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Endocrine alterations associated with ergopeptine alkaloid exposure during equine pregnancy
Vet Clin Equine 18 (2002) 371–378
Endocrine alterations associated with ergopeptine alkaloid exposure during equine pregnancy Tim J. Evans, DVM, MS Veterinary Medical Diagnostic Laboratory, Department of Veterinary Pathobiology, College of Veterinary Medicine, University of Missouri at Columbia, Columbia, MO 65211, USA
Ergopeptine alkaloid exposure is common in pregnant mares. Many mares live in geographic areas where Neotyphodium coenophialum–infected tall fescue is the dominant grass in pastures and hay. A variety of grasses and cereal grains may also be infected by Claviceps purpurea (ergot), and fungal sclerotia can contaminate forage and especially ground and pelleted feed. Ergopeptine alkaloid mycotoxins produced by the fescue endophyte and Claviceps speeres interact with D2-dopamine receptors on cells such as lactotropes and induce hypoprolactinemia as well as other endocrine alterations. Agalactia, prolonged gestation, abortion, dystocia, and placental and fetal abnormalities are all clinical manifestations of the alterations in the endocrine milieu induced by ergopeptine alkaloid exposure during late gestation. Late gestational mares are particularly sensitive to the actions of ergopeptine alkaloids on prolactin secretion and the associated abnormalities in lactogenesis and steroidogenesis. Moreover, placental function is impaired in mares exposed to ergopeptine alkaloids during late pregnancy. An understanding of the endocrine alterations associated with ergopeptine alkaloid exposure during pregnancy is necessary for the diagnosis of potential exposure to these compounds and for effective prophylaxis and therapy. Sources of ergopeptine alkaloids The ergopeptine alkaloids ergotamine, ergocristine, ergosine, ergocornine, and ergocryptine are the predominant toxins contained within the black, dark brown, or purple ergot bodies (sclerotia) of C. purpurea [1–4]. P.O. Box 6023, Columbia, MO 65203. E-mail address: [email protected] (T.J. Evans). 0749-0739/02/$ - see front matter Ó 2002, Elsevier Science (USA). All rights reserved. PII: S 0 7 4 9 - 0 7 3 9 ( 0 2 ) 0 0 0 1 9 - 6
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Claviceps sclerotia replace the individual seeds in the seed head of common pasture grasses, including fescues, bluegrasses, and bromegrasses, or cereal grains, such as oats, barley, wheat, and especially rye and triticale. The germination and growth of C. purpurea are facilitated by the cool wet spring seasons in the northwestern United States and the northern Great Plains [1,2]. Horses are most likely to be exposed to ergot bodies found in contaminated pastures or hay or to fragments of fungal sclerotia hidden in processed feed or concentrated within the screenings from ergotized grains [3,4]. The total concentration of ergopeptine alkaloids in C. purpurea sclerotia generally ranges from 2000 to 10,000 mg/kg (ppm) [4,5]. Ergovaline is thought to be the most physiologically active ergot alkaloid produced by the fungal endophyte N. coenophialum (previously known as Acremonium coenophialum or Epichlo¨e typhinia) [1,2,6]. This endophyte grows within the intercellular spaces of Festuca arundinacea as part of a symbiotic grass/endophyte association. Over 35 million acres in the upper southeastern and lower midwestern regions of the United States contain endophyte-infected tall fescue, and it has been estimated that this grass is the primary forage for more than 700,000 horses [1–3,6–8]. Although also found in other parts of fescue grass infected by N. coenophialum, ergovaline concentrations are highest in the seed heads [1–3,6,7,9,10]. Concentrations of ergovaline generally range between 0.2 and 0.6 mg/kg (ppm) in tall fescue, with seed head concentrations of ergovaline often exceeding 1 mg/kg (ppm) [7,10]. Mechanism of action of ergopeptine alkaloids Ergopeptine alkaloids cause vasoconstriction through interactions with dopaminergic, adrenergic, and serotonergic receptors [1,2,11]. The pathogenesis of the attenuated lactation or agalactia and impaired reproductive function associated with equine ‘‘fescue toxicosis’’ are generally thought to involve primarily the stimulation of D2-dopamine receptors by ergopeptine alkaloids. Dopaminergic stimulation results in decreased prolactin secretion by lactotropes located in the anterior pituitary [1–4,6,7,9,11–13]. Prolactin is involved in the endocrine regulation of many physiologic processes, including lactogenesis and steroidogenesis [11,14]. The initiation of parturition in the mare depends on the maturation and proper function of the fetal hypothalamic-pituitary-adrenal axis [14,15]. Prolonged gestation in mares exposed to ergopeptine alkaloids may be related to hypoprolactinemia-induced alterations in uterofetoplacental steroid metabolism or may potentially be mediated by direct or indirect inhibition of D2-dopamine receptors on corticotropes in the fetal anterior pituitary [11,13–15]. Clinical signs of equine ‘‘fescue toxicosis’’ and ergotism Although embryonic death has been associated with ergopeptine alkaloid exposure early in pregnancy, equine ‘‘fescue toxicosis’’ is most commonly
