admi- nistered, the delay of ovulation was noted in a preovulatory mare.23 However, this was not repeatable in a larger field setting.1 Interestingly, use of an injectable sustained-release vehicle with varying doses of altrenogest delayed ovulation substantially.24 The effect was profound when lactide-glycolide microparticles were added, delaying ovulation by 33.5 days after PGF2α treatment. However, MPA had no effect on ovulation when combined with the same vehicle or microparticle formulation. Hastening transition Mares are seasonally polyestrous long-day breeders and are therefore anovulatory in the months surrounding the winter equinox. A transitionary phase exists between anestrus and proper cyclicity; and shortening this phase has been attempted through the administration of exogenous steroids. The proposed rational for progestin treatment is believed to inhibit the release of luteinizing hormone (LH) from the pituitary, allowing for a repositol of the gonadotropin to form, thereby allowing for ovulation to be induced. Progestin treatment has been shown to hasten the first ovulation of the year dependent on the stage of transition and the specific progestin used. Altrenogest hastened the occurrence of first estrus and reduced the interval to concep- tion in late transitional mares (> March 15, 20 mm follicle) with no impact on early transitional mares (< March 15th, 14 mm follicle).5 However, when a higher dose (0.22 mg/kg) was used, results were not repeatable.25 Other progestins (1500 mg of proligestone) induced comparable ovulation and pregnancy rates to altrenogest treatment in transitional mares.13 It should be noted that only a minimal number of control mares were used in this study, and pretreatment follicular status was not assessed. Controlled release of intramuscular progesterone (LA P4; BioRelease) was effective in shortening the duration to first ovulation in late transitional mares; however, it was ineffective in the early transitional mare.26 Studies using intravaginal progesterone-releasing devices to hasten first ovulation had mixed results. Although controlled intravaginal releasing devices combined with progesterone were ineffective in improving pregnancy rates in the first estrous cycle,27 89% of anestrous mares ovulated within 10 days after progesterone-releasing Clinical Theriogenology 2021; 13: 248 intravaginal devices were remove.28 It should be noted that mild to moderate vaginitis was observed in a subset of mares after implantation of the progesterone-releasing devices. These devices are not approved for equine use in most countries (except for Cue-Mare® in New Zealand). Overall, the efficacy of progestin treatment in shortening the duration to first ovulation is inconsistent and depends on the reproductive status of the mare in addition to the progestin used. Synchronizing estrus For simplicity of breeding in addition to timing embryo transfer, progestins are commonly used to regulate both estrus and diestrus phases of the cycle. Although progestins suppress the release of LH, their effect on FSH is less evident, resulting in normal follicular development.29 Hence, progestin treatment results in a reduction of ovulations, but does not synchronize follicular growth in treated mares. In order to effectively synchronize estrus in mares, a combination of progestins and estrogens can be used. This ‘P&E’ treatment consists of 10 days of intramus- cular treatment of 150 mg progesterone in oil alongside 10 mg estradiol-17β (Figure 2). This was followed by a luteolytic dose of PGF2α on the 10th day of treatment.30 It is expected that mares will have a preovulatory follicle 8 days after cessation of treatment and respond to an ovulatory-inducing agent. This has been repeated by replacing progesterone in oil with altrenogest, and responses were comparable.31 Supporting early pregnancy Progesterone is produced in ovaries (primary and secondary corpora lutea) after ovulation until 120 - 150 days of pregnancy when pregnancy becomes fully dependent on 5α-reduced progestogens from the feto-placental unit, including 5α-DHP and 20α-5P.32-35 