It is a frustrating condition. Even though a mare is not pregnant, her body tells her that she is, and all of her reproductive systems react accordingly. She does not come into heat, and her body begins preparations for housing and nourishing a fetus that doesn't exist. All the while, valuable reproduction time is being wasted. This is called false pregnancy.
There are a number of other terms that are used to describe false pregnancy in the mare. They include prolonged luteal activity, persistence of the corpus luteum, prolonged luteal phase, prolonged diestrus, pseudopregnancy, pseudocyesis, and spurious pregnancy. Whatever the term used, the condition is the same. The mare's body has been given a signal that a pregnancy exists when it does not.
Dealing with a false pregnancy was more difficult in the past than it is today. The arrival on the equine medical scene of the diagnostic ultrasound has made the practitioner's work much easier.
There is more good news. The administration of the appropriate drug therapy usually brings about a successful resolution.
To understand how the mare's body receives the wrong signal, we must first understand what occurs when a normal pregnancy is achieved and maintained.
As the mare's brain records increased light and higher temperatures with the advent of spring and summer, the hypothalamus gland, located within tissues of the mid-brain, is stimulated. It signals the start of the reproductive system by producing a gonadotropic releasing hormone (GnRH). When GnRH is secreted in the proper quantity, the pituitary gland, located at the base of the brain, is stimulated. The pituitary then secretes two hormones that affect the ovaries. The first hormone is follicle stimulating hormone (FSH). It travels along the bloodstream to the ovaries, where it stimulates development of one or more follicles.
The follicles, when they reach 20 to 25 millimeters in diameter, secrete estrogen. This hormone stimulates estrual activity, causes relaxation of the cervix, stimulates contractions along the mare's reproductive tract, signals the pituitary gland to cease secretion of FSH, and, at the same time, stimulates release of the second gonadotropic hormone--luteinizing hormone (LH).
Luteinizing hormone facilitates maturation and ovulation of the growing egg-bearing follicle.
Ovulation occurs when the mature egg leaves the follicle and begins its trip through the oviduct. In the wake of ovulation, the estrogen level falls and the remains of the ovulated follicle are converted to form a corpus luteum (CL) or yellow body. The luteal cells secrete the hormone progesterone. It is the job of progesterone to shut down the estrous-stimulating hormones and to set the stage for maintaining a pregnancy. The mare is no longer receptive to the stallion.
Now let's look at what happens when a pregnancy doesn't occur or when it is terminated in the embryonic stage.
After 12 to 14 days, if pregnancy has not occurred, the uterus secretes the hormone prostaglandin, which causes regression of the corpus luteum. As the corpus luteum regresses, progesterone secretion declines. The reduction in the serum progesterone level allows the mare to return to estrus, and the cycle is repeated with the mare once again becoming receptive to the stallion.
There is, of course, another reason why the mare's reproductive system returns to a state of estrus after a pregnancy has been confirmed--early embryonic death.
Researchers have found that early embryonic death, if it is to occur, will happen in the first 35 to 40 days of gestation. It can occur as early as five to 14 days of gestation.
There are many reasons that early embryonic death can occur. One of the significant players in the game at this stage and throughout pregnancy is the hormone progesterone. Some mares simply don't produce enough progesterone to maintain a pregnancy and must have it administered on a regular basis throughout the pregnancy. However, there are a number of other causes of early embryonic death.
Often, there is no outward sign that early embryonic death has occurred. At an early stage of the reproductive process, the embryo is so small that it simply might be absorbed. Rarely would there be something as obvious as a vaginal discharge to alert the owner or practitioner that something had gone awry with the newly created conceptus.
When the pregnancy is terminated in a normal mare, there is a return to a state of estrus. All of the classic receptive signs are manifested, and she is ready for another infusion of semen. However, with the problem mare, that might not be the case. The mare's reproductive system must receive a message that there no longer is a pregnancy or that one didn't exist in the first place. The messenger that is to convey this information is the hormone prostaglandin.