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recognized as a condition of late gestational and early postparturient mares [6,9]. Agalactia has been observed mares fed fescue hay with ergovaline concentrations greater than 0.1 mg/kg (ppm) and in mares exposed to processed feed containing screenings of ergotized grain with total ergopeptine alkaloid concentrations ranging from 0.5 to 1.5 mg/kg (ppm) [3,4,7]. The clinical signs of fescue toxicosis and ergotism in pregnant mares include agalactia, prolonged gestation, abortion, dystocia, fetal asphyxia, weakness, dysmaturity and mortality, and placental abnormalities, such as premature placental separation, thickening and edema of fetal membranes, and retained placentae [1–4,6,7,9,11,12,16]. The signs of impending parturition in pregnant mares, such as the rapid increase in udder development, the commonly observed ‘‘waxing’’ or accumulation of colostrum at the teat orifices, and increases in the calcium levels in mammary secretions, are often attenuated in ergopeptine alkaloid–exposed late gestational mares, and these foalings are frequently unexpected and thus unattended [3,4,6,7, 9,11]. Fetal dysmaturity and increased fetal size associated with maternal exposure to ergopeptine alkaloids contribute to the development of dystocia and its sequelae in mares and foals [6,9]. In addition, failure of passive transfer and neonatal septicemia and decreased viability commonly occur in foals from agalactic mares.
Ergopeptine alkaloid–induced endocrine alterations in pregnant mares Hypoprolactinemia during late gestation Hypoprolactinemia is a hallmark feature of ergopeptine alkaloid exposure in pregnant mares [1–4,6,7,9,11,12,17]. Prolactin is secreted by lactotropic cells in the anterior pituitary gland and is commonly regarded in conjunction with its role in lactogenesis [15]. Prolactin secretion is tonically inhibited by endogenous dopamine interacting with D2-dopamine receptors on lactotropes. Large increases in maternal prolactin secretion normally occur a few days before parturition and remain elevated for several weeks postpartum. A placental lactogen is not produced during equine pregnancy, and lactation in mares, unlike cattle, is completely dependent on the production and secretion of prolactin by the anterior pituitary gland for normal lactation [14,15]. Ergopeptine alkaloids produced by N. coenophialum mimicking the actions of dopamine on lactotropic D2 receptors induce hypoprolactinemia and thus agalactia in pregnant mares. Suppression of maternal progestin levels during late gestation Beginning on or about day 40 of gestation, the uterofetoplacental unit begins to secrete progestins known as 5-a pregnanes, which differ structurally from progesterone (4-pregnene-3,20-dione) [14,15]. The predominant 5-a pregnane found in the peripheral blood of mares during late gestation,
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3, 20-dione (5a-DHP), may be found in extremely large concentrations (milligrams per milliliter versus nanograms per milliliter for progesterone). The 5-a pregnanes cross-react with the radioimmunoassay (RIA) for progesterone but are undetectable with ELISAs, which measure progesterone [14, 15,18,19]. The existence of a unique steroidogenic pathway in pregnant mares is suggested by the predominance of 5-a pregnanes in mid- to lateterm pregnant mares and the manner in which these mares metabolize exogenous progestins [14,15,18–20]. Production of 5-a pregnanes by the uterofetoplacental unit reaches a plateau by approximately midgestation, and there is normally a dramatic rise in maternal blood levels of immunoassayable progestins 30 days before parturition. This surge in maternal progestin levels peaks 2 to 3 days prepartum and declines precipitously within 24 hours of parturition [14,15]. In cases of ergopeptine alkaloid exposure, the metabolism of progestins is altered, and the dramatic increase in maternal circulating progestins observed during the last 30 days of pregnancy is suppressed or absent [3,4,6,7,9,11–13,15,17,18]. This suppression of the late gestational surge in immunoassayable progestins is frequently concurrent with prolonged gestation and may be related to maternal hypoprolactinemia and/or alterations in fetal cortisol production [6,7,11,12,15,17]. Administration of D2-dopamine receptor antagonists to ergopeptine alkaloid–exposed pregnant mares results in resolution of the hypoprolactinemia and resurgence of maternal levels of circulating progestins [6,7,11–13,17,21]. Decreased maternal relaxin levels during gestation The placenta is thought to be the sole source of relaxin in the mare. Relaxin decreases the collagen content in the extracellular matrices of the pubic symphysis and uterine cervix, inhibits uterine contractility, and may also play a role in mammary gland development [14,15]. Placental secretion of relaxin begins on about day 80 of pregnancy and peaks initially at approximately day 175. After a subsequent slight decline in blood levels, maternal levels of relaxin begin to increase gradually from about day 225 of gestation until parturition [14,15,22]. Relaxin has been demonstrated to be an indicator of placental health in the mare. In late gestational mares exposed to endophyte-infected fescue, maternal circulating levels of relaxin are decreased and may indicate placental dysfunction [22]. Alterations in maternal estrogen levels As luteal estrogen production begins to decline around days 60 to 90 of pregnancy, fetoplacental estrogen secretion begins to increase. Estrogens of fetoplacental origin increase gradually until day 200 of pregnancy and then decrease gradually until parturition [14,15]. It has been suggested that circulating levels of immunoassayable estrogens are increased in mares exposed to ergopeptine alkaloids during late gestation [11]. This endocrine alteration
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seems to be less consistent during the last 30 days of gestation than alterations in prolactin, progestins, and relaxin, however [13,17]. Alterations in maternal thyroid hormones There exists significant controversy regarding the diagnosis of equine thyroid dysfunction. A wide variety of external factors influence the hypothalamic-pituitary-thyroid axis. It has been suggested that mares exposed to endophyte-infected fescue during late gestation as well as their foals have decreased circulating levels of thyroxine (T4), and this potential effect of ergopeptine alkaloids may be worthy of further investigation [23].