Secondary luteal deficiency, as a result of endogenous release of PGF2α, is associated with systemic or uterine inflammation and endotoxemia. This premature release is detrimental to pregnancy if it occurs prior to this shift in progestogen release. Although low systemic progesterone concentrations are incompatible with maintenance of pregnancy, there are limited scientific data to support a primary failure of the corpus luteum (CL) as a cause of embryonic loss.36 Measuring circulating progeste- rone concentrations in a single blood sample to determine if pregnancy is at risk is not accurate unless it is less than 1 ng/ml, due to variations in secretion of this hormone during the day. Studies have indicated the progestogenic potential of altrenogest, while no other synthetic progestin has been found as effective. Either 150 mg progesterone in oil or 0.044 mg/kg altrenogest maintained pregnancy after a luteolytic dose of PGF2α in ovariec- tomized mares.37 Interestingly, a double dose of altrenogest (0.088 mg/kg) was more effective in pregnancy maintenance in ovariectomized recipient mares after embryo transfer.38 In this study, it was determined that the standard dose of 0.044 mg/kg produced poor uterine tone and decreased cervical competence. The authors recommended the double dose to be utilized during embryo transfer procedures. Several other synthetic progestins (1000 mg hydroxyprogesterone caproate, 1000 mg MPA, 500 mg hydroxyprogesterone hexanoate, 15 mg norgestomet, and 500 mg megestrol acetate) were unable to maintain pregnancy.39 Mares treated with altrenogest maintained pregnancy after PGF2α treatment whereas other progestins were not able to prevent abortion.1 In addition, long-acting progesterone (150 mg) compounds were effective in preventing early pregnancy loss after a luteolytic dose of PGF2α and are worth considering.40 There is no cross reactivity between altrenogest and progesterone in commercial progesterone assays; therefore, progesterone assays are helpful in determining whether a CL has undergone luteolysis (P4 < 1 ng/ml) or is still active during treatment. Assay results can guide the clinician to make a decision (discontinue or continue treatment) until the feto-placental unit provides necessary progestogen support to maintain pregnancy. However, altrenogest treatment can suppress endogenous production of progesterone and may interfere with formation of secondary CL.41,42 One study found altrenogest supplementation to decrease the production of equine chorionic gonadotropin (eCG) in mares greater than 8 years of age, but also found altrenogest treatment to maximize embryo size in the aged mare population.43 Figure 2: Standard progesterone and estrogen ‘P&E’ treatment protocol (daily 150 mg progesterone in oil or 0.044 mg/kg altrenogest with 10 mg estradiol-17β). Treatment is initiated when a 25 mm follicle is detected. A luteolytic dose of PGF2α is given on day 10 and transrectal ultrasonography is used to assess follicular development. When a 35 mm follicle (6 - 9 days after cessation of treatment) is detected, hCG is given and mares are inseminated 24 - 36 hours later. Clinical Theriogenology 2021; 13: 249 Supporting late pregnancy Treatment with a double dose of altrenogest (0.088 mg/kg) prevented late-term abortion in mares after abortion was attempted with a synthetic prostaglandin from 90 - 115 days of pregnancy.44 Progestins have been suggested to maintain pregnancy in late pregnancy by inducing myometrial quiescence and suppressing PGF2α secretion, presumably by modulation of cytokines. Therefore, altrenogest is routinely used during pregnancy for the prevention or treatment of ascending placentitis, a leading cause of infectious abortion in North America. Combined treatment of antimicrobial (trimethoprim sulfamethoxazole), immunomodulator (pentoxifylline), and progestin (altrenogest) was effective in treating experimentally induced disease (transcervical inoculation with Streptococcus zooepidemicus).45 Although none of the untreated mares carried to term, 10/12 (83%) of the treated mares produced a viable foal. Since treatments