Twin embryos in mares. I: From ovulation to fixation

Recent findings on the origin and development of twins from ovulation (Day 0) to fixation (mean: Day 16) are reviewed. Available data show that almost all twins originate from multiple ovulations. Results of recent ultrasound studies indicate that the number of days between double ovulations does not affect the conception rate per ovum or embryo survival during the first 16 days after each ovulation. Embryo reduction is the natural elimination of excess embryos so that only one embryo enters the foetal stage. In two studies, embryo reduction before or on the day of fixation was not considered an important aspect of the natural correction of twins. Diameters and growth rates on Days 11 to 16 were similar between singletons and twins and the presence of two vesicles did not have a direct effect on their diameter other than that attributable to their age. Twin and singleton embryonic vesicles were mobile within the uterine lumen from the first day of detection (Days 9 to 11) to the day of fixation (mean: Day 16). In one study, the embryonic vesicles were in the uterine body for over 50 per cent of the time during Days 9 to 12 and thereafter were most often in the uterine horns. For twins of dissimilar size, the preference for uterine body versus horns appeared to be an independent function of each vesicle based on its age or size. After Day 12, the number of entries from the uterine body into the horns increased and the vesicles began a maximum mobility phase which continued until fixation.(ABSTRACT TRUNCATED AT 250 WORDS)     

Uterine morphology and function in postpartum mares

Ultrasonically detectable characteristics of the uterus and embryo and palpable uterine tone were assessed in 10 postpartum mares. A bright fern-like pattern of ultrasonic uterine echogenicity, outlining the endometrial folds, was observed for an average of 2.1 ±0.2 days following parturition (range, 1 to 3 days). Unexpectedly, the uterus was quiescent throughout the postpartum interval, based on daily one-minute contractility scans. Contractility was maximal on Days 12 to 15 of pregnancy in both postpartum (n=7) and nonparturient (n=7) mares. The mean diameter of ultrasonically detectable intrauterine fluid collections increased (P<0.05) abruptly between days 1 and 2 postpartum and gradually decreased (P<0.05) between days 4 and 7; no collections were detected after day 16. There was no effect of day on echogenicity of the intrauterine fluid collections; on all days, fluid was relatively black or nonechogenic, suggesting that puerperal endometritis was not a problem in this group. Because the increase in intraluminal fluid occurred after parturition and in temporal association with a decrease in diameter and tone of the uterus, the fluid collections apparently represented a physiologic influx from the involuting uterus rather than residual placental fluid. Involution of the horns was completed by day 27 (formerly nongravid horn) and day 31 postpartum (formerly gravid horn), based on failure to detect further significant decreases in diameter. However, the formerly gravid horn was larger (P<0.05) in diameter than the formerly nongravid horn on each of Days 1 to 35 postpartum (end of experiment), indicating residual effects on uterine size. When averaged over both horns, uterine diameters were larger on Days 0 to 24 (Day 0=day of ovulation) of pregnancy in postpartum mares than in nonparturient mares; by Day 25, diameters were similar between statuses. By approximately Day 6 of pregnancy, uterine contractility and ultrasonic endometrial exhotexture were similar between postpartum mares and nonparturient mares. Uterine tone was greater (P<0.05) in postpartum mares than in nonparturient mares on all days between Day 0 and 25. An unexpected, transient increase in uterine tone was detected on Day 5 of pregnancy in both postpartum mares and nonparturient mares. No differences were found between reproductive statuses in patterns of embryo mobility, the day of fixation of the embryonic vesicle (postpartum, Day 15.3 ±0.4; nonparturient, Day 15.0 ±0.3), and diameter of the embryonic vesicle on the day of fixation (postpartum, 22.1 ±1.4 mm; nonparturient, 19.4 ±l.6mm). However, mean uterine tone and mean horn diameters on the side of fixation were greater (cranial and middle cornual segments; P<0.05) or tended to be greater (caudal segment; P<0.1 ) on the day of fixation in postpartum mares than in nonparturient mares. In all postpartum mares, fixation occurred in the formerly nongravid horn. Enhanced uterine tone in postpartum mares may account for the occurrence of fixation on the same day for the two reproductive statuses, despite the larger uterus in postpartum mares.     

