Effect of prolonged use of altrenogest on behaviour in mares

Oral administration of altrenogest for oestrus suppression in competition horses is believed to be widespread in some equestrian disciplines, and can be administered continuously for several months during a competition season. To examine whether altrenogest has any anabolic or other potential performance enhancing properties that may give a horse an unfair advantage, we examined the effect of oral altrenogest (0.044 mg/kg), given daily for a period of eight weeks, on social hierarchy, activity budget, body-mass and body condition score of 12 sedentary mares. We concluded that prolonged oral administration of altrenogest at recommended dose rates to sedentary mares resulted in no effect on dominance hierarchies, body mass or condition score.     

Efficacy of short-term administration of altrenogest to postpone ovulation in mares

Estrogen from a growing follicle stimulates the preovulatory surge of luteinizing hormone (LH) while progesterone (P) is known to suppress LH. The possibility exists that administration of P, in the presence of an ovulatory follicle, would sufficiently suppress LH and, therefore, delay ovulation. The objective of this research was to elucidate the potential for oral administration of altrenogest (17-Allyl-17β-hydroxyestra-4,9,11-trien-3-one) to postpone ovulation of a preovulatory follicle (35 mm) for approximately two days. Fourteen light-horse mares, ranging in age from two to 19 years, were randomly assigned to one of three treatments (A-.044 mg/kg BW altrenogest for two days; B-.088 mg/kg BW altrenogest for two days; and C- no altrenogest). Mares began treatment when a 35-mm or greater follicle was observed via real-time transrectal ultrasonography. Both number of days until ovulation and follicular maintenance differed between treated and control mares. Number of days until ovulation was increased (P<.05) for mares in treatment A when compared with the control mares. Follicular diameter maintenance, a measurement of follicular diameter throughout treatment, also increased (P<.05) for mares in treatment A when compared with the control mares. Mean LH concentration was not different between mares treated with altrenogest at either treatment dose when compared with the control mares. Pregnancy rates and embryonic vesicle size change were also measured to determine potential effects of altrenogest administration. No differences (P>.05) were found in either characteristic.Short-term administration of altrenogest increased the number of days to ovulation. Further study is warranted to prove conclusively that altrenogest increases follicular maintenance, alters the preovulatory LH surge, and has no detrimental effects upon reproductive efficiency.     

Safety of altrenogest in pregnant mares and on health and development of offspring

Fifty-one light-horse mares were utilized to evaluate the safety of an oral progestin, altrenogest, administered throughout gestation on: gestation length, embryonic and fetal loss, periparturient events, health and development of offspring, and future reproductive capabilities of the mares. Pregnancies were established by inseminating mares with 250 × 10⁶ progressively motile spermatozoa from the same stallion every other day throughout estrus or by non-surgical transfer of embryos. Mares were randomly assigned to 1 of 2 treatments upon confirmation of pregnancy on day 20: 1) controls, 2 ml of neobee oil orally per 44.5 kg of body weight; and 2) treated, 2 ml of altrenogest dissolved in neobee oil at a concentration of 2.2 mg/ml orally per 44.5 kg of body weight. Treatments were administered daily from day 20 to 320 of gestation.There were no significant differences between treatment groups for duration of gestation, placental weight, time to placental expulsion and incidence of retained placental membranes. Number of female foals born from altrenogest treated mares (14 of 23) was greater (P<.05) than the number from untreated control mares (4 of 16). Female foals born from altrenogest treated mares had larger clitori (P<.05) than those from control mares. Times to sternal recumbency, standing and nursing were similar for the 2 groups (P>.05). Body weight and height at withers, heart girth circumference and length and width of cannon were measured at time of birth and at 2, 4, 6, 8, 12 and 16 weeks of age. Measurements did not differ (P>05) between treated and control foals for any development parameters.Beginning on day 20 postpartum, mares were teased daily. During estrus, mares were inseminated every other day with 250 × 10⁶ motile spermatozoa. Teasing and/or insemination was continued for 2 cycles or until mares were 35 days pregnant. The number of mares pregnant after 1 cycle and after 2 cycles of insemination was similar (P>.05) for treated and control mares. Nineteen of 21 treated mares and 15 of 16 control mares were pregnant after 2 cycles of insemination. Number of cycles per pregnancy was similar (P>.05) for treated and control mares (1.37 vs 1.13) as was number of days mares exhibited estrus (6.30 vs 6.13). Number of inseminations per cycle did not differ (P>.05) between treated and control mares (2.92 vs 3.00). In summary, there was no effect of treatment with altrenogest from day 20 to 320 of gestation on periparturient events, viability and growth of offspring and subsequent reproductive performance of mares.     