were not assessed individually, it is unclear if altrenogest treatment alone prevented abortion. This confers with recent research that demonstrated a withdrawal of PR preceded premature parturition in experimentally induced disease, indicating that altrenogest supplementation may be of no benefit.46 This was further supported by a study47 that demonstrated that addition of altrenogest treatment had no effect on pregnancy length, interval from inoculation to delivery, or neonatal outcome in comparison to mares solely treated with antimicrobials and antiinflammatories. In that study, the addition of estradiol cypionate to antimicrobial/antiinflamma- tory treatment increased pregnancy length from induction of disease, in addition to improving neonatal viability. Furthermore, altrenogest treatment has been shown to shorten pregnancy length.48 Altrenogest supplementation was also unnecessary in pregnancy maintenance after late ovariectomy (> 100 days), regardless of concentration or time of treatment.49 There are some discrepancies with regard to safety of altrenogest treatment in mid- to late-pregnancy. Mares treated with altre- nogest in late pregnancy had shorter gestation, poorer neonatal outcomes,48 and fillies were born with an enlarged clitoris.8 Additionally, after treatment in late pregnant mares, altrenogest was detectable within both fetal fluid compartments and in grossly higher concentrations in fetal compared to maternal serum.50 Yet, foals born to mares receiving altrenogest in late pregnancy had normal onset of puberty, comparable fertility, and reproductive function to controls.51,52 Based on these findings, the efficacy and necessity of progestin supplementation in mid- to late-pregnancy needs to be further investigated. Hastening postpartum involution In mares, first postpartum estrus occurs within 5 - 20 days53,54 while the uterus is still undergoing repair from tissue alterations during pregnancy and foaling. Uterine involution is not complete until 15 days postpartum and mares ovulating > 10 days postpartum are more likely to achieve and maintain pregnancy compared to those that ovulated earlier.55 This is attributed to the interval of time (5.5 days) that the embryo resides within the oviduct after fertilization before migrating to uterine lumen, at which point involution should be complete. Therefore, progestins were used56-58 in an attempt to delay ovulation in order to improve fertility at first postpartum estrus, with mixed results. One study found altrenogest to delay ovulation and increased pregnancy rates in postpartum mares when administered for the initial 8 days.56 Additionally, altrenogest had no effect on other aspects of uterine involution, including uterine tone, size, or retention of fluid. A second study found altrenogest treatment to delay first ovulation by 10 days but did not improve pregnancy rates when fed for 15 days postpartum.57 Progesterone (100 mg) in oil treatment from days 5 to 14 postpartum successfully inhibited ovulation in 6/9 mares.58 Additionally, the researchers noted no histological changes in treated mares, and pregnancy was achieved in 66% of treated mares, comparable to other estrous cycles. Addition of estradiol-17β to this treatment was tried.59 There was no delay in ovulation when mares were treated with either 150 mg progesterone in oil + 10 mg estradiol-17β or 300 mg progesterone in oil + 20 mg estradiol-17β, and pregnancy rates were comparable among the groups and controls.59 It was noted that none of the P&E treated mares ovulated prior to 10 days postpartum, whereas 20% of control mares ovulated early. Additionally, less variability from time to ovulation was noted in treated mares. Overall, progestin treatment appears to successfully delay ovulation postpartum; however, there are conflicting data on its ability to improve first estrous cycle pregnancy rates. Potential side effects of progestins to immune system Progestins affect the immune system in a variety of species.60 In humans, similar synthetic progestins to those used in mares are available for contraception, many of which have detrimental