The nature of embryo reduction in mares with twin conceptuses: deprivation hypothesis

Ultrasonography was used to determine whether there is embryo reduction in mares with unilaterally fixed twins when a major portion of the vascularized area of the wall of one of the embryonic vesicles is in apposition with the wall of the adjacent vesicle, rather than with the endometrium (deprivation hypothesis). In addition, the effect of ovulatory pattern (synchronous and asynchronous) on the incidence of embryo reduction was studied. Twin vesicles were ultrasonically detected on days 11 to 15 (ovulation = day 0) and were examined daily until there was embryo reduction or until day 40. In 31 mares with twin embryonic vesicles, unilateral fixation (71%) was more frequent (P less than 0.05) than was bilateral fixation (29%). In 28 mares with known ovulatory patterns, synchronous ovulations did not affect the type of fixation (9/17 unilateral and 8/17 bilateral); however, for asynchronous ovulators the frequency of unilateral fixation (10/11) was greater (P less than 0.01) than the frequency of bilateral fixation (1/11). The incidence of embryo reduction was greater (P less than 0.01) for unilateral fixation (14/19) than for bilateral fixation (0/9) and was greater (P less than 0.05) for asynchronous ovulators (9/11) than for synchronous ovulators (5/17). In mares with embryo reduction, the reduction was complete before detection of both embryo propers (early reduction) in 10/14 and after detection of both embryo propers (late reduction) in 4/14.(ABSTRACT TRUNCATED AT 250 WORDS)     

Diagnosis and correction of twin pregnancy in the mare

Reproductive Physiology 1. Twin pregnancies result in high rates of abortion, stillbirth, and neonatal mortality. 2. Twins develop subsequent to multiple ovulations. Multiple ovulations are related to breed, parity, and mare history. Multiple ovulations are most frequently seen in Thoroughbred and Draft mares. Multiple ovulations are more common in barren and perhaps maiden mares than in lactating mares, and they are more common in certain individual mares. 3. Equine embryos are motile in the uterus from the time of first detection (Days 9 to 10) until fixation (Day 16). They are frequently located in the uterine body on Days 9 and 10. 4. Twin embryos have a pattern of motility and fixation similar to that of single embryos, and fixation is more frequently unilateral than bilateral (70 per cent versus 30 per cent, respectively). 5. Mares have an efficient natural embryo-reduction mechanism to eliminate excess (greater than 1) embryos resulting from multiple ovulations. Natural embryo reduction is more successful in unilateral than bilateral twin pregnancies (89 per cent versus 11 per cent successful reduction, respectively). 6. After the establishment of endometrial cups (Days 35 to 40), mares that are aborted will frequently not cycle for several months. Management of Twin Pregnancy 1. Breed all mares regardless of the number of preovulatory follicles. Withholding mares with preovulatory follicles from breeding does not decrease the incidence of twin pregnancy, but it decreases the overall pregnancy rate and results in a loss of breeding time. 2. Check all mares for twins, regardless of the number of detected ovulations.(ABSTRACT TRUNCATED AT 250 WORDS)     