Reproductive traits, lactation and foal growth in mares fed altrenogest

Lactating mares were assigned as controls or fed altrenogest (.044 mg.kg body wt-1.d-1) for 15 d after foaling. Mares (n = 6) fed altrenogest were inseminated during the first estrus after treatment and mares (n = 6) in the control group were inseminated during the second postpartum estrus. Ovulation during the estrus in which mares were inseminated occurred 26 +/- 1 d postpartum for treated mares and 36 +/- 1 d postpartum for control mares. The percentage of mares conceiving was not different for control (67%) and alternogest-treated (100%) mares. No differences were observed in tone and size of the uterus or size of the ovulatory follicle between treated and control groups. Uterine cultures and biopsies collected on d 7 and 15 postpartum were similar between treatment and control groups in bacterial populations or endometrial epithelial cell height. Blood was collected on d 7, 11, 15, 19 and 23 postpartum, and concentrations of estradiol-17 beta in serum were determined by radioimmunoassay. Mean concentrations of estradiol-17 beta across days were 10 +/- .8 and 12 +/- .6 pg/ml for control and treated mares, respectively. Concentrations of serum estradiol-17 beta were higher (P less than .05) in treated mares on d 23 postpartum. Daily milk yields, determined by the weigh-suckle-weigh method, and milk composition were similar between treatment groups on each collection day. Altrenogest can be used to predictably delay estrus in the postpartum mare without altering fertility, yield and composition of milk, or foal growth.     

Efficacy of altrenogest administration to postpone ovulation and subsequent fertility in mares

A recent report suggested administration of altrenogest during the follicular phase could postpone ovulation. Based on these results, two questions were generated. We first hypothesized that by initiating a altrenogest treatment earlier in the estrous cycle, a greater and/or more consistent delay in ovulation would result. Second, we hypothesized that exposure to elevated progestin concentrations might alter viability of the ovulatory follicle and oocyte. The focus of the first experiment was to determine if initiation of altrenogest treatment at different stages of the estrous cycle would yield a more predictable time to ovulation, whereas the second experiment was designed to determine whether mares receiving altrenogest during estrus had compromised fertility. In the first experiment thirty mares of mixed light breed, ranging in age from 5-15 years, were randomly assigned to one of three groups. The two treated groups received altrenogest (0.088 mg/kg of body weight) for two days once a follicle of 30 or 35 mm in diameter was detected. Control mares were not treated. Mares treated with altrenogest whether initiated at the detection of a 30 or 35 mm follicle demonstrated similar (P>.05) day to ovulation interval when adjusted to 35 mm (5.4 and 5.6 days, respectively). Both treated groups demonstrated a delayed interval (P<.05) when compared to control (3.9 days). Thirty-six mares of similar breed and age, were randomly assigned to two groups for use in the second experiment. All mares were monitored daily via transrectal ultrasonography from the time a 35 mm or greater follicle was detected until ovulation. Treated mares received daily doses of altrenogest (0.088 mg/kg of body weight) for two days once a follicle of 35 mm or greater was detected. Control mares received no treatment. Fertility data were collected from mares inseminated every other day with 500 million motile spermatozoa from one of two stallions with proven fertility. Pregnancy data were collected via transrectal ultrasonography at days 12, 14 and 16 post-ovulation. Ovulation data were collected from 27 control cycles and 26 treated cycles. Contrary to previous reports and Experiment 1, no difference (P=0.35) was noted between groups with respect to days to ovulation. Control mares averaged 4.14 days and treated mares averaged 4.7 days to ovulation from initial detection of a 35 mm follicle. Fertility data were also similar (P=0.8) between control and treated mares (66.6% and 61.5% per cycle, respectively). Interestingly, a greater number (P=0.017) of treated cycles (5/26) resulted in follicular regression than did control cycles (0/27). While these data suggest that this dosage of altrenogest may not postpone ovulation, it did appear related to increased incidence of follicular regression. Fertility was unaffected, however, in those mares that ovulated. Further studies are needed in which initiation at different stages of estrus and different doses of altrenogest are used.     