effect on the immune system. In most species, progesterone is highly antiinflammatory and enacts this function on the immune system through signaling on the PR.61-64 In contrast, synthetic progestins such as MPA and levonorgestrel (LNG) increase the expression or production of a variety of proinflammatory mediators.60,65-67 Additionally, the specific function of the indi- vidual progestin appears to be affected by the receptor through which it enacts, as MPA activates both PR and glucocorticoid receptors, whereas LNG solely activates the PR.68,69 Through this increase in proinflammatory cytokine activation, in addition to recruitment of varying immune cells, women on synthetic progestins were found to be at higher risk for a variety of diseases, including HIV,70-75 HSV-2,76-78 chlamydia,79,80 gonorrhea,81 and even influenza.60,82 It is believed that the increased risk of viral infection is due to proinflammatory activation of NFκβ that binds to the terminal repeat of viral DNA, thereby promoting viral replication. A recent study investigated effects of altrenogest on the immune system of nonpregnant mares, with similar results to those reported in humans.83 In vitro, altrenogest caused a dose-dependent Clinical Theriogenology 2021; 13: 250 increase in the expression of the proinflammatory cytokine IL-1β in addition to the pleiotropic cytokine IL-6, in peripheral blood mononuclear cells. This was also noted in vivo, wherein under the diestrus influence of endogenous progesterone, control mares had decreased expression of proinflammatory cytokines (IFNγ, IL-1β, and IL-8) while the peripheral blood mononuclear cells of altrenogest-treated mares did not decrease in the expression of either IL-1β or IL-8. Additionally, altrenogest altered expression of a variety of cytokines within the endome- trium, including IFNγ, IL-10, and IL-1RN, indicating both local and systemic effects of altrenogest on the immune system of the mare. This may be clinically relevant, as altrenogest treat- ment in nonpregnant mares has been associated with vaginal candidiasis84 and uterine inflammation.85 Interestingly, it was determined that altrenogest alters the immune system of the nonpregnant mare without activating glucocorticoid receptors. As it is structurally a 19-nortestosterone, and more comparable to LNG, this was not surprising. In early pregnancy, altrenogest treatment decreased endometrial expression of the PR.86 Additionally, an increase in the number of polymorphonuclear neutrophils was noted within the endo- metrium after treatment. In late pregnancy, minimal changes were observed in endometrial gene expression of IL-6, IL-8, or COX-2 after altrenogest treatment.85 In this same study, altrenogest treatment had no effect on the expression of the progesterone, estrogen, or oxytocin receptors, conflicting with early pregnant mare data. This is interesting, as progesterone itself inhibited the NFκβ pathway, including downstream COX-2 and IL-1β,87 again indicating a differing pathway of effect for altrenogest. Foals born to mares that were treated with altrenogest during late pregnancy up to the day of foaling were affected by treatment. This included an imbalanced neutrophil to lymphocyte ratio, increased cortisol, decreased potassium, and decreased calcium within hours after birth.88 In an additional study, neonates born to mares receiving late-term altrenogest, had decreased respiratory function and substantially more problems in the perinatal period.48 Conclusion Progestins are valuable clinical tools in managing several aspects of equine reproduction. Although only natural and synthetic (altrenogest) progestins are currently available, both compounds are efficacious in suppressing estrus, hastening transition, and maintaining early pregnancy. In the past, progestin treatment has been considered to be without side effects and perhaps overused as an insurance policy for pregnancy maintenance. Based on recent studies, long-term use of altrenogest, particularly during late pregnancy, may be associated with immune-related side effects in mares and foals. This possibility should be considered when benefits are weighed against risks in clinical situations (Figure 3). Figure 3. Risk to benefit ratio of altrenogest treatment Clinical Theriogenology 2021; 13: 251 Conflict of interest Author has no affiliations, memberships, funding, or financial holdings that might be perceived as affecting the objectivity of this review. References 1. Bruemmer JE, Coy RC, Olson A, et al: Efficacy of altrenogest admi- nistration to postpone ovulation and subsequent fertility in mares. J Eq Vet Sci 2000;20:450-453. 