ISRAEL JOURNAL OF
VETERINARY MEDICINE
VOLUME 54 (2), 1999
ESTROGEN AND PROGESTERONE LEVELS IN PURE BRED
ARABIAN HORSES DURING PREGNANCY
M. E. Naber1, M. Shemesh2, L. S. Shore2 and C. Rios21.
Ministry of Agriculture, Division of Veterinary Laboratories, Unit of Biochemistry,
Amman, Jordan
2. Dept. of Hormone Research, Kimron Veterinary Institute, Bet Dagan, POB 12, Israel
Summary
Hormonal profiles of progesterone and estrogen were determined in 45 pregnant Arabian mares of
45 to 200 days of gestation. Approximately 30% of the mares had lower progesterone concentrations
(1.0 to 1.9 ng/ml; n=9 for the low group vs 2.3-18.9 ng/ml; n=24 for the normal group). Mares with
low progesterone concentrations also had low estrogen concentrations (727±102 pg/ml; vs 1104±120
pg/ml; P<0.01). All the mares of gestational age older than 120 days, had progesterone levels greater
than 6.0 ng (6.0-16 ng/ml; n=9) and estrogen levels greater than 500 pg/ml (546-2015 pg/ml).
Introduction
The reproductive endocrinology of the horse differs radically from those of the ruminant,
canine and feline species. In early pregnancy (10-30 days) there is a rise in progesterone due
to the activity of the corpus luteum (1,2). This rise in progesterone is followed by a short
period of decline before the ”rescue” of the corpus luteum and formation of accessory
corpora lutea by PMSG. PMSG is usually detectable from 45 to 150 days of pregnancy and
with its cessation progesterone values again begin to decline till the pre-parturient rise.
Peripheral serum estrogen levels begin to rise around 45 days of pregnancy. This rise in
estrogen during pregnancy originates from the fetal placental unit.
Due to the large variation among various breeds of horses as well as difference in laboratory
techniques, it is necessary to determine the normal values for each breed. The present study
was conducted (a) to determine the hormonal profile of progesterone and estrogen
(radioimmunoassay) in 45 pure-bred Arabian mares at various stages of gestation, and (b) to
determine if insufficient progesterone in early pregnancy could be responsible for some of
the losses seen in these mares.
Materials and Methods
Animals: Sera were taken from 45 mares at various stages of gestation (45-55 days, N=13;
60-70 days, N=8; 70-80 days, N=8; 90-120 days, N=7; 140-200 days, N=9) from pure bred
Arabian mares located at the Royal Stables, Amman, Jordan.
Pregnancy determination: Pregnancy was confirmed either by PMSG (utero-ovarian
response in the immature mouse bioassay (3) or ultrasound examination.
Hormone determinations: Estrogen and progesterone were measured by
radioimmunoassay (4,5) . Sera (0.5 ml) were first diluted with 3 volumes of 0.1 M sodium
acetate buffer, pH 5.0, and extracted on C-18 extract colunms. The columns were eluted
with 2 ml 100% methanol, evaporated to dryness and re-dissolved in 0.5 ml assay buffer. 50
µl aliquots were used for the assays and all determinations were done in duplicate.
Results
Progesterone
Peripheral progesterone values increased gradually from 4-6 µg/ml at 45-70 days gestation to
about 8-10 ng/ml at 200 days of gestation (Fig. 1). However in all of the mares of 140 to 200
days of gestation, progesterone values were greaer than 6.0 ng/ml (6.6 to 16.0 ng/ml; n=9).
Peripheral serum progesterone values showed great variation from 45 to 120 days
gestation.
There appeared to be three groups of progesterone concentration. Low levels
from 1.0 to 1.9 (n=9); middle range of 2.3-10.5 ng/ml (n=22) and high levels of
12.5-18.9 ng/ml (n=5) (Fig. 2). These data would indicate that 30% of the
mares were progesterone deficient in early pregnancy.
Figure 1. Peripheral sera progesterone concentrations in mares
at 45-55 days (N=13); 60-70 days (N=8); 70-80 days (N=8); 90
to 120 day (N=7) and 140 to 200 days (N=9) of gestation.
Figure 2. Progesterone concentrations in pregnant mares of 45 to 120 days of gestation. Progesterone concentrations were
classified in three groups: Low = < 2.0 ng/ml; middle range >2.0 but <11.0 ng/ml; and high >12.5 ng/ml.
Estrogen
Peripheral serum estrogen values did not show any dramatic changes during the period of study
(45 to 200 days). The estrogen values were reflective of the progesterone concentrations,
particularly in early pregnancy (45-140 days). Mares with serum progesterone of less than 2.0
ng/ml had significantly lower estrogen concentrations than mares with progesterone above 2.0
ng/ml (727±102; n=9 vs 1104±120;n=24; P<0.01). Nevertheless, three mares (45, 96, 98 days
of gestation) had very low estrogen values (<200 pg/ml) even though they had high
progesterone (<5 ng/ml). Mares of gestational age of 140-200 days of age had estrogen
concentrations ranging from 546-2015 pg/ml; n=9.