Ovarian, uterine and embryo dynamics in horses versus ponies

Characteristics of the internal genitalia were compared between horses and ponies contemporaneously in a single study by asingle operator. Diameter of the preovulatory follicle for 10 days before ovulation (Day 0) and cross-sectional area of the corpus luteum for 10 days after ovulation were measured in 12 horses and 12 ponies. Follicle growth rate and maximum diameter attained during growth and luteal tissue area were not different between mare types. However, ultrasonic echogenicity of luteal tissue was less on Day 0 (P<0.09) and Day 1 (P<0.01) in ponies than in horses, suggesting that under field conditions mare type should be considered when using the extent of luteal gland echogenicity for estimating the number of days postovulation and luteal function. Circulating concentrations of progesterone averaged over Days 3 to 10 were greater in ponies (10.3 ng/ml) than in horses (8.7 ng/ml) as suggested by a tendency (P<0.1) towards a type-by-day interaction.Luteal, uterine and embryonic characteristics during Days 0 to 40 of pregnancy were compared between 7 horses and 9 ponies. Luteal tissue area of the primary corpus luteum was not different between mare types during Days 0 to 30 and during luteal resurgence after Day 30. The interaction between mare type and day was significant for circulating progesterone concentrations; the interaction seemed to result primarily from higher progesterone levels in ponies during Days 6 to 12 and Days 34 to 40. Progesterone concentrations were about 25% higher in ponies on the approximate days that levels were at their highest in the two types of mares. These results indicate, a need for further studies on the effect of mare size or type on circulating concentrations of progesterone, and the consideration of size or type in clinically evaluating progesterone levels in pregnant mares. Apparent partial recovery of the corpus luteum of the previous estrous cycle occurred during early pregnancy in one pony.Increasing diameter of the embryonic vesicle and the extent of uterine tone were similar between horses and ponies. Although diameter of the uterine horns decreased in parallel for the two mare types during Days 0 to 40, the diameter was significantly less (2.2 mm difference averaged over all days) for ponies than horses. The temporal relationship between an expanding embryonic vesicle concurrently with decreasing uterine horn size and enhanced uterine turgidity is compatible with the postulated mechanism of embryo fixation. The smaller uterus in ponies with no difference in size of the conceptus between mare types could account for earlier fixation of the embryonic vesicle in ponies than in horses.     

Embryonic loss in mares. Incidence, possible causes, and diagnostic considerations

Fertilization rates were similar for normal and subfertile mares, and much of the difference in fertility between normal and subfertile mares was due to embryonic loss. Fertilization rate estimates for mares ranged from 71 to 96 per cent. The incidence of embryonic loss detected by ultrasonography between Days 11 and 50 was approximately 9 per cent for normal mares, and the estimated incidence of embryonic loss before Day 14 was also 9 per cent. Therefore, the estimated incidence of embryonic loss in normal mares between fertilization and Day 50 is approximately 18 per cent (Fig. 1). In subfertile mares, the corresponding estimate for embryonic loss between fertilization and Day 50 is 80 per cent, with most embryonic losses occurring before Day 14 in subfertile mares (Fig. 1). The high rate of early embryonic loss in subfertile mares could be related to embryonic defects, oviductal environment, or uterine environment. Oviductal embryos from subfertile mares were less viable than embryos from normal mares; therefore, embryonic defects were important in early embryonic losses in subfertile mares. These defects might be inherent within the embryo or might arise from the early oviductal environment. The uterine environment of subfertile mares was adequate to support normal embryos in early gestation; however, the relationship between the uterine environment and the increased metabolic demands of the conceptus in the late embryonic or early fetal periods requires further study. The uterine environment is also altered in mares with endometritis; therefore, endometritis may also be an important factor in embryonic loss in some mares. Uterine-induced luteolysis, as well as the effect of the pathogen or the resulting inflammation, may lead to embryonic loss. An increased susceptibility of some subfertile mares to endometritis could result in embryonic loss secondary to a postcoital endometritis that persists until the embryo reaches the uterus at Days 5 or 6 postovulation. Although progesterone is critical to embryonic survival, the cause-and-effect relationship between progesterone and spontaneous embryonic loss remains unclear. Reduced progesterone concentrations could be related to endometritis, failure of maternal pregnancy recognition, or luteal insufficiency. Progesterone supplementation may be indicated for some mares, but the value of exogenous progesterone for prevention of spontaneous embryonic loss has not been critically tested. A number of other factors have been associated with embryonic loss in mares.(ABSTRACT TRUNCATED AT 400 WORDS)     

Effects of time of insemination relative to ovulation on pregnancy rate and embryonic-loss rate in mares

