Nothing is more frustrating than dealing with a “problem mare” when it comes to getting that particular mare in foal. There is a wide range of causes that can underlie subfertility/infertility and management of a problem mare to maximize reproductive performance varies from case to case. There are obvious conditions that can lead to subfertility such as age, poor vaginal conformation and breeding induced endometritis. A decline in fertility with age is a natural process in mares. Oocytes (i.e. eggs) are aging which results in a higher incidence of early embryonic death. Unfortunately there is no treatment available for this phenomenon. The uterus as well undergoes age-dependent degenerative processes, the degree of which can be determined through obtaining a uterine biopsy. The so called Kenney and Doig scoring system provides an estimation of the likelihood of a mare to carry a pregnancy to term. A grade IIb for example correlated to a 10-50% chance of maintaining a pregnancy until term. A Grade IIII correlates to less than 10% chance of maintaining a pregnancy until term. If offspring is desired from a mare that with advanced uterine degeneration, carrying out an embryo transfer (ET) should be considered. In this case, the embryo will be removed from the hostile uterine environment and transferred to a younger recipient mare. Poor vaginal conformation is another obvious cause of subfertility. Poor vaginal conformation often time leads to ascending uterine infections that can be diagnosed through a uterine culture and cytology. Management comprises the correction of the poor conformation (either through a Caslick’s surgery or a more advanced reconstructive surgery) along with a treatment of the uterine infection. Breeding induced endometritis is a condition occurring in young and in old mares. Once a mare is diagnosed as “susceptible”, this condition can be managed to maximize the chances of this mare to get pregnant. Management includes the administration of an ecbolic agent such as oxytocin as early as 4 to 6 hours after artificial insemination and uterine lavages if required.

In a small number of mares, anovulatory follicles can occur repeatedly (either in the form of an anovulatory follicle that luteinizes, or in the form of a persistent follicle that does not luteinize). A mare with a known history of anovulatory follicles that luteinize may be managed through the administration of prostaglandins during estrus until ovulation occurs. In some cases this can prevent the occurrence of an anovulatory, luteinizing follicle. Anovulatory follicles that do not luteinize can persist for weeks and interfere with breeding management. As long as this anovulatory follicle is present, the mare will not come back in heat. These persistent follicles can be managed through transvaginal ultrasound-guided aspiration of the follicular fluid. This will result in luteinization of the follicle and the mare can then be short cycled so that she comes back into heat and can bred.

Less obvious conditions that can lead to subfertility are blocked oviducts, impatent cervix, and karyotype abnormalities. Recent studies indicate that mares can suffer from blocked oviducts. This condition cannot be diagnosed directly but indirect evidence suggests that blocked oviducts can occur in mares. A typical clinical picture would be a mare that has no obvious reasons for infertility but does not get pregnant despite on time breeding with semen of good fertility. After the laparoscopic administration of PGE2 gel onto the utero-tubal junction in 15 mares presenting with this clinical picture, 14 mares became pregnant (PGE2 relaxes the utero-tubal junction, providing indirect evidence that the latter were blocked). Lacerations of the cervix occurring during parturition can go by unnoticed and lead to an impatent cervix, i.e. a cervix that does not close properly. This differential diagnosis should be included when carrying out a breeding soundness exam. The intactness of the cervix and whether the cervix closes properly should be evaluated during diestrus, i.e. when levels of progesterone are high (the cervix is relaxed and open when a mare is in heat and is closed when a mare is in diestrus). When upon manual evaluation of the cervix the concern is raised that the latter does not close properly, enrolling the mare in an embryo transfer program should be considered. If extensive cervical lacerations are present that lead to uterine infections prognosis for future fertility is poor.

Karyotype abnormalities can lead to early embryonic loss and early fetal loss. Mares experiencing repeated early fetal loss should be evaluated for karyotype abnormalities, i.e. for an abnormal count of chromosomes. On rare occasions mares suffering from repeated embryonic/fetal loss suffer from karyotype abnormalities themselves. Unfortunately there are no management options for these mares and a mare affected from a karyotype abnormality cannot be used as broodmare. Some cases of early embryonic death are attributable to karyotype abnormalities of the embryo itself.

Assisted reproductive technologies can be used to manage a problem mare that cannot get in foal through routine breeding management. A mare suffering from a persistent uterine infection due to Pseudomona aeruginosa or fungi for example, that cannot be resolved through appropriate treatment, can be managed through oocyte aspiration and subsequent transfer of the oocyte into the oviduct of a synchronized (so called “gamete intrafallopian transfer”). Embryo transfer is the most well established assisted reproductive technique that is most commonly used for performance horses that are in competition. Embryo transfer can be used to manage problem mares such as aged mares that suffer from advanced uterine degeneration. In this case the embryo is removed from a uterine environment that interferes with carrying a pregnancy to term and transferred to a uterine environment of a younger recipient mare. A mare being a carrier for hyperkalemic periodic paralysis (HYPP) can be considered a problem mare as she can produce a foal affected by HYPP. If this of concern the embryo can be flushed, a biopsy can be obtained from the embryo following cryopreservation of the embryo. The embryo can then be transferred to a recipient mare pending the genetic testing result of the embryo biopsy.


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