Persistent Luteal Phase in the Mare
History. You have been called to examine a mare that foaled this spring but was not bred at the foal “heat” because of a retained placenta. It has been 40 days since the foal “heat,” and the owner wants to know why the mare has not returned to estrus.
Clinical Examination. The main clinical findings are a cervix that is found (through speculum examination) to be relatively small and tightly constricted and (through palpation per rectum) to have considerable tone. Rectal palpation also reveals a uterus that has considerable tone. The ovaries are normal in size; in fact, one ovary has a 35-mm follicle. This prompts you to ask the owner whether the mare has been vigorously teased by a stallion for the detection of estrus. The owner brings the teasing stallion to the mare to demonstrate the farms teasing technique, and as predicted, the mare vigorously rejects the stallion.
Comment. A history of a mare that has been previously in estrus and has not returned to estrus within 30 days usually indicates the presence of a persistent corpus Iuteum (CL). The CL persists because of inadequate PGF2o synthesis and release, which normally occurs approximately 14 days after ovulation and causes regression of the CL in the absence of pregnancy. The incidence of the syndrome may be as high as 15% to 20%. The CL can remain active for as long as 3 months before the mare is able to synthesize and release PGF2α in amounts sufficient to cause regression of the CL. It is difficult to palpate a persistent CL per rectum because it tends to shrink into the interior of the ovary. The structure may be visualized by ultrasonography, but this is not always possible. The appearance of the cervix and the tone of the cervix and uterus suggest that the genital tubular system is under the influence of progesterone; these findings, together with the history, support a tentative diagnosis.
A tentative diagnosis can also be made if the mare returns to estrus within a few days after the administration of PGF2α. A definitive diagnosis can be made by progesterone analysis of blood; values are often 1 to 2 ng/mL in this syndrome, versus 3 ng/mL or more in mares with normal- estrous-cycle CLs. /Xdditional supportive diagnostics would be to repeat the palpation and ultrasound examination in several days. If the mare maintains her uterine tone, does not have edema, and maintains cervical tone, these findings would also support a persistent CL.The clinical finding that can be confusing in this syndrome is the presence of a large follicle in the absence of estrus. Ovarian follicles develop in this syndrome, and sometimes ovulation even occurs. However, mares do not show sexual receptivity in the presence of large follicles if luteal-phase concentrations of progesterone are present. Additionally, they do not develop marked uterine edema or cervical relaxation if progesterone is still present. One possibility that should be considered in a differential diagnosis is that ovarian activity has stopped (i.e., the mare has become anestrous). Although this does not occur often in foaling mares, mares that foal early can be adversely affected by the relatively short photoperiod that is present. In this case the clinical signs do not support the diagnosis of anestrus.
Treatment. The administration of PGF2α (or one of its analogues) usually initiates regression of the persistent CL and results in the appearance of estrus within 5 to 7 days. The early return to estrus is based on the fact that ovarian follicles tend to develop on a continuous basis throughout the persistent IuteaLphase syndrome. Regression of the CL allows the current dominant follicle to continue to develop and produce estrogen, which brings the mare into estrus. One caveat: If a large follicle (e.g., 40-45 mm) is present at treatment, the follicle may ovulate before the mare manifests estrus, and the treatment will be judged as failing. In this case the animal needs to be monitored daily; if ovulation occurs within a few days of treatment, the animal may need to be inseminated artificially if breed rules allow.