Veterinarians see a wide variety of ovarian abnormalities when evaluating broodmare reproductive tracts. Knowing what’s benign and what’s pathogenic–causing disease or damage–can help them best manage these mares.
At the 2017 American Association of Equine Practitioners convention, held Nov. 17-21 in San Antonio, Texas, Patrick McCue, DVM, PhD, Dipl. ACT, reviewed common and uncommon ovarian issues that veterinarians might encounter when performing transrectal ultrasound exams. McCue is the Iron Rose Ranch Chair in Equine Reproduction at Colorado State University’s Equine Reproduction Laboratory.
Common Ovarian Problems
Anovulatory follicles “Approximately 4% to 8% of large dominant follicles (those faster-growing that are next-up for ovulating) in a mare in estrus fail to ovulate,” said McCue. Of these anovulatory follicles, about 85% luteinize (transform into a structure that produces progesterone–the pregnancy-maintaining hormone) and 15% remain as persistent anovulatory follicles.
“Persistent anovulatory follicles are often slow to regress and may remain for weeks,” said McCue. Affected mares go out of heat, and the follicles eventually get replaced.
With luteinized anovulatory follicles, veterinarians can administer the hormone prostaglandin to cause them to regress.
Persistent corpus luteum If a mare does not become pregnant after ovulation, her corpus luteum (ovarian follicle after ovum discharge) should disappear after about two weeks. “A corpus luteum that fails to regress within 14 to 16 days … is considered to be pathologically persistent,” said McCue. Causes can include embryonic loss, chronic uterine infection, and late diestrus (not in heat) ovulation. A single dose of prostaglandins will cause the corpus luteum to regress and allow the mare an opportunity to come back into heat.
Premature luteolysis The most common cause of premature destruction of the corpus luteum that forms after ovulation is bacterial endometritis (inflammation of the uterine lining), said McCue. Veterinarians can perform uterine culture and cytology (examining the sample microscopically) to confirm infection and treat it appropriately.
Parovarian cysts These fluid-filled structures, also known as fimbrial cysts, near the ovary most often occur in middle-aged (10 to 15 years) to older (over 15) mares, said McCue, but do not typically affect fertility. He said veterinarians should learn to recognize a large parovarian cyst and not confuse it with a pre-ovulatory follicle.
Less-Common Ovarian Problems
Ovarian tumors The most common ovarian tumor in mares is the often-benign granulosa cell tumor, which can cause behavior issues and regression of the contralateral ovary (the one opposite it). Other ovarian tumors, such as cystadenomas, teratomas, and dysgerminomas, are very rare and don’t cause behavior problems, says McCue.
Ovarian hematoma “The term ovarian hematoma refers to excessive post-ovulation hemorrhage resulting in ovarian enlargement,” he said. Affected mares can still become pregnant.
Ovarian senescence Mares 20 years of age and older can experience ovarian dysfunction (lack of follicular development and ovarian shutdown) resulting in reduced fertility. Once mares reach this point there’s no treatment that will improve fertility, said McCue.
Postpartum anestrus Most mares develop follicles and ovulate on what is called foal heat. “However, some mares may exhibit a temporary failure of follicular development or fail to ovulate after foaling,” said McCue, adding that this anovulatory period can last weeks or months.
“Mares that foal early in the year (i.e., winter) are more prone to exhibit postpartum anestrus than mares foaling later in the spring or summer,” he said.
Exogenous hormone therapy The administration of hormones such as anabolic steroids (potentially used to increase weight or muscle mass or to improve recovery from injury) can inhibit ovarian follicular activity. The mare will eventually return to cyclicity once the hormone therapy effects wear off.
GnRH vaccination Immunizing mares against gonadotropin-releasing hormone (used to suppress ovulation, usually for behavior reasons) can reduce their follicular activity, suppress estrus, and lower fertility. “In most instances the effect is reversible over time … and vaccinated mares eventually resume ovarian function and return to normal fertility,” said McCue, noting that in rare instances mares might fail to cycle for several years.
Luteal insufficiency With this condition, the corpus luteum doesn’t produce sufficient progesterone to support the pregnancy, likely due to the mare’s body not recognizing the pregnancy, said McCue. When this occurs, veterinarians can administer supplemental progestins (synthetic hormones) to maintain the pregnancy.
Epithelial inclusion cysts These cysts develop within the ovulation fossa (the concave side of the ovary). They’re more common in older mares and can affect fertility if they obstruct the fossa, said McCue. However, they usually don’t warrant treatment.
Chromosomal abnormalities Affected mares typically have small, inactive ovaries that rarely, if ever, cycle, said McCue.
Wrapping up, he said, “transrectal ultrasonography can be used to diagnose a majority of ovarian issues and differentiate between normal and abnormal conditions of the ovary.”