Date Published: July 26, 2006
Publisher: BioMed Central
Author(s): Ylva Hedberg, Anne-Marie Dalin, Malin Santesson, Hans Kindahl.
Strong oestrous symptoms in the mare can cause problems with racing, training and handling. Since long-acting progesterone treatment is not permitted in mares at competition (e.g. according to FEI rules), there is a need for methods to suppress unwanted cyclicity. Spontaneous dioestrous ovulations in the late luteal phase may cause a prolongation of the luteal phase in mares.
In this preliminary study, in an attempt to induce ovulation during the luteal phase, human chorionic gonadotropin (hCG) (3000 IU) was injected intramuscularly in four mares (experimental group) in the luteal phase when a dioestrous follicle ≥ 30 mm was detected. A fifth mare included in this group was not treated due to no detectable dioestrous follicles ≥ 30 mm. Four control mares were similarly injected with saline. The mares were followed with ultrasound for 72 hours post injection or until ovulation. Blood samples for progesterone analysis were obtained twice weekly for one month and thereafter once weekly for another two to four months.
Three of the hCG-treated mares ovulated within 72 hours after treatment and developed prolonged luteal phases of 58, 68 and 82 days respectively. One treated mare never ovulated after the hCG injection and progesterone levels fell below 3 nmol/l nine days post treatment. Progesterone levels in the control mares were below 3 nmol/l within nine days after saline injection, except for one mare, which developed a spontaneously prolonged luteal phase of 72 days.
HCG treatment may be a method to induce prolonged luteal phases in the mare provided there is a dioestrous follicle ≥ 30 mm that ovulates post-treatment. However, the method needs to be tested on a larger number of mares to be able to draw conclusions regarding its effectiveness.
According to many mare owners, strong oestrous symptoms result in difficulties with training and handling of the animal [1,2]. One method used earlier was to treat such mares with long-acting progesterone [3,4]. However, this treatment is not permitted for use in racing or competition mares according to many European racing authorities, such as the British Jockey Club  and the Swedish Trotting Association (STC) , and also according to the rules of the International Federation for Equestrian Sports (FEI) . In addition, in Sweden, short-acting progesterone treatment, such as altrenogest orally, is prohibited for use at competition in riding horses  and has a recommended drug withdrawal time of 14 days in Standardbred racehorses (personal communication, Peter Kallings). Another method, the crushing of embryos to achieve a prolonged progesterone phase, has recently been presented ; also, the insertion of glass or plastic balls into the uterus has been used to prolong the luteal phase in mares [10,11]. However, some consider these methods to be improper and the use of intrauterine balls is not permitted in Sweden in racing horses . Therefore there is a need for alternative techniques. Mares, in contrast to females of other mammal species, have the ability to ovulate during dioestrus, when serum concentrations of progesterone are high. The frequency of dioestrous ovulations varies according to breed; in Thoroughbreds and Quarter Horses as many as 20 % of ovulations occur during the luteal phase , whereas the frequency of dioestrous ovulations is very low in ponies . The follicles that grow and ovulate during dioestrus do not cause oestrous signs, due to the suppressive effects of progesterone on oestrous behaviour . Spontaneous dioestrous ovulations that occur between one to four days prior to luteolysis may cause a prolonged luteal phase, due to the immature corpus luteum being refractory to prostaglandins, as shown by exogenous prostaglandin treatment . A refractory state to endogenous prostaglandin for up to four days after ovulation was also demonstrated by the use of intrauterine saline infusion [15,16]. The normal mean dioestrous length, derived from several different reports, is 14.9 days (range 12.1 to 16.3 days)  and in one study, prolonged luteal phases lasted for a mean of 63 days (range 35–96 days) . Daels et al.  demonstrated that ovulation induced by human chorionic gonadotropin (hCG) during altrenogest treatment could lead to a prolonged luteal phase (5/6 of the mares that ovulated). The present study was designed to evaluate if hCG-induced ovulation during the late luteal phase in mares will cause a prolonged luteal phase. Both the method of Daels et al.  and the method used in the present study would eliminate oestrous behaviour for a period of time due to high endogenous progesterone levels. However, an advantage with the method presented in this paper, as compared to the method of Daels et al. , is that exogenous altrenogest treatment is not used.