The effects of pre-ovulatory and post ovulatory insemination on pregnancy rate and embryonic-loss rate were studied in 268 mares in two experiments. Within each experiment mares were randomised within replicates as follows: to be inseminated on the day the pre-ovulatory follicle reached 35 mm (pre-ovulatory group), to be inseminated on the day of ovulation (Day 0 group), and to be inseminated on the day after ovulation (Day 1 group). Ultrasonic pregnancy diagnoses were performed on Days 11, 12, 13 and 14 (Experiment 1) and Days 11, 12, 13, 14, 15, 20 and 40 (Experiment 2). Combined for the two experiments, pregnancy rates were different (P less than 0.01) among the three groups. For Experiment 2, pregnancy rate within the pre-ovulatory group was lower (P less than 0.05) for insemination 4 days or more before ovulation than for up to 3 days before ovulation. Pregnancy rate was lower (P less than 0.05) for the Day 0 group than for the pre-ovulatory group inseminated up to 3 days before ovulation. In Experiment 2, ovulation was detected by examinations every 6 h; pregnancy rate was greater (P less than 0.05) for mares inseminated 0 to 6 h after ovulation than for mares inseminated at 18 to 24 h. No pregnancies occurred when mares were inseminated 30 h or more after ovulation. The mean day of first detection of the embryonic vesicle was different (P less than 0.0001) among the three groups. Diameter of embryonic vesicle averaged over Days 11 to 15 also differed (P less than 0.0001) among groups.(ABSTRACT TRUNCATED AT 250 WORDS)     

The effects of equine somatotropin (eST) on follicular development and circulating plasma hormone profiles in cyclic mares treated during different stages of the estrous cycle

The effects of exogenous equine somatotropin (eST) administration on ovarian activity and plasma hormone levels were evaluated on horse and pony mares. The objectives of this study were to determine the effects of eST on follicular development and circulating concentrations of leutinizing hormone (LH), estradiol, progesterone, and insulin-like growth factor I (IGF-I) in cyclic horse and pony mares. Sixteen mares received daily injections (i.m.) of eST at a concentration of 25 micrograms/kg body weight on either Days 6 through 12 (Treatment A) or 13 through 19 (Treatment B) postovulation. In addition, contemporary mares were similarly given the carrier vehicle and served as controls (Treatments C and D). Blood samples were collected at 24-hr intervals and ultrasonographic evaluations were performed on the ovaries of each mare at 48-hr intervals beginning on the first day of treatment and ending either on the day of ovulation or 5 d postovulation. Circulating levels of insulin-like growth factor-I (IGF-I) were increased in treated mares by Day 3 post-treatment (P < 0.05). Also, mares in Treatment B exhibited a decrease in plasma estradiol concentrations (P < 0.05) when compared with control mares on Days 1 through 5 postovulation of the post-treated estrous cycle. In addition, circulating leutinizing hormone levels were different for mares in Treatment A compared with controls on Days--8 through--1 pre-ovulation (P < 0.05). All follicles present on the ovaries of each mare were measured and placed into one of five categories based on their diameter. Neither the mean number of follicles per size category > or = 8 mm in diameter nor the mean follicular diameter within each size category differed among treatment and control mares. However, eST treatment significantly increased the number of follicles < or = 7 mm on the ovaries of mares treated early in the estrous cycle when compared with control mares on Days 3 and 7 post-treatment and at the onset of standing estrus.     

Changes in plasma progesterone concentrations from days 17 to 42 of gestation in mares maintaining or losing pregnancy

Plasma progesterone concentrations were measured in 179 mares bled on alternate days commencing with a positive pregnancy diagnosis on Days 17 to 18 after ovulation and concluding on Days 42 to 45. During this period 17 mares (10 per cent) lost their pregnancies, 11 before Day 25. In 15 mares the timing of the pregnancy loss could be determined with adequate accuracy; in only one did a decline in progesterone precede the loss. Thus pregnancy loss between Days 17 and 42 was rarely caused by a fall in plasma progesterone.     