2. Loy RG, Swan SM: Effects of exogenous progestogens on reproductive phenomena in mares. J Anim Sci 1966;25:821-826. 3. Van der Holst W, van Laar PH, Oldenkamp EP: Prolonged spring oestrus in mares: the use of progestogens with specific reference to proligestone. Theriogenology 1985;24:609-617. 4. Lopez-Bayghen C, Zozaya H, Ocampo L, et al: Melengestrol acetate as a tool for inducing early ovulation in transitional mares. Acta Vet Hung 2008;56:125-131. 5. Webel SK, Squires EL: Control of the oestrous cycle in mares with altrenogest. J Reprod Fertil Suppl 1982;32:193-198. 6. Bauer ER, Daxenberger A, Petri T, et al: Characterisation of the affinity of different anabolics and synthetic hormones to the human androgen receptor, human sex hormone binding globulin and to the bovine progestin receptor. APMIS 2000;108:838-846. 7. Hodgson D, Howe S, Jeffcott L, et al: Effect of prolonged use of altrenogest on behaviour in mares. Vet J 2005;169:322-325. 8. Shoemaker CF, Squires EL, Shideler RK: Safety of altrenogest in pregnant mares and on health and development of offspring. J Equine Vet Sci 1989;9:69-72. 9. Machnik M, Hegger I, Kietzmann M, et al: Pharmacokinetics of altrenogest in horses. J Vet Pharmacol Ther 2007;30:86-90. 10. McConaghy FF, Green LA, Colgan S, et al: Studies of the pharma- cokinetic profile, in vivo efficacy and safety of injectable altrenogest for the suppression of oestrus in mares. Aust Vet J 2016;94:248-255. 11. Ellis KE, Council-Troche RM, Von Dollen KA, et al: Pharmacokinetics of intrarectal altrenogest in horses. J Equine Vet Sci 2019;72:41-46. 12. Hawkins DL, Neely DP, Stabenfeldt GH: Plasma progesterone concentrations derived from the administration of exogenous proges- terone to ovariectomized mares. J Reprod Fertil Suppl 1979:211-216. 13. McCue PM: Estrus suppression in performance horses. J Equine Vet Sci 2003;23:341-345. 14. Storer WA, Thompson DL, Gilley RM, et al: Evaluation of injectable sustained release progestin formulations for suppression of estrus and ovulation in mares. J Equine Vet Sci 2009;29:33-36. 15. Squires EL, Heesemann CP, Webel SK, et al: Relationship of altre- nogest to ovarian activity, hormone concentrations and fertility of mares. J Anim Sci 1983;56:901-910. 16. Shideler RK, Voss JL, Aufderheide WM, et al: The effect of altrenogest, an oral progestin, on hematologic and biochemical parameters in mares. Vet Hum Toxicol 1983;25:250-252. 17. Gee EK, DeLuca C, Stylski JL, et al: Efficacy of medroxyprogeste- rone acetate in suppression of estrus in cycling mares. J Equine Vet Sci 2009;29:140-145. 18. Wiepz GJ, Squires EL, Chapman PL: Effects of norgestomet, altre- nogest, and/or estradiol on follicular and hormonal characteristics of late transitional mares. Theriogenology 1988;30:181-193. 19. Neely DP: Progesterone/progestin therapy in the broodmare. Proc Am Assoc Equine Pract1988. p. 203-218. 20. Bristol F: Studies on estrous synchronization in mres. Proc Soc of Therio1981. p. 258-264. 21. Canisso IF, Gallacher K, Gilbert MA, et al: Preovulatory progestagen treatment in mares fails to delay ovulation. Vet J 2013;197:324-328. 22. James AN, Vogelsang MM, Forest DW, et al: Efficacy of short-term administration of altrenogest to postpone ovulation in mares. J Eq Vet Sci 1998;18:329-331. 23. Bruemmer JE, Coy RC, Olson A, et al: Efficacy of altrenogest admi- nistration to postpone ovulation and subsequent fertility in mares. J Equine Vet Sci 2000;20:450-453. 24. Storer WA, Thompson DL, Gilley RM, et al: Evaluation of injectable sustained release progestinformulations for suppression of estrus and ovulation in mares. J Equine Vet Sci 2009;29. 