Using oxytocin to prolong diestrus

It's common for mares to display different and even aberrant behaviors during estrus, including attitude changes, stubbornness, "horsing" and potentially impaired performance.

"Some mares display such profound signs of estrus that the behavior itself impairs performance," says Dirk Vanderwall, DVM, PhD, DACT, associate professor at Utah State University's Department of Animal, Dairy and Veterinary Sciences. "Even under saddle, some mares may 'break down' and show estrus in response to being around other horses or other stimuli. For these mares, estrus suppression is clearly warranted."1

Veterinarians are now looking to alternative methods, such as prolonging the functioning of the corpus luteum, to suppress estrus in mares. (GETTY IMAGES/FOCUS_ON_NATURE)

Equine practitioners have explored a variety of techniques to suppress estrus or extend the diestrus period, therefore limiting problematic behaviors and maintaining performance, especially for racing horses in training. Historically, an orally active exogenous synthetic progestin (altrenogest) has been administrated effectively to mares at a dose of 0.044 mg/kg/day to accomplish estrus suppression. Though this method is considered the gold standard for suppressing estrus in mares, the expense of supplemental progesterone, the need for daily administration and safety concerns over the use of steroids in performance horses and for personnel during handling and administration have caused veterinarians to look for alternative techniques.

A promising alternative

The most promising alternative method to suppress estrus is to prolong the functioning of the corpus luteum (CL), causing it to naturally continue to produce endogenous progesterone and accomplish the same goal without the concerns of administering the exogenous compound.

Transrectal ultrasound image of a mid-diestrus corpus luteum (arrows) in a mare. (PHOTO COURTESY OF DR. VANDERWALL)

A common method of prolonging CL function and suppressing estrus is insertion of a glass or plastic intrauterine ball. But this methodology comes with drawbacks and concerns, primarily anecdotal reports of complications with the intrauterine marbles. While Vanderwall was at the University of Pennsylvania's New Bolton Center, one mare with two marbles in her uterus was referred for treatment. The current owner was unaware of the marbles, and upon removal, the glass balls showed pitting on their surfaces from prolonged contact in the uterine lumen. Others have reported glass marbles fragmenting in the uterus.

At the 2013 Society for Theriogenology Meeting, the theriogenology group at Kansas State University described a case of pyometra in a mare that was associated with the presence of an intrauterine glass marble (this condition could occur with either glass or plastic marbles).2 "Those types of complications could horrendously affect the mare's fertility," Vanderwall says. "Although you could avoid the fragmentation of a glass marble with the use of a plastic one, that does not eliminate all the concerns of this procedure."

A corpus hemmorhagicum (i.e. new corpus luteum and an adjacent corpus albicans from the previous cycle). (PHOTO COURTESY OF DR ROB LÖFSTEDT, PROFESSOR OF REPRODUCTIVE ENDOCRINOLOGY AND THERIOGENOLOGY, ATLANTIC VETERINARY COLLEGE AT UNIVERSITY OF PRINCE EDWARD ISLAND)

With these reported complications, Vanderwall says he felt the procedure warranted serious reconsideration, and he began his search for alternatives.

Taking a new approach

Normally, in nonpregnant mares, the CL secretes progesterone for the two weeks of diestrus. Then, secretion of PGF2α causes luteolysis, which brings mares back to estrus. Developing therapeutic strategies for prolonging CL function (i.e. maintaining progesterone > 1 ng/ml) as a means of long-term estrus suppression has gained increased interest, Vanderwall says, because it obviates the need for administering exogenous progesterone or progestins to mares.

Vanderwall and his colleagues found that administering exogenous oxytocin during diestrus blocked luteolysis, thus prolonging CL function. In their initial study,3 they found that administering 60 units of oxytocin intramuscularly (IM) twice daily on days seven to 14 after ovulation effectively blocked luteolysis. In a subsequent study,4 they found that once daily administration of oxytocin was as effective as twice-daily administration, with both treatments resulting in 60 to 70 percent of the mares developing prolonged CL function.