The Relationship Between the Positive Identification of the Embryo Proper in Equine Pregnancies Aged 18-28 Days and its Future Viability: A Field Study

Early embryonic loss (EEL) negatively affects the reproductive efficiency of equine reproduction. A sign of future EEL is when the embryo proper (EP) fails to develop within the embryonic vesicle after 30 days of gestation. The earlier the identification of impending EEL, the earlier the mare can be rebred to allow a second chance of pregnancy. The objectives of this study were to determine the percentage of embryonic vesicles with a visible EP at 18-28 days of gestation and to study the association between the presence/absence of the EP at different days of gestation and the future viability of the pregnancy. A total of 1,256 pregnancies were identified and followed by transrectal B-mode ultrasonography 12-45 days post ovulation in mares of the same Thoroughbred farm. The identification of the EP was attempted once during days 18-28. Pregnancy reconfirmation was performed on days 35-45. The percentage of vesicles with an EP increased gradually from days 18 (2.8%) to 21 (86.9%) (P < .05). From day 20 onward, the EEL rate of mares with vesicles without an EP was significantly higher (P < .05) than that of vesicles with a positive identification of an EP. In conclusion, the EP of the equine vesicle can be identified reliably with B-mode ultrasonography in the majority of mares (>71%) on day 20 of gestation. The lack of a positive identification of an EP from day 20 onward is associated with a higher EEL rate.     

Relationship between estrous behavior in pregnant mares and the presence of a female conceptus

Unsolicited reports of estrous behavior in mares thought to be pregnant were received from owners or caretakers of Arabian mares. Estrous behavior was confirmed and mares were examined for pregnancy. Gender of the conceptus was determined at foaling in 11 mares in which estrous behavior was confirmed while an apparently viable, ultrasonically normal-appearing conceptus was present. In 9 mares in which the day of ovulation was known (Day 0), the estrous behavior occurred on Day 12, 13 or 14 (5 mares), Day 18 or 20 (2 mares), Day 40 (1 mare) and Day 60 (1 mare). In another study, 55 pony mares were observed for estrous behavior every 3 days for 20 minutes during Days 11 to 40. Estrous behavior was observed in 1 mare (2%) on Day 24. Combined for the 2 studies, the incidence of a female conceptus (12/12) was greater (P<0.01) than the incidence of a male conceptus (0/12) in mares that exhibited estrous behavior.     

Ultrasonographic appearance of the bovine conceptus from days 10 through 20

The bovine conceptus was monitored in 19 heifers by intrarectal ultrasonic imaging from the day an embryonic vesicle was first detected (mean, day 11.7 +/- 0.4; range, days 10 to 17) until detection of the embryo proper (mean, day 20.3 +/- 0.3). Fifteen heifers maintained the conceptus and 4 heifers apparently lost the conceptus. In the heifers that maintained the conceptus, 73% of the embryonic vesicles were classified as spherical (mean diameter, 2.8 +/- 0.2 mm) and 27% were classified as oblong (mean dimensions, 2.0 +/- 0.0 mm and 4.5 +/- 1.0 mm) on the day of first detection. All vesicles were in the uterine horn ipsilateral to the corpus luteum. The vesicles increased in length from the day of first detection. On the average, the vesicle occupied all of the ipsilateral uterine horn on day 16.9 +/- 0.6 and all of the contralateral horn on day 19.6 +/- 0.9. During elongation, the vesicle remained at an approximate height of 2 mm throughout its length, but developed a localized enlargement or bulge on mean day 19.7 +/- 0.2. The embryo proper was detected within the bulge in all 15 heifers. A heartbeat (mean, 188 +/- 4.8 beats/min) was detected on the first day of detection of the embryo proper (8 heifers) or on the following day (7 heifers). The mean length of the interovulatory interval in the 4 heifers that apparently lost the embryonic vesicle was not significantly different from that of nonbred heifers. The vesicles were lost (not ultrasonographically detectable) on days 17 (2 heifers) and 19 (2 heifers).(ABSTRACT TRUNCATED AT 250 WORDS)     

Videoendoscopic evaluation of the mare's uterus: III. Findings in the pregnant mare.