25. Colbern GT, Squires EL, Voss JL: Use of altrenogest and human chorionic gonadotropin to induce normal ovarian cyclicity in transi- tional mares. J Equine Vet Sci 1987;7:69-72. 26. Staempfli SA, Clavier S, Thompson DL, et al: Effect of a single injection of long-acting progesterone on the first ovulation in early and late spring transitional mares. J Equine Vet Sci 2011;31:744-748. 27. Cuervo-Arango J, Clark A: The first ovulation of the breeding season in the mare: the effect of progesterone priming on pregnancy rate and breeding management (hCG response rate and number of services per cycle and mare). Anim Reprod Sci 2010;118:265-269. 28. Newcombe JR: Field observations on the use of a progesterone-re- leasing intravaginal device to induce estrous and ovulation in seasonally anestrous mares. J Equine Vet Sci 2002;22:378-382. 29. Evans MJ, Loy RG, Taylor TB, et al: Effects of exogenous steroids on serum FSH and LH, and on follicular development in cyclic mares. J Reprod Fertil Suppl 1982;32:205-212. 30. Loy RG, Evans MJ, Pemstein R, et al: Effects of injected ovarian steroids on reproductive patterns and performance in post-partum mares. J Reprod Fertil Suppl 1982;32:199-204. 31. Squires EL, Martin JM, Jasko DJ: Reproductive response of mares after treatment with progestogens with and without the additional of estradiol. J Equine Vet Sci 1992;12:28-32. 32. Conley A, Ball BA: Steroids in the establishment and maintenance of pregnancy, and at parturition in the mare. Reproduction 2019. 33. Conley AJ: Review of the reproductive endocrinology of the pregnant and parturient mare. Theriogenology 2016;86:355-365. 34. Fowden AL, Forhead AJ, Ousey JC: The endocrinology of equine parturition. Exp Clin Endocrinol Diabetes 2008;116:393-403. 35. Ousey JC: Peripartal endocrinology in the mare and foetus. Reprod Domest Anim 2004;39:222-231. 36. Irvine CH, Sutton P, Turner JE, Mennick PE: Changes in plasma progesterone concentrations from days 17 to 42 of gestation in mares maintaining or losing pregnancy. Equine Vet J 1990;22:104-106. 37. Shideler RK, Squires EL, Voss JL, et al: Progestagen therapy of ovariectomized pregnant mares. J Reprod Fertil Suppl 1982;32:459-464. 38. Hinrichs K, Sertich PL, Kenney RM: Use of altrenogest to prepare ovariectomized mares as embryo transfer recipients. Theriogenology 1986;26:455-460. 39. McKinnon AO, Tarrida del Marmol Figueroa S, Nobelius AM, et al: Failure of hydroxyprogesterone caproate to maintain pregnancy in Clinical Theriogenology 2021; 13: 252 ovariectomised mares. Equine Vet J 1993;25:158-160. 40. Vanderwall DK, Marquardt JL, Woods GL: Use of a compounded long-acting progesterone formulation for equine pregnancy mainte- nance. J Equine Vet Sci 2007;27:62-66. 41. Daels PF, DeMoreas J, Stabenfeldt GH, et al: The effect of altrenogest on the development of secondary corpora lutea. Proc 12th ICAR1992. p. 1855-1857. 42. DeLuca CA, MCCue PM, Patten ML, et al: Effect of nonsurgical embryo transfer procedure and/or altrenogest therapy on endogenous progesterone concentration in mares. J Equine Vet Sci 2011;31:57-62. 43. Willmann C, Schuler G, Hoffmann B, et al: Effects of age and altrenogest treatment on conceptus development and secretion of LH, progesterone and eCG in early-pregnant mares. Theriogenology 2011;75:421-428. 44. Daels PF, Besognet B, Hansen B, et al: Effect of progesterone on prostaglandin F2 alpha secretion and outcome of pregnancy during cloprostenol-induced abortion in mares. Am J Vet Res 1996;57:1331-1337. 45. Bailey CS, Macpherson ML, Pozor MA, et al: Treatment efficacy of trimethoprim sulfamethoxazole, pentoxifylline and altrenogest in expe- rimentally induced equine placentitis. Theriogenology 2010;74:402-412. 46. El-Sheikh Ali H, Legacki EL, Loux SC, et al: Equine placentitis is associated with a downregulation in myometrial progestin signaling- dagger. Biol Reprod 2019. 47. Curcio BR, Canisso IF, Pazinato FM, et al: Estradiol cypionate aided treatment for experimentally induced ascending placentitis in mares. Theriogenology 2017;102:98-107. 