In two mares which miscarried, estrogen was still high (1141, 1435 pg/ml) three weeks after the
miscarriage even though the progesterone levels were below 1.0 ng/ml.
Discussion
Peripheral progesterone and estrogen values can provide information on managing the
pregnancy. The values reported here agree with those in the literature except that for saddle
mares it was reported that progesterone begins to drop at 150 to 200 days (8), while in our
Arabian mares and in pony mares (9,10) progesterone concentrations remained elevated over
the 140-200 day period.
Low progesterone can be used to identify mares with progesterone insufficiency. About 30%
of the mares in this study had low progesterone (<2.0 ng/ml) in early pregnancy (up to 120 days
of gestation). Progesterone deficiency in mares is usually associated with abortion from 45 days
to 8 months of gestation (6). Although supplementation with progesterone during gestation is
not thought to be efficacious, these mares should receive special attention, i.e., stress reduction.
Since progesterone deficient mares continue to have the syndrome in subsequent pregnancies,
these mares should be supplemented with progesterone early in gestation (Altrenogest either
orally or intramuscularly). In cattle, progesterone supplementation four days after insemination
was found to improve fertility (7).
As opposed to progesterone which reflects corpus luteum activity, low estrogen values are an
indication that the fetal-placental unit is compromised. This may well be the case in the three
mares identified with high progesterone and low estrogen levels.
Estrogen concentrations (<500 pg/ml) can remain high several weeks after miscarriage. Since
high levels of estrogen may interfere with implantation, it seems logical to wait until estrogen is
below 300 pg/ml before attempting re-insemination. Hormone values are of use also in the nonpregnant mare. High testosterone (< 200 pg/ml) is a sign of granulosa tumor while high
estrogen (<0.5 ng/ml) could be an indication of tumors or ovarian cysts. In any event, the
presence of the tumor or cyst should be confirmed manually or by ultrasound.
The present report indicates that progesterone profiles in pregnant mares can be used as a tool
to improve fertility in Arabian mares. This could be done by providing progesterone to deficient
mares in early stages of pregnancy.
Acknowledgment
We would like to thank Her Royal Highness, Princess Alyia, for allowing the use of sera from
her mares at the Royal Stables in Amman, Jordan and to Dr. A. Rawashdeh, Veterinarian,
Royal Stables. We would also like to thank Dr. S. Marcus, Kimron Veterinary Institute for his
assistance in performing the bioassays. We would also like to acknowledge the support of
Mashav - Center for International Cooperation, Israeli Ministry of Foreign Affairs for Dr.
M.E. Naber.
References
1.Nett T.M.and Pickett B.W. (1979). Effect of diethylstilbestrol on the relationship between
LH, PMSG and progesterone during pregnancy in the mare. J. Reprod. Fert., Suppl 27: 465470.
2. Terqui M. and Palmer E. (1979). Oestrogen pattern during early pregnancy in the mare. J.
Reprod. Fert. Suppl. 27: 441-446.
3.Walker D. (1977). Laboratory methods of equine pregnancy diagnosis. Vet. Rec. 100: 396399.
4.Shore, L.S. and Shemesh, M. (1981). Altered steroidogenesis by the fetal bovine freemartin
ovary. J. Reprod. Fertil. 63: 309-314 .
5. Shemesh, M., Ayalon, N. and Mazor, T. (1979). Early pregnancy diagnosis in the ewe,
based on milk progesterone levels. J. Reprod. Fertil. 56: 301-304.
6. Roberts S.J. (1971). Veterinary Obstetrics and Genital Diseases. Published by
Author:Ithaca, NY.
7. Shemesh, M., Ayalon, N., Marcus, S., Danielli, Y., Shore, L. and Lavi, S. (1981).
Improvement of early pregnancy diagnosis based on milk progesterone by use of progestinimpregnated sponges. Theriogenology 15: 459-462.
8. Holtan, D.W., Nett, T.M. and Estergreeen V.L., (1975). Plasma progestagens in pregnant
mare. J. Reprod. Fert., Suppl. 23:419-424.
9. Squires, E. L., Wentworth, B.C. and Ginther, O.J. (1975). Progesterone concentration in
blood of mares during the estrous cycle, pregnancy and after hysterectomy. J. Anim. Sci.
39:759-767.
10. Squires, E.L. and Ginther, O.J. (1975). Collection technique and progesterone
concentration of ovarian and uterine venous blood in mares. J. Anim. Sci. 40:275-281.