Transitional Spring Estrus in Mares: Diagnosis and Treatment

Transitional spring estrus in mares refers to the period between winter anestrus and the onset of regular ovulatory cycles as daylight increases in spring.

During this transitional phase, mares often exhibit irregular and prolonged signs of estrus without consistent ovulation. Their ovaries produce large, developing follicles that may regress without releasing an egg, leading to unpredictable reproductive behavior.

Pressure to breed mares early in the year, before the onset of their natural breeding season, can pose problems for the veterinarian. Because of considerable variation in the duration of estrus during the transitional period, efficient handling of the mare can be difficult.

Shortly after the winter solstice, changes in the pineal/hypothalamic-pituitary axis result in some follicular growth; however, follicles remain small, do not ovulate, and regress. Eventually, after a variable transitional period of up to 2 months, larger follicles (>35 mm) will develop and ovulate, usually marking the onset of normal cyclical ovarian activity.

Diagnosis

The diagnosis is by thorough ultrasonographic examination and rectal palpation, which can diagnose transitional follicles reaching a preovulatory size of >30 mm.

Visual identification of a corpus luteum or progesterone levels above 4 ng/ml indicate that the first ovulation has occurred and the onset of normal ovarian cyclical activity.

Treatment

The treatment of mares in the transitional stage is based on progesterone or progestogens, with or without the addition of estradiol esters. Progesterone can be administered as an oil-based intramuscular injection, orally the synthetic progestogen altrenogest or by using a silastic progesterone releasing intravaginal device.

Progesterone exerts a negative feedback on gonadotrophin secretion which is followed by an decreased release of FSH and luteinising hormone (LH). When the source of progesterone is withdrawn or its effect wanes, because of the withdrawal of the negative feedback effect there is follicular growth, maturation and ovulation.

Progesterone treatment is more effective in mares that are in late transitional stage and is ineffective in mares with minimal follicular activity, particularly during deep anestrus.

Currently, the most effective treatment is the use of in-feed medication with the potent progestogen altrenogest (Equine Regumate):

• This liquid contains 2.2 mg/ml of the active substance and should be added to the food once per day at a dose rate of 0.044 mg/kg body weight for 10 consecutive days.
• Estrus occurs within 6 days and ovulation between 7 and 13 days after the last treatment.
• Because of the possibility of ovulation occurring during treatment, an injection of PGF2α on the last day of in-feed medication may be necessary to cause luteolysis of any corpus luteum that may be present.

The use of intramuscular injections of progesterone and estradiol-17β in oil for 10 days produces a similar response to altrenogest, but the interval to estrus is longer due to the suppression of follicular development by the estradiol.

The use of 0.04 mg of buserelin (Receptal) given twice daily by intramuscular injection is also quite successful. It is expensive, as treatment is necessary for atleast 1-2 weeks.

Regardless of the hormones used, mares undergoing treatment early in the season need 16 hours of adequate light and good housing and nutrition to ensure success.

During the transitional period before the first ovulation of the year, mares demonstrate erratic estrus behaviour of varying intensity:

• The presence of multiple large follicles, possibly as large as 30 mm, makes detection of ovulation difficult by palpation alone.
• Even outside this transitional period, misinterpretation of ovulation, even by experienced clinicians, has been shown to be as high as 50%.
• Visualization of the corpus haemorrhagicum/early corpus luteum ultrasonographically when the anechoic follicle is replaced by an intensely echoic area representing the early corpus luteum is much easier.
• It is recommended that the interval between matings should not exceed 2 or 3 days, although there have been no critical studies on the survival time of sperm in the mares genital tract. It is important not to begin breeding too early or this will result in the mare being mated many times.
• The appearance of uterine edema is an indication that the follicle should ovulate within a few days.