Ergopeptine alkaloid–induced endocrine disturbances in neonatal foals Neonatal foals from mares exposed to ergopeptine alkaloids during late gestation frequently exhibit signs of dysmaturity [3,4,6,7,9,13]. Plasma levels of immunoassayable progestins, cortisol, T4, and tri-iodothyronine (T3) are generally decreased in these foals [3,4,6,7,23,24]. These endocrine alterations in the neonate may reflect the effects of ergopeptine alkaloids on blood supply to the fetus, placental structure and metabolism, and/or fetal hypothalamic and pituitary function.
Diagnosis of ergopeptine alkaloid exposure in horses Hormonal alterations in pregnant mares during the last 30 days of gestation may be helpful in diagnosing ergopeptine alkaloid exposure, but assays for immunoassayable progestins, prolactin, and relaxin are not available at all diagnostic laboratories [3,4,7]. In many cases, prolonged gestation and, especially, decreased mammary development and agalactia may be the best diagnostic criteria available for ergopeptine alkaloid exposure [3,4,6,7,9]. Calcium concentrations in mammary secretions rarely exceed 50 ppm in mares exposed to ergopeptine alkaloids. ELISA testing for urinary excretion of fescue ergot alkaloids, if performed within 24 to 48 hours of animal removal from suspect pasture, hay, or grain-containing products, has provided a method of definitively confirming exposure to ergot alkaloids in cattle. Research is being performed to determine the usefulness of this technique in the confirmation of ergopeptine alkaloid exposure in horses [3,4,7]. The determination of ergopeptine alkaloid concentrations in forage or grain by ELISA or high performance liquid chromatography (HPLC) does not confirm ingestion of ergopeptine alkaloids by horses, but it may be useful in establishing exposure to these compounds, especially in geographic areas where fescue pastures and C. purpurea infection are uncommon [3,4,6,7].
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Prophylactic and therapeutic considerations in ergopeptine alkaloid toxicosis Knowledge of breeding dates, confirmation of pregnancy, and careful monitoring of mammary gland development are critical steps in the identification of mares most susceptible to the effects of exposure to ergopeptine alkaloids [3,4,7]. Exposure of pregnant mares to heavily ergotized feedstuffs should be avoided. Pastures, hays, and grains should be monitored for the presence of C. purpurea sclerotia, and grain screenings should not be incorporated in feedstuffs intended for consumption by horses [1–4,7]. Analysis for ergopeptine alkaloid concentrations in suspect forages or rations may be advisable [3,4,7]. Avoidance of the use of toxigenic N. coenophialum–infected tall fescue in pastures or hays may be the best prophylactic approach to fescue toxicosis but is extremely challenging. Complete pasture renovation and reseeding with endophyte-free fescue or other grass species is limited by the symbiotic nature of tall fescue grass/Neotyphodium interactions. Endophyte-free tall fescue may not grow as well under some environmental conditions as fescue infected with N. coenophialum [1–4,7,9]. Tall fescue infected with a genetically altered ‘‘friendly’’ endophyte has shown promise in the prevention of the clinical signs of fescue toxicosis in horses [25]. This approach to fescue toxicosis prevention and control may be limited by economic constraints as well as by the possible reintroduction of ‘‘unfriendly’’ endophyte-infected fescue grass, however [3,7]. Strategic timing of withdrawal of horses from endophyte-infected pasture or hay for periods as long as 60 to 90 days before anticipated foaling dates has been recommended for pregnant mares. Because the most significant endocrine alterations associated with ergopeptine alkaloid exposure occur after day 300 of gestation, the removal of pregnant mares from potential sources of ergopeptine alkaloids 30 days before the expected foaling date has generally been successful in controlling the incidence of equine fescue toxicosis [3,6,7,9,11]. Frequent mowing, heavy grazing