In the case of false pregnancies, the messenger does not arrive because the corpus luteum either has not shut down or something else has occurred that inhibits the production of prostaglandin. Whatever the reason, there is no messenger to bring the news that a pregnancy no longer exists or never existed in the first place.
The importance of prostaglandin's role in bringing a mare into estrus when pregnancy does not occur or when it is ended rivals that of progesterone's role of helping to maintain pregnancy. We can learn about prostaglandin's important and powerful role from an expert in equine reproduction.
Michelle M. LeBlanc, DVM, Dipl. ACT, University of Florida, a frequent speaker at AAEP conventions and the author of a chapter on "Diseases of the Female System, Hormonal Therapy for Reproduction" in the equine medical textbook, The Horse, Diseases and Clinical Management, outlines the role that prostaglandin can and does play in the reproductive process.
"Prostaglandin and a variety of synthetic prostaglandin analogs are effective luteolytic agents in mares. These compounds, administered directly into the uterus or parenterally, result in rapid demise of the corpus luteum and subsequent return to estrus. In equine reproductive practice, prostaglandins are valuable therapeutic tools for terminating diestrus and for inducing early abortion.
"Prostaglandins also may be used to hasten or synchronize ovulation in mares. When 250 milligrams of feprostalene, a long-acting prostaglandin, were given 60 hours after the onset of estrus, 81% of treated mares ovulated within 48 hours versus 31% of untreated control mares.
"Indications for clinical use of prostaglandins include interrupting diestrus in spontaneously prolonged diestrus and treatment of pyometra (accumulation of pus in the uterus) accompanied by a persistent corpus luteum. Estrus can be synchronized in donor and recipient mares for embryo transfer, and pregnancy can be terminated at up to 38 days gestation.
"Prostaglandins can be given to change the distribution of estrus in groups of mares booked to the same stallion to avoid over-breeding, and can be used to shorten the interovulatory interval when breeding is passed or missed (i.e., twin ovulation, passed foal heat) or when diagnostic or therapeutic procedures require cervical relaxation.
"Prostaglandin also has a direct effect on the myometrium (the smooth muscle coat of the uterus which forms the main mass of the organ), where it induces high-amplitude contractions. Repeated administration of prostaglandins may be helpful in treating mares with uterine infections caused by decreased physical clearance of bacteria."
LeBlanc noted that the time from prostaglandin treatment to the onset of estrus and ovulation in mares varies with corpus luteum maturation and follicular development.
"The best response is seen when prostaglandins are administered between Days 6 and 9 of the estrous cycle," she explains. "Follicular status of the ovary at the time of prostaglandin treatment affects the interval to ovulation. When follicles of 40 millimeters or more in diameter are present, the time to ovulation varies from 24 hours to 10 days.
"In most cases, the interval from treatment to ovulation is less than six days. In a few instances, a follicle may ovulate within 24 to 72 hours, with the mare showing an abbreviated estrus or no estrus at all."
She notes that about one-third of the time when a large diestrous follicle is present, it does not ovulate, but regresses and is replaced by a smaller follicle.
"Less variation," she points out, "is seen in mares with smaller follicles (less than 40 millimeters in diameter) on the day of treatment. The average time between treatment and ovulation in these mares is six days.
"Variation in response to prostaglandin treatment stresses the importance of careful assessment of ovarian activity by rectal palpation at the time of injection. If large follicles are present, careful teasing and palpation should be performed daily to identify estrus and impending ovulation."
Prostaglandins available for clinical use are the natural compound and several analogs.
"The main differences in these compounds," LeBlanc tells us, "are the adverse effects produced, none of which are serious when the drugs are used at levels recommended for luteolysis. Sweating, increased gastrointestinal motility, and slight caudal ataxia (failure of muscular coordination) are the main adverse reactions seen when natural prostaglandin is used. Slight to profuse sweating is evident in most mares treated with natural prostaglandins, and usually ends in 30 to 45 minutes."