48. Neuhauser S, Palm F, Ambuehl F, et al: Effects of altrenogest treatment of mares in late pregnancy on parturition and on neonatal viability of their foals. Exp Clin Endocrinol Diabetes 2008;116:423-428. 49. Knowles JE, Squires EL, Shideler RK, et al: Progestins in mid- to late- pregnant mares. J Equine Vet Sci 1994;14:659-63. 50. Palm FM, Schenk I, Neuhauser S, et al: Concentrations of altrenogest in plasma of mares and foals and in allantoic and amniotic fluid at parturition. Theriogenology 2010;74:229-235. 51. Squires EL, Shideler RK, McKinnon AO: Reproductive performance of offspring from mares administered altrenogest during gestation. J Equine Vet Sci 1989;9:73-76. 52. Naden J, Squires EL, Nett TM, et al: Effect of maternal treatment with altrenogest on pituitary response to exogenous GnRH in pubertal stallions. J Reprod Fertil 1990;88:177-83. 53. Gygax AP, Ganjam VK, Kenney RM: Clinical, microbiological and histological changes associated with uterine involution in the mare. J Reprod Fertil Suppl 1979:571-578. 54. Blanchard TL, Thompson JA, Brinsko SP, et al: Mating mares on foal heat: a five-year retrospective study. Proc Am Assoc Equine Pract 2004. p. 525-530. 55. Loy RG: Characteristics of postpartum reproduction in mares. Vet Clin North Am Large Anim Pract 1980;2:345-349. 56. McKinnon AO, Squires EL, Harrison LA, et al: Ultrasonographic studies on the reproductive tract of mares after parturition: effect of involution and uterine fluid on pregnancy rates in mares with normal and delayed first postpartum ovulatory cycles. J Am Vet Med Assoc 1988;192:350-353. 57. Sigler DH, Ericson DE, Gibbs PG, et al: Reproductive traits, lactation and foal growth in mares fed altrenogest. J Anim Sci 1989;67:1154-1159. 58. Loy RG, Hughes JP, Richards WP et al: Effects of progesterone on reproductive function in mares after parturition. J Reprod Fertil Suppl 1975:291-295. 59. Bruemmer JE, Brady HA, Blanchard TL: Uterine involution, day and variance of first postpartum ovulation in mares treated with progeste- rone and estradiol-17beta for 1 or 2 days postpartum. Theriogenology. 2002;57:989-995. 60. Hall OJ, Nachbagauer R, Vermillion MS, et al: Progesterone-based contraceptives reduce adaptive immune responses and protection against sequential influenza A virus infections. J Virol 2017;91. 61. Hardy DB, Janowski BA, Corey DR, et al: Progesterone receptor plays a major antiinflammatory role in human myometrial cells by antagonism of nuclear factor-kappaB activation of cyclooxygenase 2 expression. Mol Endocrinol 2006;20:2724-733. 62. Jones LA, Kreem S, Shweash M, et al: Differential modulation of TLR3- and TLR4-mediated dendritic cell maturation and function by progesterone. J Immunol 2010;185:4525-4534. 63. Lei K, Chen L, Georgiou EX, et al: Progesterone acts via the nuclear glucocorticoid receptor to suppress IL-1beta-induced COX-2 expression in human term myometrial cells. PLoS One. 2012;7:e50167. 64. Butts CL, Shukair SA, Duncan KM, et al: Progesterone inhibits mature rat dendritic cells in a receptor-mediated fashion. Int Immunol 2007;19:287-296. 65. Deese J, Masson L, Miller W, et al: Injectable Progestin-Only Contraception is Associated With Increased Levels of Pro-Inflammatory Cytokines in the Female Genital Tract. Am J Reprod Immunol 2015;74:357-367. 66. Coleman JS, Mwachari C, Balkus J, et al: Effect of the levonorgestrel intrauterine device on genital HIV-1 RNA shedding among HIV-1- infected women not taking antiretroviral therapy in Nairobi, Kenya. J Acquir Immune Defic Syndr 2013;63:245-248. 67. Eastman AJ, Bergin IL, Chai D, et al: Impact of the Levonorgestrel- Releasing Intrauterine System on the progression of Chlamydia trachomatis Infection to Pelvic Inflammatory Disease in a baboon model. J Infect Dis 2018;217:656-666. 68. Attardi BJ, Zeleznik A, Simhan H, et al: Comparison of progesterone and glucocorticoid receptor binding and stimulation of gene expression by progesterone, 17-alpha hydroxyprogesterone caproate, and related progestins. Am J Obstet Gynecol 2007;197:599 e1-7. 