pressure, and chemical treatment to prevent or retard seed head development have been recommended as ways to decrease ergopeptine alkaloid concentrations in fescue pastures. Seeding fescue pastures with at least 20% palatable legumes such as clovers may also be a means of decreasing levels of ergopeptine alkaloids, and consultation with local agronomists or extension personnel may be helpful [1,3,7,9]. Successful treatment of fescue toxicosis and ergotism in horses is dependent on early recognition of the clinical signs as well as on preparturient monitoring and assistance during foaling. D2-dopamine receptor antagonists, such as domperidone (1.1 mg/kg administered orally once daily), sulpiride (3.3 mg/kg administered orally once daily), perphenazine (0.3–0.5 mg/kg administered orally twice daily), and acepromazine (20 mg per horse administered intramuscularly four times daily), have been used with some success
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in the treatment of agalactia in mares exposed to ergopeptine alkaloids [3,4,6,7,9,11–13,21]. The Rauwolfian alkaloid reserpine (2.0–5.0 mg per 450-kg horse administered orally once daily), which depletes brain depots of dopamine, serotonin, and/or norepinephrine, can be used for the treatment of postpartum agalactia in mares with a history of exposure to ergopeptine alkaloids, but its use may be associated with sedation and diarrhea [3,4,6,7,9,13]. Domperidone does not cross the blood–brain barrier like other D2 antagonists and has been demonstrated to be effective for the treatment of ergopeptine alkaloid–related prolonged gestation in mares [3,4,6,7,9,11,13]. Pharmacologic intervention can also be used in late gestational mares to prevent the clinical signs of ergopeptine alkaloid toxicosis. This prophylactic approach may be advisable in environments favoring the growth of endophyte-infected fescue and the germination and development of C. purpurea. Domperidone, sulpiride, and perphenazine have all been used experimentally at their therapeutic doses, beginning on day 300 of gestation, to prevent the endocrine alterations and clinical signs associated with ergopeptine alkaloid toxicosis in pregnant mares [3,4,6,7,9,11,12,21]. In clinical settings, administration of domperidone 10 to 14 days before the expected foaling date has proven to be a useful prophylactic approach to ergopeptine alkaloid toxicosis in the pregnant mare [3,4,6,7,9,11]. Recently, the D2-dopamine receptor antagonist fluphenazine (25 mg administered intramuscularly in pony mares on day 320 of gestation) has also been used to prevent decreases in relaxin related to ergopeptine alkaloid–induced placental dysfunction [3,4,7,22].
References [1] Burrows GE, Tyrl RJ. Toxic plants of North America. Ames (IA): Iowa State University Press; 2001. [2] Cheeke PR. Natural toxicants in feeds, forages, and poisonous plants. 2nd edition. Danville (IL): Interstate Publishers; 1998. [3] Evans TJ, Rottinghaus GE, Casteel SW. Ergopeptine alkaloid mycotoxins. In: Robinson NE, editor. Current therapy in equine medicine 5. Philadelphia: WB Saunders [in press]. [4] Evans TJ, Rottinghaus GE, Casteel SW. Ergotism. In: Plumlee KH, editor. Clinical veterinary toxicology. St. Louis: Mosby [in press]. [5] Rottinghaus GE, Schultz LM, Ross PF, et al. An HPLC method for the detection of ergot in ground and pelleted feeds. J Vet Diagn Invest 1993;5:242–7. [6] Brendemuehl JP. Reproductive aspects of fescue toxicosis. In: Robinson NE, editor. Current therapy in equine medicine 4. Philadelphia: WB Saunders; 1997. p. 571–3. [7] Evans TJ, Rottinghaus GE, Casteel SW. Fescue toxicosis. In: Plumlee KH, editor. Clinical veterinary toxicology. St. Louis: Mosby [in press]. [8] Hoveland CS. Importance and economic significance of the Acremonium endophytes to performance of animals and grass plant. In: Joost RE, Quisenberry SS, editors. Acremonium/ grass interactions. Amsterdam: Elsevier; 1993. p. 3–12. [9] Green EM, Raisbeck MF. Fescue toxicosis. In: Robinson NE, editor. Current therapy in equine medicine 4. Philadelphia: WB Saunders; 1997. p. 670–3.