Now that we have an understanding of the role of prostaglandin in the reproductive process, it is time to take an in-depth look at what occurs or doesn't occur that results in a false pregnancy. This time our expert on the subject is Patrick J. Meyers, DVM, MS, Dipl. ACT, University of Guelph in Ontario. He prefers using the term prolonged luteal activity rather than false pregnancy.
"To understand this condition," he writes, "the following points must be considered. Luteal tissue can originate from anovulatory (not ovulating) as well as ovulatory follicles. The corpus luteum resulting from the primary ovulation during the follicular phase of the estrous cycle is present during diestrus (not in heat), pregnancy, and embryonic loss. New or accessory corpus luteums develop while under the influence of a normal (diestrus and pregnancy) or an abnormal (prolonged luteal activity) progestational state. These unique physiologic intricacies of the mare make understanding the term prolonged luteal activity difficult."
Meyers further explains that his rather long statement means that continued function of the corpus luteum to produce progesterone even though pregnancy has not occurred or has been terminated, is brought about by failure of the uterine luteolytic mechanism.
In the absence of an embryo, it is the endometrium of the uterus in the normal horse that produces the prostaglandin that becomes the messenger that informs the mare's reproductive system that something has gone awry, and it is time to start over.
The prostaglandin normally travels from its production site in the uterus to the corpus luteum and presents its message. When the message is received and heeded, the production of progesterone ceases, and the mare returns to estrus.
Let us interject here a word about what happens when a viable embryo exists in the uterus. The embryo attempts to ensure its preservation by traversing and contacting a large portion of the surface of the endometrium during the mobility phase (Days 11 to 15 post-ovulation). By so doing, it prevents the release of prostaglandin, so the corpus luteum continues to function.
"This physiologic phenomenon," Meyers explains, "may be referred to as the maternal recognition of a pregnancy. Loss of an embryo after maternal recognition of a pregnancy causes a prolongation of luteal activity because embryonic secretory products or remnants may remain in contact with the endometrium for several days after embryonic death. When early pregnancy loss could not be easily established before the widespread use of ultrasound, mares may have been diagnosed as 'pseudo-pregnant' if there appeared to be an extended period of anestrus during the ovulatory season. Any severe uterine anomalies or pathology that eliminates or upsets the uterine luteolytic mechanism may also result in prolonged luteal activity through uteropathic causes."
That being said, Meyers goes to what he considers to be the heart of the problem in a number of cases of false pregnancies--undetected ovulations during the diestrous (out of heat) period.
With ultrasound monitoring of ovaries, he tells us, "it was determined that previously undetected diestrous ovulations were responsible for prolonged luteal activity."
The occurrence of these ovulations toward the end of diestrus, he explains, probably created a corpus luteum that was too immature to respond to the prostaglandin sent its way from the uterus.
Diagnosis And Treatment
"If a mare experiences long periods of anestrus combined with irregular estrous cycles during the ovulatory season without being mated," Meyers explains, "diestrous ovulations may be suspected. Careful and constant monitoring of follicular and luteal dynamics through the use of ultrasound is the only way that diestrous ovulations will be detected. Monitoring must be performed every 24 to 48 hours.
"The results of monitoring luteal progression and dynamics through ultrasound and daily plasma progesterone determination are valuable information. These two techniques assist the practitioner in determining the optimal time for the assignment of prostaglandin treatment to induce a mare into a fertile estrus on the subsequent estrus."
Administration of the correct amount of prostaglandin, Meyers says, will "suppress follicular activity and may allow a more orderly 'restart' of the endocrinologic clock."