The main results are summarised in Table 1. Three of nine mares developed follicles that were above 30 mm (and thus could be treated) in the first luteal phase examined. The remaining mares developed large enough follicles in the second (n = 4), third (n = 1) or in none (n = 1) of the luteal phases studied. Depending on if a mare was treated in the first, second or third dioestrous period after the start of the experiment (due to follicle size), mares were followed for between 90 to 160 days after treatment. Three of the four hCG-treated mares ovulated, resulting in luteal phases, as defined in materials and methods, lasting for 58, 68 and 82 days post-treatment respectively (Table 1). Figure 1 shows their post-treatment progesterone concentrations. These mares were treated between eight and ten days after ovulation was confirmed by ultrasound (every other day) or after last detected oestrous signs (mare Ev), which means their actual luteal phases lasted for at least 69, 77 and 90 days. Two of the hCG-treated mares (mares A and Ev) were never moved to pasture and were therefore teased daily for symptoms of oestrus and mare A examined with ultrasound. The mares showed oestrous signs at days 62 (mare Ev) and 88 (mare A) post treatment and their progesterone levels were then below 3 nmol/l. Mare A had two additional luteal phase ovulations which were associated with two peaks in her progesterone levels (see Fig. 1). The third mare (mare Ex) did not show any symptoms of oestrus during the time she was still kept at the department (37 days). One additional mare in the experimental group (mare O) never developed a dioestrous follicle above 30 mm in any of the three luteal phases studied and was thus never treated. Another mare in this group (mare T) injected with hCG in the third luteal phase studied did not ovulate as a result, although she had a follicle above 30 mm at treatment. This mare showed symptoms of oestrus eight days after the injection had been given. The progesterone level was then low (< 3 nmol/l). In the present study, the fact that immature corpora lutea do not respond to the luteolytic effect of endogenous prostaglandin was utilised in an attempt to produce a prolonged luteal phase [15,16,22]. The mares in the hCG treatment group that developed prolonged luteal phases ovulated between days ten and 13 in dioestrus. Thus, the corpora lutea were a maximum of four days old, and presumably refractory, at the time of endogenous prostaglandin release which occurs around days 14 to 17 of dioestrus [22,23]. Failure of the endometrium to release prostaglandin at the time of luteolysis (for example due to chronic endometritis, maternal recognition of pregnancy or for idiopathic reasons) can also cause a prolonged luteal phase in the mare [23-27]. It would have been of interest to measure prostaglandin metabolite levels to rule out that impaired prostaglandin release was not a cause of the prolonged luteal phases. Unfortunately, this was for practical reasons not possible in the present study. Although all of the mares had shown normal cycle lengths prior to the study, one mare (mare Me) developed a prolonged luteal phase in the first cycle studied and also, in the subsequent, saline treated, cycle. As a method of inducing prolonged luteal phase in the mare, the results of this preliminary study may suggest the use of hCG in the late luteal phase, providing a follicle greater than 30 mm is present and ovulates as a result of the treatment. The method does not involve the use of substances banned for competition purposes and could thus potentially be used for competition mares whose performance is affected by the oestrous cycle. However, to conclude that the method is effective and applicable to practice, it is necessary to repeat the study using a larger number of mares. The author(s) declare that they have no competing interests. All authors participated in the design of the study. YH, MS and AMD examined the mares and participated in sample collection. MS analysed the blood samples. YH and MS drafted the manuscript. AMD and HK conceived of the study, participated in its coordination and helped to draft the manuscript. All authors read and approved the final manuscript. Source: http://doi.org/10.1186/1751-0147-48-12