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[10] Rottinghaus GE, Garner GB, Cornell CN, et al. An HPLC method for quantitating ergovaline in endophyte-infested tall fescue: seasonal variation of ergovaline levels in stems with leaf sheaths, leaf blades, and seed heads. J Agric Food Chem 1991;39:112–5. [11] Cross DL, Redmond LM, Strickland JR. Equine fescue toxicosis; signs and solutions. J Anim Sci 1995;73:899–908. [12] Ireland FA, Loch WE, Worthy K, et al. Effects of bromocriptine and perphenazine on prolactin and progesterone concentrations in pregnant mares during late gestation. J Reprod Fertil 1991;92:179–86. [13] Evans TJ, Youngquist RS, Loch WE, et al. A comparison of the relative efficacies of domperidone and reserpine in treating equine ‘‘fescue toxicosis.’’ Proc Am Assoc Equine Pract 1999;45:207–9. [14] Ginther OJ. Reproductive biology of the mare: basic and applied aspects. 2nd edition. Cross Plains (WI): Equiservices; 1992. [15] Evans TJ, Constantinescu GM, Ganjam VK. Clinical reproductive anatomy and physiology of the mare. In: Youngquist RS, editor. Current therapy in large animal theriogenology. Philadelphia: WB Saunders; 1997. p. 43–70. [16] Riet-Correa F, Mendez MC, Schild AL, et al. Agalactia, reproductive problems and neonatal mortality in horses associated with ingestion of Claviceps purpurea. Aust Vet J 1988;65:192–3. [17] Evans TJ. The effects of bromocriptine, domperidone, and reserpine on circulating maternal levels of progestins, estrogens, and prolactin in pregnant pony mares [master’s thesis]. Columbia (MO): University of Missouri; 1996. [18] Brendemuehl JP. Effects of Acremonium coenophialum-infected tall fescue on progestagen production in pregnant mares. In: Proceedings of the Annual Meeting of the Society for Theriogenology; 1992. p. 108–12. [19] Holtan DW, Houghton E, Silver M, et al. Plasma progestagens in the mare, fetus, and newborn foal. J Reprod Fertil Suppl 1991;44:517–28. [20] Schutzer WE, Holtan DW. Steroid transformations in pregnant mares: metabolism of exogenous progestins and unusual metabolic activity in vivo and in vitro. Steroids 1996;61:94–9. [21] Redmond LM, Cross DL, Strickland JR, et al. Efficacy of domperidone and sulpiride as treatments for fescue toxicosis in horses. Am J Vet Res 1995;55:722–9. [22] Ryan PL, Bennet-Wimbush K, Vaala WE, et al. Systemic relaxin in pregnant pony mares grazed on endophyte-infected fescue: effects of fluphenazine treatment. Theriogenology 2001;56:471–83. [23] Messer NT, Riddle T, Traub-Dargatz JL, et al. Thyroid hormone levels in Thoroughbred mares and their foals at parturition. Proc Am Assoc Equine Pract 1998;44:248–51. [24] Boosinger TR, Brendemuehl JP, Bransby DL, et al. Prolonged gestation, decreased triiodothyronine concentration, and thyroid gland histomorphologic features in newborn foals of mares grazing Acremonium coenophialum infected fescue. Am J Vet Res 1995;56: 66–9. [25] Ryan PL, Rude B, Warren B, et al. Effects of exposing late-term pregnant mares to toxic and non-toxic endophyte-infected tall fescue pastures [abstract]. Biol Reprod 2001;64 (Suppl 1):612.
Endocrine alterations associated with ergopeptine alkaloid exposure during equine pregnancy Tim J. Evans, DVM, MS Veterinary Medical Diagnostic Laboratory, Department of Veterinary Pathobiology, College of Veterinary Medicine, University of Missouri at Columbia, Columbia, MO 65211, USA
Ergopeptine alkaloid exposure is common in pregnant mares. Many mares live in geographic areas where Neotyphodium coenophialum–infected tall fescue is the dominant grass in pastures and hay. A variety of grasses and cereal grains may also be infected by Claviceps purpurea (ergot), and fungal sclerotia can contaminate forage and especially ground and pelleted feed. Ergopeptine alkaloid mycotoxins produced by the fescue endophyte and Claviceps speeres interact with D2-dopamine receptors on cells such as lactotropes and induce hypoprolactinemia as well as other endocrine alterations. Agalactia, prolonged gestation, abortion, dystocia, and placental and fetal abnormalities are all clinical manifestations of the alterations in the endocrine milieu induced by ergopeptine alkaloid exposure during late gestation. Late gestational mares are particularly sensitive to the actions of ergopeptine alkaloids on prolactin secretion and the associated abnormalities in lactogenesis and steroidogenesis. Moreover, placental function is impaired in mares exposed to ergopeptine alkaloids during late pregnancy. An understanding of the endocrine alterations associated with ergopeptine alkaloid exposure during pregnancy is necessary for the diagnosis of potential exposure to these compounds and for effective prophylaxis and therapy. Sources of ergopeptine alkaloids The ergopeptine alkaloids ergotamine, ergocristine, ergosine, ergocornine, and ergocryptine are the predominant toxins contained within the black, dark brown, or purple ergot bodies (sclerotia) of C. purpurea [1–4]. P.O. Box 6023, Columbia, MO 65203. E-mail address: [email protected] (T.J. Evans). 0749-0739/02/$ - see front matter Ó 2002, Elsevier Science (USA). All rights reserved. PII: S 0 7 4 9 - 0 7 3 9 ( 0 2 ) 0 0 0 1 9 - 6