Pharmacokinetics of altrenogest in horses

The Federation Equestre Internationale has permitted the use of altrenogest in mares for the control of oestrus. However, altrenogest is also suspicious to misuse in competition horses for its potential anabolic effects and suppression of typical male behaviour, and thus is a controlled drug. To investigate the pharmacokinetics of altrenogest in horses we conducted an elimination study. Five oral doses of 44 mug/kg altrenogest were administered to 10 horses at a dose interval of 24 h. Following administration blood and urine samples were collected at appropriate intervals. Altrenogest concentrations were measured by liquid chromatography-tandem mass spectrometry. The plasma levels of altrenogest reached maximal concentrations of 23-75 ng/mL. Baseline values were achieved within 3 days after the final administration. Urine peak concentrations of total altrenogest ranged from 823 to 3895 ng/mL. Twelve days after the final administration concentrations were below the limit of detection (ca 2 ng/mL).     

Pharmacokinetics of altrenogest in gilts

Altrenogest, a synthetic progestogen, is characterized by its estrus synchronization in mares, ewes, sows, and gilts. To investigate the pharmacokinetic profile and evaluate its accumulation in gilts, 18 oral doses of 20 mg altrenogest/gilt/day were given to eight healthy gilts at an interval of 24 hr. Plasma samples were collected, and altrenogest was determined by ultra‐high‐performance liquid chromatography with mass spectrometry. WinNonlin 6.4 software was used to calculate the pharmacokinetic parameters through noncompartmental model analysis. After the first administration (D 1), the pharmacokinetic parameters, including T_max, C_max, and the elimination half‐life (T_1/2λz), were similar to those observed after the final administration (D 18). However, the mean residence time at D 1 was significantly lower than D 18. As a whole, the mean steady‐state plasma concentration (C_ss), degree fluctuation (DF), accumulation factor (R_ac), and area under the plasma concentration–time curve in steady state (AUC_ss) were 22.69 ± 6.15 ng/ml, 270.64 ± 42.51%, 1.53 ± 0.23, and 544.63 ± 147.49 ng hr/ml, respectively. These results showed that after 18 consecutive days of oral administration of altrenogest, plasma concentrations of altrenogest had a certain degree of fluctuation, without significant accumulations.     

Effect of yohimbine on xylazine-induced hypoinsulinemia and hyperglycemia in mares

Serum insulin and plasma glucose concentrations were determined in 8 mares. Four IV treatments were studied: xylazine (1.1 mg/kg of body weight); yohimbine (0.125 mg/kg); yohimbine (0.125 mg/kg) followed 5 minutes later by xylazine (1.1 mg/kg); and 5 ml of isotonic saline solution as a control. Blood samples were collected before (time 0) and at 5, 15, 30, 60, 120, and 180 minutes after drug administration. Serum insulin concentration decreased and plasma glucose concentration increased in mares given xylazine. Plasma glucose concentration was unchanged in control mares and in mares given yohimbine or yohimbine followed by xylazine. Serum insulin concentration was unchanged in mares given saline solution, but transiently increased in mares given yohimbine alone. Treatment with yohimbine prevented xylazine-induced hypoinsulinemia and hyperglycemia.     