During a 5-year period 65 Pony and 20 Thoroughbred pregnant mares were subjected to videoendoscopic hysteroscopy from 10 to 266 days of gestation. The aims of these examinations were to 1) observe foetal and placental development in vivo (60 Pony and 10 Thoroughbred mares); 2) eliminate one of unicornuate twin conceptuses (9 Thoroughbred mares); 3) recover embryonic foetal and placental tissues non-surgically for experimental purposes (47 Pony mares); 4) induce focal separation of the placenta in late gestation as an experimental model of placentitis (5 pony mares and 1 Thoroughbred mare). It was possible to view the embryo and all constituent extra-embryonic membranes of the conceptus between Days 10 and 87 of gestation without having to perforate the allantochorion. This allowed study of physiological processes such as the coordinated uterine contractions responsible for conceptus motility between Days 7 and 17, active foetal movements, which began as early as Day 34, and invasion of chorionic girdle cells into the endometrium, which occurred between Days 34 and 38. From Day 90, vision of the foetus was reduced or prevented by the increased thickness of the allantochorion. Transendoscopic recovery of the conceptus was successful in all of 10 mares under 30 days of gestation, whereas only 10 of 18 attempts between Days 30 and 45 produced the conceptus without resorting to uterine lavage after initial rupture of the allantochorion. All 9 attempts to eliminate one of unilateral twin conceptuses were unsuccessful and the technique was abandoned as clinically unsuitable. Nevertheless, 9 of 22 (41%) single conceptuses remained viable after one or more hysteroscopic examinations.(ABSTRACT TRUNCATED AT 250 WORDS)     

Die Entwicklung des Dottersackes beim Wiederkäuer (Schaf und Rind)

Yolk sac development was investigated in 69 ovine and 10 bovine embryos from the blastocyst stage to the 7th week of gestation. Light and electron microscopical findings are reported. The yolk sac in sheep and cattle is composed of an enlarged sac-like portion lying below the embryo and two ends which follow the elongated course of the trophoblast. In sheep, an open connection exists between the intestines and the yolk sac up to a crown-rump length (CRL) of 9 mm. It is closed by 12 mm CRL. The wall of the yolk sac is especially well vascularized in the enlarged, sac-like portion. Primary erythropoiesis occurs within the blood capillaries. In the blastocyst, the yolk sac entoderm is made up of elongated, flat cells. It becomes cuboidal in the 3 mm embryo (ovine) and later columnar. The up to 20 microns tall cells stain darkly and contain numerous light-colored vesicles. At 4.5 mm CRL light cells appear between the dark ones. Both cells are rich in rough endoplasmic reticulum (rER). The increased staining of the darker cells is due to an osmophilic cytoplasm and numerous, often parallel lamella of rER. The rER of the light cells is enlarged to irregularly-shaped cisternae, which nearly fill the entire cytoplasm and give them a rounded appearance. The dark cells contain polygonal nuclei, whereas those in the light cells are round with one or two nucleoli. The oval mitochondria have only a few peripheral cristae. Golgi fields are not very common. Cells of the entoderm are connected to one another over zonulae occludentes. They possess microvilli on the luminal surface and are supported by a basement membrane. From 5 mm CRL onwards (ovine), the yolk sac entoderm folds itself between the capillaries, thereby becoming stratified. The intercellular space between the cells expands as projections between neighboring cells interlock. Canaliculi arise between adjacent epithelia. The wall of the yolk sac thickens as a result of this infolding and the densely packed capillaries. Infoldings are especially predominant in the sac-like portion of the yolk sac, and only suggested in the ends. Involution of the yolk sac begins in the peripheral end segments and proceeds centripetally. Numerous glycogen particles appear in the yolk sac entoderm cells of the ovine fetus at a CRL of 36 mm, and by a CRL of 42 mm, the sac-like portion has also begun to show signs of degeneration. Mesenchyme is very sparse within the wall of the yolk sac throughout the entire period of development.(ABSTRACT TRUNCATED AT 400 WORDS)     

Twin pregnancy experimental model for transvaginal ultrasound-guided twin reduction in mares