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Claviceps sclerotia replace the individual seeds in the seed head of common pasture grasses, including fescues, bluegrasses, and bromegrasses, or cereal grains, such as oats, barley, wheat, and especially rye and triticale. The germination and growth of C. purpurea are facilitated by the cool wet spring seasons in the northwestern United States and the northern Great Plains [1,2]. Horses are most likely to be exposed to ergot bodies found in contaminated pastures or hay or to fragments of fungal sclerotia hidden in processed feed or concentrated within the screenings from ergotized grains [3,4]. The total concentration of ergopeptine alkaloids in C. purpurea sclerotia generally ranges from 2000 to 10,000 mg/kg (ppm) [4,5]. Ergovaline is thought to be the most physiologically active ergot alkaloid produced by the fungal endophyte N. coenophialum (previously known as Acremonium coenophialum or Epichlo¨e typhinia) [1,2,6]. This endophyte grows within the intercellular spaces of Festuca arundinacea as part of a symbiotic grass/endophyte association. Over 35 million acres in the upper southeastern and lower midwestern regions of the United States contain endophyte-infected tall fescue, and it has been estimated that this grass is the primary forage for more than 700,000 horses [1–3,6–8]. Although also found in other parts of fescue grass infected by N. coenophialum, ergovaline concentrations are highest in the seed heads [1–3,6,7,9,10]. Concentrations of ergovaline generally range between 0.2 and 0.6 mg/kg (ppm) in tall fescue, with seed head concentrations of ergovaline often exceeding 1 mg/kg (ppm) [7,10]. Mechanism of action of ergopeptine alkaloids Ergopeptine alkaloids cause vasoconstriction through interactions with dopaminergic, adrenergic, and serotonergic receptors [1,2,11]. The pathogenesis of the attenuated lactation or agalactia and impaired reproductive function associated with equine ‘‘fescue toxicosis’’ are generally thought to involve primarily the stimulation of D2-dopamine receptors by ergopeptine alkaloids. Dopaminergic stimulation results in decreased prolactin secretion by lactotropes located in the anterior pituitary [1–4,6,7,9,11–13]. Prolactin is involved in the endocrine regulation of many physiologic processes, including lactogenesis and steroidogenesis [11,14]. The initiation of parturition in the mare depends on the maturation and proper function of the fetal hypothalamic-pituitary-adrenal axis [14,15]. Prolonged gestation in mares exposed to ergopeptine alkaloids may be related to hypoprolactinemia-induced alterations in uterofetoplacental steroid metabolism or may potentially be mediated by direct or indirect inhibition of D2-dopamine receptors on corticotropes in the fetal anterior pituitary [11,13–15]. Clinical signs of equine ‘‘fescue toxicosis’’ and ergotism Although embryonic death has been associated with ergopeptine alkaloid exposure early in pregnancy, equine ‘‘fescue toxicosis’’ is most commonly
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recognized as a condition of late gestational and early postparturient mares [6,9]. Agalactia has been observed mares fed fescue hay with ergovaline concentrations greater than 0.1 mg/kg (ppm) and in mares exposed to processed feed containing screenings of ergotized grain with total ergopeptine alkaloid concentrations ranging from 0.5 to 1.5 mg/kg (ppm) [3,4,7]. The clinical signs of fescue toxicosis and ergotism in pregnant mares include agalactia, prolonged gestation, abortion, dystocia, fetal asphyxia, weakness, dysmaturity and mortality, and placental abnormalities, such as premature placental separation, thickening and edema of fetal membranes, and retained placentae [1–4,6,7,9,11,12,16]. The signs of impending parturition in pregnant mares, such as the rapid increase in udder development, the commonly observed ‘‘waxing’’ or accumulation of colostrum at the teat orifices, and increases in the calcium levels in mammary secretions, are often attenuated in ergopeptine alkaloid–exposed late gestational mares, and these foalings are frequently unexpected and thus unattended [3,4,6,7, 9,11]. Fetal dysmaturity and increased fetal size associated with maternal exposure to ergopeptine alkaloids contribute to the development of dystocia and its sequelae in mares and foals [6,9]. In addition, failure of passive transfer and neonatal septicemia and decreased viability commonly occur in foals from agalactic mares.