Effect of repeated transvaginal ultrasound-guided follicle aspiration on fertility in mares

OBJECTIVE: To determine whether performance of transvaginal ultrasound-guided follicle aspiration (TVUFA) repeatedly in mares adversely affects their fertility. DESIGN: Historical prospective study. ANIMALS: 23 mares that had never undergone TVUFA and 59 mares that had undergone TVUFA on 1 to 11 occasions. PROCEDURE: Mares were classified into 4 groups according to the number of TVUFA procedures previously performed on the ovary in which ovulation occurred at the time of insemination as follows: group 1, 0 TVUFAs (control group, n = 23 mares); group 2, 1 or 2 TVUFAs (40 mare-cycles); group 3, 3 or 4 TVUFAs (21 mare-cycles); and group 4, 5 to 11 TVUFAs (13 mare-cycles). Each ovary and its associated number of TVUFAs were considered separately; therefore, some of the mares that underwent TVUFA were represented in > 1 group (1 mare was included in group 2 twice [once for each ovary]), and the sample size in groups 2, 3, and 4 was denoted as mare-cycles. Fertility was assessed as pregnancy rates in cycles in which mares were inseminated with fresh or cooled semen from 1 fertile stallion. RESULTS: There were no significant differences in pregnancy rates among groups 1, 2, 3, and 4 (83%, 90%, 81%, and 85%, respectively). CONCLUSIONS AND CLINICAL RELEVANCE: Results indicated that repeated performance of TVUFA (as many as 11 times) had no detectable adverse effect on fertility in mares. This finding is clinically important for situations when TVUFA is performed on fertile mares, whether for oocyte collection or other purposes.     

Effect of photoperiod on reproductive activity and hair in mares.

The effects of photoperiod on reproductive activity and hair changes in pony mares were studied in 2 experiments. In experiment I, the effect of a fixed daily photoperiod on the onset of the breeding season was studied in 36 mares from Nov 13, 1973, to June 13, 1974. The 4 treatment groups were as follows: daily photoperiod equivalent to the normal day length (control group); constant light 24 hours a day with no dark (L24:D0 group); 16-hour daily photoperiod with 8 hours of dark (L16:D8 group); and 9-hour daily photoperiod with 15 hours of dark (L9:D15 group). The intervals from beginning of experiment to 1st ovulation of breeding season, to shedding of hair in tufts, and to appearance of a smooth coat were shorter (P less than 0.05) for L16:D8 group (107.1 +/- 11.1, 56.0 +/- 0, and 145.8 +/- 4.0 days, respectively) than for control, L24:D0, and L9:D15 groups and were shorter (P less than 0.05) for L24:D0 group (less than 156.1 +/- 12.2, 99.5 +/- 9.5, and 173.9 +/- 9.9 days, respectively) than for control group (192.1 +/- 3.3, 134.9 +/- 8.9, and 205.0 +/- 0 days, respectively) or L9:D15 group (less than 200.3 +/- 5,8, 150.6 +/- 12.9, and 201.7 +/- 3.3 days, respectively). These intervals were not significantly different between the control group and the L9:D15 group, but fewer (P less than 0.05) mares in the L9:D15 group had at least 1 ovulation by termination of the project. In experiment II, the effect of photoperiod on onset of anestrus was studied in 3 groups of 7 mares each. Mares in group A, as part of a previous experiment, were induced to enter the breeding season earlier than normal by a gradual increase in daily photoperiod beginning on Oct 13, 1972. From Feb 16, 1973, to June 22, 1973, group A mares were maintained at a fixed daily photoperiod of 15 hours 23 minutes. Mares in group B, as part of a previous experiment, were kept under environmental conditions simulating normal conditions in southern Wisconsin. On June 22, 1973 (beginning of the present experiment), the following treatments began: groups A and B were exposed to natural day length. In addition, 7 mares (group C) were allotted from a band of mares that had been exposed to natural day length and were exposed to 15-hour 23-minute daily photoperiod from the beginning of the present experiment (June 22, 1973) to the end (June 22, 1974). The interval to onset of anestrus was longer (P less than 0.05) for group C mares (234.6 +/- 35 days) than for group B mares (133.6 +/- 16.5 days). Significant difference did not exist between group A (144.0 +/- 45.9 days) and group B. A fixed daily photoperiod of 16 or 24 hours induced early onset of the breeding season and early shedding of hair, with development of a smooth coat. A photoperiod of 9 hours retarded the onset of the breeding season. Mares induced to begin the breeding season earlier than normal did not become anestrous earlier than normal. Mares kept on a long daily photoperiod in the fall became anestrous later than normal.     