Multiple pregnancies are still an important cause of noninfectious abortion, stillbirth, neonatal mortality, and significant delays in reproductive performance in mares. Despite new management techniques, reduction in multiple pregnancies is an ongoing preoccupation and challenge for the equine veterinarian. The aim of the present study was to establish a twin pregnancy experimental model in the mare to study the effectiveness of a transvaginal ultrasound-guided embryonic vesicle injection. Mares in heat were inseminated and then received an embryo at day 7 of the estrous cycle. At days 14 and 30, 53.5% (n = 23) and 23% (n = 10) of the mares, respectively, were carrying twins. Twin pregnancies were reduced at day 30 by transvaginal ultrasound-guided puncture of the embryonic vesicle (control, n = 5) or by transvaginal ultrasound-guided injection (TVUEVI) of 25 mg of amikacin into the embryonic vesicle (n = 5). The TVUEVI treatment had a 40% success rate and no significant variations in progesterone and prostaglandin metabolite were observed. Even though the technique does not seem very effective, the experimental model could be useful for clinical research in embryo reduction and early embryonic loss.     

Progesterone and equine chorionic gonadotropin concentrations around the time of pregnancy loss in mares impregnated by donkeys or stallions

The objective of this study was to compare the incidence of pregnancy loss of mares carrying a mule embryo with that of mares carrying a horse embryo. The possible causes of such mortality were evaluated through serial ultrasonographic evaluations and hormonal monitoring, paying special attention to the role of premature regression of the endometrial cups and its relation to inadequate luteal function.

Twenty-eight mares impregnated by stallions and 19 mares impregnated with donkey semen were evaluated ultrasonographically every week from day 20 to day 150 of pregnancy. The viability of the product was assessed each time, and the diameter of the embryonic vesicle was measured from day 25 to day 60. Blood samples for progesterone and equine chorionic gonadotropin (eCG) determination were taken every week.

Both progesterone and eCG concentrations during normal pregnancies wegre significantly lower in the mares inseminated with donkey semen than in the mares impregnated by stallions (P<.05). In 7 of the mares carrying mule conceptuses to term, the concentrations of eCG remained basal throughout the study. In the other animals from this group, the levels of this hormone did increase but returned to baseline much earlier (on day 77 of pregnancy) than in the mares served by stallions (on day 126 day of pregnancy). There was no significant difference between the growth rate of embryonic vesicles of mares carrying mule embryos and that of mares carrying horse embryos (P>.05).

The incidence of pregnancy loss was significantly higher (P<.05) in mares carrying a mule embryo (36.8 %) than in mares carrying a horse embryo (21.4%); it occurred on average on day 93 of pregnancy in mares carrying mule embryos and on day 43 on mares carrying horse embryos. There was only 1 case in which pregnancy loss was associated with concentrations of both eCG and progesterone that were much lower than the average for the normal pregnancies of the same group, and this was in a mare carrying a horse embryo. The most frequent cause of pregnancy loss was premature luteal regression due to primary luteolysis, as evaluated via peripheral progesterone concentrations. This occurred in 2 mares carrying horse embryos and in 4 mares carrying mule embryos. Three mares carrying mule embryos and 1 carrying a horse embryo had abortions that were not preceded or accompanied by any alteration in progesterone or eCG levels and were thus classified as fetal deaths of non-endocrine origin.

It is concluded that the incidence of pregnancy loss is higher in mares carrying a mule embryo than in mares carrying a horse embryo. However, this is not due to the low progesterone concentrations associated with the premature regression of the endometrial cups that occurs in mares with interspecific pregnancy.     

Circulating concentrations of immunoreactive inhibin and FSH during the estrous cycle of mares

A heterologous radioimmunoassay developed to measure inhibin in rat plasma was validated and used to characterize changes in peripheral concentrations of immunoreactive inhibin (ir-inhibin) in relation to follicle stimulating hormone (FSH) concentrations during the estrous cycle of mares. The primary antiserum used in the assay was developed against a synthetic porcine inhibin -subunit [(1-26)-Gly-Tyr] fragment. The same synthetic peptide was used for preparation of standards and tracer. Slopes of the dose-response curves for pooled estrus and diestrus mare plasma and equine follicular fluid were similar to the slopes for the porcine inhibin -subunit standard curve and porcine follicular fluid dose-response curve. Twelve mares were bled once daily beginning when diameter of the largest follicle reached ≤25 mm and continuing until 3 days after the end of an interovulatory interval (ovulation=Day 0). Each of the 12 interovulatory intervals were normalized to the mean length of the interovulatory interval (22.2 days; range, 19 to 26). There was an effect of day for concentrations of ir-inhibin (P<0.001) and FSH (P<0.006). Significant mean changes were as follows: 1) ir-inhibin decreased between Days 0 and 1, whereas FSH increased between Days 0 and 5; and 2) ir-inhibin increased between Days 7 and 12, whereas FSH decreased between Days 11 and 14. Mean concentrations of ir-inhibin and FSH were negatively correlated (r=-0.548; P<0.002). In conclusion, mean peripheral concentrations of ir-inhibin and FSH were inversely related during the estrous cycle of mares.     