Ergopeptine alkaloid–induced endocrine alterations in pregnant mares Hypoprolactinemia during late gestation Hypoprolactinemia is a hallmark feature of ergopeptine alkaloid exposure in pregnant mares [1–4,6,7,9,11,12,17]. Prolactin is secreted by lactotropic cells in the anterior pituitary gland and is commonly regarded in conjunction with its role in lactogenesis [15]. Prolactin secretion is tonically inhibited by endogenous dopamine interacting with D2-dopamine receptors on lactotropes. Large increases in maternal prolactin secretion normally occur a few days before parturition and remain elevated for several weeks postpartum. A placental lactogen is not produced during equine pregnancy, and lactation in mares, unlike cattle, is completely dependent on the production and secretion of prolactin by the anterior pituitary gland for normal lactation [14,15]. Ergopeptine alkaloids produced by N. coenophialum mimicking the actions of dopamine on lactotropic D2 receptors induce hypoprolactinemia and thus agalactia in pregnant mares. Suppression of maternal progestin levels during late gestation Beginning on or about day 40 of gestation, the uterofetoplacental unit begins to secrete progestins known as 5-a pregnanes, which differ structurally from progesterone (4-pregnene-3,20-dione) [14,15]. The predominant 5-a pregnane found in the peripheral blood of mares during late gestation,
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3, 20-dione (5a-DHP), may be found in extremely large concentrations (milligrams per milliliter versus nanograms per milliliter for progesterone). The 5-a pregnanes cross-react with the radioimmunoassay (RIA) for progesterone but are undetectable with ELISAs, which measure progesterone [14, 15,18,19]. The existence of a unique steroidogenic pathway in pregnant mares is suggested by the predominance of 5-a pregnanes in mid- to lateterm pregnant mares and the manner in which these mares metabolize exogenous progestins [14,15,18–20]. Production of 5-a pregnanes by the uterofetoplacental unit reaches a plateau by approximately midgestation, and there is normally a dramatic rise in maternal blood levels of immunoassayable progestins 30 days before parturition. This surge in maternal progestin levels peaks 2 to 3 days prepartum and declines precipitously within 24 hours of parturition [14,15]. In cases of ergopeptine alkaloid exposure, the metabolism of progestins is altered, and the dramatic increase in maternal circulating progestins observed during the last 30 days of pregnancy is suppressed or absent [3,4,6,7,9,11–13,15,17,18]. This suppression of the late gestational surge in immunoassayable progestins is frequently concurrent with prolonged gestation and may be related to maternal hypoprolactinemia and/or alterations in fetal cortisol production [6,7,11,12,15,17]. Administration of D2-dopamine receptor antagonists to ergopeptine alkaloid–exposed pregnant mares results in resolution of the hypoprolactinemia and resurgence of maternal levels of circulating progestins [6,7,11–13,17,21]. Decreased maternal relaxin levels during gestation The placenta is thought to be the sole source of relaxin in the mare. Relaxin decreases the collagen content in the extracellular matrices of the pubic symphysis and uterine cervix, inhibits uterine contractility, and may also play a role in mammary gland development [14,15]. Placental secretion of relaxin begins on about day 80 of pregnancy and peaks initially at approximately day 175. After a subsequent slight decline in blood levels, maternal levels of relaxin begin to increase gradually from about day 225 of gestation until parturition [14,15,22]. Relaxin has been demonstrated to be an indicator of placental health in the mare. In late gestational mares exposed to endophyte-infected fescue, maternal circulating levels of relaxin are decreased and may indicate placental dysfunction [22]. Alterations in maternal estrogen levels As luteal estrogen production begins to decline around days 60 to 90 of pregnancy, fetoplacental estrogen secretion begins to increase. Estrogens of fetoplacental origin increase gradually until day 200 of pregnancy and then decrease gradually until parturition [14,15]. It has been suggested that circulating levels of immunoassayable estrogens are increased in mares exposed to ergopeptine alkaloids during late gestation [11]. This endocrine alteration
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seems to be less consistent during the last 30 days of gestation than alterations in prolactin, progestins, and relaxin, however [13,17]. Alterations in maternal thyroid hormones There exists significant controversy regarding the diagnosis of equine thyroid dysfunction. A wide variety of external factors influence the hypothalamic-pituitary-thyroid axis. It has been suggested that mares exposed to endophyte-infected fescue during late gestation as well as their foals have decreased circulating levels of thyroxine (T4), and this potential effect of ergopeptine alkaloids may be worthy of further investigation [23].
Ergopeptine alkaloid–induced endocrine disturbances in neonatal foals Neonatal foals from mares exposed to ergopeptine alkaloids during late gestation frequently exhibit signs of dysmaturity [3,4,6,7,9,13]. Plasma levels of immunoassayable progestins, cortisol, T4, and tri-iodothyronine (T3) are generally decreased in these foals [3,4,6,7,23,24]. These endocrine alterations in the neonate may reflect the effects of ergopeptine alkaloids on blood supply to the fetus, placental structure and metabolism, and/or fetal hypothalamic and pituitary function.
Diagnosis of ergopeptine alkaloid exposure in horses Hormonal alterations in pregnant mares during the last 30 days of gestation may be helpful in diagnosing ergopeptine alkaloid exposure, but assays for immunoassayable progestins, prolactin, and relaxin are not available at all diagnostic laboratories [3,4,7]. In many cases, prolonged gestation and, especially, decreased mammary development and agalactia may be the best diagnostic criteria available for ergopeptine alkaloid exposure [3,4,6,7,9]. Calcium concentrations in mammary secretions rarely exceed 50 ppm in mares exposed to ergopeptine alkaloids. ELISA testing for urinary excretion of fescue ergot alkaloids, if performed within 24 to 48 hours of animal removal from suspect pasture, hay, or grain-containing products, has provided a method of definitively confirming exposure to ergot alkaloids in cattle. Research is being performed to determine the usefulness of this technique in the confirmation of ergopeptine alkaloid exposure in horses [3,4,7]. The determination of ergopeptine alkaloid concentrations in forage or grain by ELISA or high performance liquid chromatography (HPLC) does not confirm ingestion of ergopeptine alkaloids by horses, but it may be useful in establishing exposure to these compounds, especially in geographic areas where fescue pastures and C. purpurea infection are uncommon [3,4,6,7].