69. Kontula K, Paavonen T, Luukkainen T, et al: Binding of progestins to the glucocorticoid receptor. Correlation to their glucocorticoid-like effects on in vitro functions of human mononuclear leukocytes. Biochem Pharmacol 1983;32:1511-1518. 70. Huijbregts RP, Helton ES, Michel KG, et al: Hormonal contraception and HIV-1 infection: medroxyprogesterone acetate suppresses innate and adaptive immune mechanisms. Endocrinology. 2013;154:1282-1295. 71. Lavreys L, Chohan V, Overbaugh J, et al: Hormonal contraception and risk of cervical infections among HIV-1-seropositive Kenyan women. AIDS 2004;18:2179-2184. 72. Lavreys L, Baeten JM, Martin HL, Jr., et al: Hormonal contraception and risk of HIV-1 acquisition: results of a 10-year prospective study. AIDS 2004;18:695-697. 73. Lavreys L, Baeten JM, Kreiss JK, et al: Injectable contraceptive use and genital ulcer disease during the early phase of HIV-1 infection increase plasma virus load in women. J Infect Dis 2004;189:303-311. 74. McClelland RS, Lavreys L, Katingima C, et al: Contribution of HIV-1 infection to acquisition of sexually transmitted disease: a 10-year prospective study. J Infect Dis 2005;191:333-338. Clinical Theriogenology 2021; 13: 253 75. Ralph LJ, McCoy SI, Shiu K, et al: Hormonal contraceptive use and women’s risk of HIV acquisition: a meta-analysis of observational studies. Lancet Infect Dis 2015;15:181-189. 76. Quispe Calla NE, Vicetti Miguel RD, Boyaka PN, et al: Medroxyprogesterone acetate and levonorgestrel increase genital mucosal permeability and enhance susceptibility to genital herpes simplex virus type 2 infection. Mucosal Immunol 2016;9:1571-1583. 77. Kaushic C, Ashkar AA, Reid LA, et al: Progesterone increases suscep- tibility and decreases immune responses to genital herpes infection. J Virol. 2003;77:4558-4565. 78. Gillgrass AE, Fernandez SA, Rosenthal KL, et al: Estradiol regulates susceptibility following primary exposure to genital herpes simplex virus type 2, while progesterone induces inflammation. J Virol 2005;79:3107-3116. 79. Kaushic C, Zhou F, Murdin AD, et al: Effects of estradiol and proges- terone on susceptibility and early immune responses to Chlamydia trachomatis infection in the female reproductive tract. Infect Immun 2000;68:4207-4216. 80. Kaushic C, Murdin AD, Underdown BJ, et al: Chlamydia tracho- matis infection in the female reproductive tract of the rat: influence of progesterone on infectivity and immune response. Infect Immun 1998;66:893-898. 81. Xu L, Dong B, Wang H, et al: Progesterone suppresses Th17 cell responses, and enhances the development of regulatory T cells, through thymic stromal lymphopoietin-dependent mechanisms in experimental gonococcal genital tract infection. Microbes Infect 2013;15:796-805. 82. Hall OJ, Limjunyawong N, Vermillion MS, et al: Progesterone-Based Therapy Protects Against Influenza by Promoting Lung Repair and Recovery in Females. PLoS Pathog 2016;12:e1005840. 83. Fedorka CE, Ball BA, Walker OF, et al: Alteration of the mare’s immune system by the synthetic progestin, altrenogest. Am J Reprod Immunol 2019:e13145. 84. Montes AJ, Montes LF, Vaughan JT, et al: Vulvo vaginal candidiasis in thoroughbred mares following progestogen administration intra- vaginal treatment with clotrimazole. J Equine Vet Sci. 2001;21:68-70. 85. Palm F, Walter I, Nowotny N, et al: Progestin treatment does not affect expression of cytokines, steroid receptors, oxytocin receptor, and cyclooxygenase 2 in fetal membranes and endometrium from pony mares at parturition. Theriogenology 2013;79:59-68. 86. Willmann C, Budik S, Walter I, et al: Influences of treatment of early pregnant mares with the progestin altrenogest on embryonic development and gene expression in the endometrium and conceptus. Theriogenology 2011;76:61-73. 87. Lei K, Georgiou EX, Chen L, et al: Progesterone and the Repression of Myometrial Inflammation: The Roles of MKP-1 and the AP-1 System. Mol Endocrinol 2015;29:1454-1467. 88. Neuhauser S, Palm F, Ambuehl F, et al: Effect of altrenogest-treatment of mares in late gestation on adrenocortical function, blood count and plasma electrolytes in their foals. Equine Vet J 2009;41:572-577. Clinical Theriogenology 2021; 13: 254 Progestin use in mares