Reduction of Twin Pregnancy in the Mare by Transvaginal Ultrasound-Guided Aspiration

Diagnosis and management of twin pregnancies in the mare are an ongoing challenge in equine reproduction. Early detection of twin and manual crush of one vesicle are the main steps in the management of twins. Few studies were carried out about the use of transvaginal ultrasound-guided aspiration (TUGA) for reduction of twins in the mare. In this study, the efficiency of TUGA for management of twin pregnancies was investigated. Reduction of unicornuate twins between 16 and 25 days of gestation gave a success rate of 70.0% (14 viable foals/20 twin pregnancies); when reduction was performed after day 40 of gestation, all mares (one unicornuate; three bicornuate) lost both. For those cases in which the window for early crush of a twin vesicle has been missed, TUGA can be successfully used to reduce twin pregnancy between 16 and 25 days of gestation.     

Evaluation of progesterone treatment to create a model for equine endometritis.

To investigate a model for equine endometritis, 12 mares with normal reproductive tracts were divided into 2 groups. All mares received progesterone in oil, 250 mg im, daily. At 5 days after initiation of progesterone administration, the uteri were inoculated with 10(6) colony forming units of Pseudomonas aeruginosa. The day of inoculation was designated Day 0. On Day 6, endometrial swab samples yielded P. aeruginosa in 5 mares; samples from the other 7 mares yielded heavy growth of Escherichia coli, Streptococcus zooepidemicus, Klebsiella pneumoniae, Enterobacter spp., Citrobacter diversus, Staphylococcus epidermidis and Streptococcus morbillorum. On Days 6, 7 and 8, Group A mares received intrauterine infusions of 6 g ticarcillin disodium and 0.2 g clavulanate potassium in 100 ml sterile saline. Group B mares received infusions of saline only. The incidence of swab specimens yielding no bacterial growth was significantly higher in Group A than Group B mares on Days 8 and 13 (4/6 vs 0/6). Swab samples from 5 of the 6 mares in Group A yielded growth of fungi on Days 13 and 19. Mares in Group B were then similarly treated with ticarcillin/clavulanate infusions, on Days 19, 20 and 21. The incidence of swab specimens yielding no bacterial growth was 2/6 and 1/6 on Days 21 and 26, respectively; fungi were not recovered from these mares at any time. The incidence of no-growth swabs after antibiotic treatment tended to be higher in Group A and incidence of fungal recovery after antibiotic treatment was significantly higher in Group A.(ABSTRACT TRUNCATED AT 250 WORDS)     

Methods for the treatment of twin pregnancy in the mare

Within a group of 130 mares which, in the event, carried twin pregnancies, 29 (22.3 per cent) produced their twins following diagnosis of a single palpable follicle at service and a single pregnancy 42 days later. The whole group of 130 twin-pregnant mares gave birth to only 17 live foals (13 per cent). Breeding results for the following season were available for 102 of these mares, when 38 (37 per cent) produced live foals. Thus over the two seasons an average 23 per cent of this group produced live foals. In a second group, comprising 70 mares, twin pregnancy had been diagnosed in all of them by rectal palpation. Five methods of treatment were then applied to five subgroups of mares and 20 (28 per cent) of the 70 mares produced live foals that season. Breeding results for the following season were available for 53 of these mares; 33 (61 per cent) then produced live foals. Thus over the two seasons an average 46 per cent of the treated group produced live foals. The administration of a prostaglandin analogue at less than 35 days of gestation was outstandingly successful as a method of treatment.