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Prophylactic and therapeutic considerations in ergopeptine alkaloid toxicosis Knowledge of breeding dates, confirmation of pregnancy, and careful monitoring of mammary gland development are critical steps in the identification of mares most susceptible to the effects of exposure to ergopeptine alkaloids [3,4,7]. Exposure of pregnant mares to heavily ergotized feedstuffs should be avoided. Pastures, hays, and grains should be monitored for the presence of C. purpurea sclerotia, and grain screenings should not be incorporated in feedstuffs intended for consumption by horses [1–4,7]. Analysis for ergopeptine alkaloid concentrations in suspect forages or rations may be advisable [3,4,7]. Avoidance of the use of toxigenic N. coenophialum–infected tall fescue in pastures or hays may be the best prophylactic approach to fescue toxicosis but is extremely challenging. Complete pasture renovation and reseeding with endophyte-free fescue or other grass species is limited by the symbiotic nature of tall fescue grass/Neotyphodium interactions. Endophyte-free tall fescue may not grow as well under some environmental conditions as fescue infected with N. coenophialum [1–4,7,9]. Tall fescue infected with a genetically altered ‘‘friendly’’ endophyte has shown promise in the prevention of the clinical signs of fescue toxicosis in horses [25]. This approach to fescue toxicosis prevention and control may be limited by economic constraints as well as by the possible reintroduction of ‘‘unfriendly’’ endophyte-infected fescue grass, however [3,7]. Strategic timing of withdrawal of horses from endophyte-infected pasture or hay for periods as long as 60 to 90 days before anticipated foaling dates has been recommended for pregnant mares. Because the most significant endocrine alterations associated with ergopeptine alkaloid exposure occur after day 300 of gestation, the removal of pregnant mares from potential sources of ergopeptine alkaloids 30 days before the expected foaling date has generally been successful in controlling the incidence of equine fescue toxicosis [3,6,7,9,11]. Frequent mowing, heavy grazing pressure, and chemical treatment to prevent or retard seed head development have been recommended as ways to decrease ergopeptine alkaloid concentrations in fescue pastures. Seeding fescue pastures with at least 20% palatable legumes such as clovers may also be a means of decreasing levels of ergopeptine alkaloids, and consultation with local agronomists or extension personnel may be helpful [1,3,7,9]. Successful treatment of fescue toxicosis and ergotism in horses is dependent on early recognition of the clinical signs as well as on preparturient monitoring and assistance during foaling. D2-dopamine receptor antagonists, such as domperidone (1.1 mg/kg administered orally once daily), sulpiride (3.3 mg/kg administered orally once daily), perphenazine (0.3–0.5 mg/kg administered orally twice daily), and acepromazine (20 mg per horse administered intramuscularly four times daily), have been used with some success
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in the treatment of agalactia in mares exposed to ergopeptine alkaloids [3,4,6,7,9,11–13,21]. The Rauwolfian alkaloid reserpine (2.0–5.0 mg per 450-kg horse administered orally once daily), which depletes brain depots of dopamine, serotonin, and/or norepinephrine, can be used for the treatment of postpartum agalactia in mares with a history of exposure to ergopeptine alkaloids, but its use may be associated with sedation and diarrhea [3,4,6,7,9,13]. Domperidone does not cross the blood–brain barrier like other D2 antagonists and has been demonstrated to be effective for the treatment of ergopeptine alkaloid–related prolonged gestation in mares [3,4,6,7,9,11,13]. Pharmacologic intervention can also be used in late gestational mares to prevent the clinical signs of ergopeptine alkaloid toxicosis. This prophylactic approach may be advisable in environments favoring the growth of endophyte-infected fescue and the germination and development of C. purpurea. Domperidone, sulpiride, and perphenazine have all been used experimentally at their therapeutic doses, beginning on day 300 of gestation, to prevent the endocrine alterations and clinical signs associated with ergopeptine alkaloid toxicosis in pregnant mares [3,4,6,7,9,11,12,21]. In clinical settings, administration of domperidone 10 to 14 days before the expected foaling date has proven to be a useful prophylactic approach to ergopeptine alkaloid toxicosis in the pregnant mare [3,4,6,7,9,11]. Recently, the D2-dopamine receptor antagonist fluphenazine (25 mg administered intramuscularly in pony mares on day 320 of gestation) has also been used to prevent decreases in relaxin related to ergopeptine alkaloid–induced placental dysfunction [3,4,7,22].
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