He concludes his discourse on diagnosis and treatment protocol this way:
"Again, ultrasound is an invaluable tool in the diagnosis of cases of prolonged luteal activity and its causation. Endometrial biopsy may be extremely helpful in the diagnosis of prolonged luteal activity associated with repeated bouts of early embryonic mortality. An endometrial biopsy is also required for ruling out the possibility of uteropathic persistence of the corpus luteum (i.e., endometrial underdevelopment, chronic low-grade endometritis). If prolonged luteal activity can be ascribed to idiopathic (of unknown causes) persistence or early embryonic death, prostaglandin should be administered to induce a return to estrus."
The role of ultrasound in the diagnostic procedure is highly important because rectal palpation of the corpus luteum often is ineffective.
In the textbook, Equine Reproduction, researchers Elaine M. Carnevale, DVM, MS; Angus O. McKinnon, BVSc, MSc, Dipl. ABVP, Dipl. ACT; Edward L. Squires, PhD; and James Voss, DVM, MS, one of the book's editors, collaborated on a chapter that discusses the use of ultrasound. They noted the following information:
"Some of the reasons for ultrasonographic evaluation of corpa lutea are to 1) detect ovulation; 2) evaluate corpus luteum formation; 3) determine size and characteristics of the corpus luteum; 4) determine if failure of a mare to display estrus is caused by prolonged maintenance of a corpus luteum or absence of a corpus luteum and follicular activity; 5) distinguish between anovulatory hemorrhagic follicles, luteinized unruptured follicles, and corpus luteum, and 6) determine if a mare has ovulated more than one follicle."
The four then zeroed in on the use of ultrasound to determine if a pseudopregnancy (false pregnancy) is being dealt with. They point out that, with ultrasound, prolonged maintenance of the corpus luteum, resulting in false pregnancy, can be differentiated from an anovulator or anestrous condition. The corpus luteum is first visible on the day of ovulation (Day 0) as a strongly echogenic (giving rise to echoes of ultrasound waves), circumscribed mass of tissue.
The echogenicity, they reported, gradually decreases throughout diestrus. However, just before regression of the corpus luteum, echogenicity increases.
The researchers explained it this way:
"Initially, the corpus luteum is highly echogenic on the day of ovulation. At this time, it is easiest to identify. The echogenicity decreases over the first six days of diestrus, remains at a minimum level for several days during the middle of diestrus, then increases over Days 12 to 16. An increase in brightness of the corpus luteum during the time of corpus luteum regression was also observed. The ultrasonographic changes are apparently indicative of changes in luteal hemodynamics and may be indicative of changes in patterns of blood flow within the corpus luteum as well as changes in tissue density.
"With experience, the practitioner can become accurate at using ultrasonography to confirm ovulation and detect the presence of a corpus luteum. Ultrasonography can also be used to diagnose pseudopregnant mares. A persistent corpus luteum and absence of an embryonic vesicle are evidence of a pseudopregnancy. Once these mares are identified, prostaglandins can be safely given to induce estrus."
Thus, we can conclude, the condition of false pregnancy, though simple on the surface, is a bit more complicated when given an in-depth examination. The good news for the horse owner is that the problem can be diagnosed with ultrasound and normally can be treated effectively with administration of prostaglandins.
The take home message for the horse owner is to make use of the technology that is available. Most major breeding farms today have veterinarians on staff or on call who routinely monitor the pregnancy progress of the broodmare band. A number of smaller breeders, however, put stallion to mare, check them through teasing to determine whether they have come into heat again, and if they do not show signs of estrus, declare the mare to be pregnant. As has been learned from the experts quoted above, this is not always the case.
The mare's reproductive system might have failed to receive the signal that no pregnancy exists and does not return to estrus. The result can be a wasted year. An ultrasound examination can prevent that from occurring, and can be a valuable tool in monitoring fetal development.
About the Author
Les Sellnow is a free-lance writer based near Riverton, Wyo. He specializes in articles on equine research, and operates a ranch where he raises horses and livestock. He has authored several fiction and non-fiction books, including Understanding Equine Lameness and Understanding The Young Horse, published by Eclipse Press and available at www.exclusivelyequine.com or by calling 800/582-5604.