Effect of probenecid administration on cephapirin pharmacokinetics and concentrations in mares

Cephapirin (20 mg/kg of body weight, IV) was administered before and after 3 doses of probenecid (25, 50, or 75 mg/kg, intragastrically, at 12-hour intervals) to 2 mares. Clearance and apparent volume of distribution, based on area under the curve, were negatively correlated with probenecid dose. Clearance of cephapirin was decreased by approximately 50% by administration of 50 mg of probenecid/kg. Serum, synovial fluid, peritoneal fluid, CSF, urinary, and endometrial concentrations of cephapirin were determined after 5 doses of cephapirin (20 mg/kg, IM, at 12-hour intervals) without and with concurrently administered probenecid (50 mg/kg, intragastrically) to 6 mares, including the 2 mares given cephapirin, IV. Highest mean serum cephapirin concentrations were 16.1 +/- 2.16 micrograms/ml at 0.5 hour after the 5th cephapirin dose [postinjection (initial) hour (PIH) 48.5] in mares not given probenecid and 23.7 +/- 1.30 micrograms/ml at 1.5 hours after the 5th cephapirin dose (PIH 49.5) in mares given probenecid. Mean peak peritoneal fluid and synovial fluid cephapirin concentrations were 6.2 +/- 0.57 micrograms/ml and 6.6 +/- 0.58 micrograms/ml, respectively, without probenecid administration and 12.3 +/- 0.46 micrograms/ml and 10 +/- 0.78 micrograms/ml, respectively, with concurrent probenecid administration. Mean trough cephapirin concentrations for peritoneal and synovial fluids in mares given probenecid were 2 to 3 times higher than trough concentrations in mares not given probenecid. Overall mean cephapirin concentrations were significantly higher for serum, peritoneal fluid, synovial fluid, and endometrium when probenecid was administered concurrently with cephapirin (P less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of food deprivation on D-xylose absorption test results in mares

A D-xylose absorption test was conducted on 4 healthy mares deprived of food for 12, 36, 72, and 96 hours before the test, with a 13- to 15-day adjustment period between each test. Maximal plasma concentrations after 72 and 96 hours of food deprivation were approximately 36% lower than those obtained after the 12- and 36-hour periods (P = 0.0001). Absorption curves were flatter and the decrease in plasma concentration was slower after the 72- and 96-hour periods of food deprivation. The rate of D-xylose absorption (P = 0.0108) and the initial rate of urinary excretion (P = 0.0117) were slower at 72 and 96 hours. Gastric emptying appeared to be progressively delayed with food deprivation, as evident by the delay in peak D-xylose excretion in urine (P = 0.0268). Areas under the plasma concentration-time curves and quantitites of D-xylose excreted in urine were similar for all periods of food deprivation, evidence that the same amounts of D-xylose were absorbed, despite changes in the plasma curve. A 15-hour collection period was sufficient to recover all D-xylose excreted in the urine, and during all periods 9.8 +/- 0.6% (mean +/- SEM) of the oral dose was eliminated in the urine.