A recent report5 from the Gluck Equine Center at the University of Kentucky described the results of a study in which IM treatment of 60 units of oxytocin was initiated on day eight and continued for two, four or six days (i.e. days eight to 10, eight to 12 and eight to 14) postovulation. The proportion of mares with prolonged CL function increased (P < 0.01) as the number of days of oxytocin administration increased, confirming the need to continue oxytocin treatment until the expected time of luteolysis on day 14 for maximum effectiveness.5 This method was shown to prolong diestrus for up to two months.

"What's unique about the hormone oxytocin is that, at the end of the estrous cycle, endogenous or exogenous oxytocin actually promotes luteolysis, or regression of the CL, to bring a mare into heat," Vanderwall says. "Oxytocin is proluteolytic at the end of diestrus, but by starting treatment at mid-diestrus, oxytocin has a different effect, essentially derailing the subsequent luteolytic process. If given on days seven to 14 postovulation, it will prolong CL function. But, if given late during the period, it will actually promote luteolysis, counter to our goal of estrus suppression."

Mechanism of action

"Some work has been done to determine the mechanism of action of the oxytocin treatment," Vanderwall says. "We hypothesized that the oxytocin treatment suppresses the upregulation of oxytocin receptors on the endometrial cells at the end of the estrous cycle, preventing endogenous oxytocin from initiating luteolysis. However, when we experimentally tested that hypothesis, it was not supported—there was no difference in the concentration of endometrial oxytocin receptors between oxytocin-treated mares and control-treated mares."4

A study by Keith and colleagues5 at the University of Kentucky looked at the levels of prostaglandin synthetic enzymes in the endometrial cells of oxytocin-treated mares versus control-treated mares. They found that in the oxytocin-treated mares, there were lower levels of the prostaglandin synthetic enzyme COX-2, necessary to synthesize PGF2α at the endometrial cell level.

"It seems that the oxytocin treatment is having a postreceptor effect, suppressing the ability of the endometrial cells to synthesize PGF2α, resulting in a failure of the luteolytic process," Vanderwall says. "That's an important piece of new information that helps explain how and why the oxytocin treatment is blocking the luteolytic process, resulting in prolonged CL function for two to three months. The time frame of two to three months is not part of the oxytocin mechanism, but by nature, the CL has a lifespan of a few months if the luteolytic process does not occur. This occurs similarly with the uterine marble insertion. Regardless of how you prolong luteal function, it seems to have a lifespan of two to three months."

A drawback to the use of oxytocin to prolong diestrus is determining the day of ovulation in order to administer it on days seven to 14 postovulation. For optimal use, you do need to know the day of ovulation and give oxytocin IM from day seven to 14, so it's not a one-time shot or simply done during any phase of the estrous cycle.

Additional oxytocin research

Melinda Roche, DVM, had good results using the oxytocin protocol (60 units b.i.d. days seven to 14 postovulation) for one problem mare, with successful suppression for more than two months. Roche, owner of Roche Equine Veterinary Services in Twin Falls, Idaho, says the horse was a performance barrel mare that had a difficult time when she was in estrus (i.e. she wouldn't turn barrels well).

"I followed up to see that she had her CL postprotocol for two months, and she still had not cycled for almost three months," Roche notes. "With such success, this year I'm going to use it for several more mares as part of Vanderwall's upcoming field study, using once-daily administration. I've been really happy with it, and from my interaction with other veterinarians, I know it's been used extensively with good results. One colleague had success of estrus suppression in 11 of 12 mares."

Roche says she has tried intrauterine marbles in the past and has quite a few performance horses on altrenogest, but the oxytocin protocol is a good alternative. "It's easy for the owners to administer, easy for me to offer directions and it has had good results with no untoward side effects. Other than exogenous progestin supplementation, it's my new first line of therapy for estrus suppression in those mares that require it."

Evidence of the role of endogenous oxytocin in promoting luteolysis goes back to studies in the 1980s and 1990s, which demonstrated the role of oxytocin in the luteolytic process. "This foundational knowledge regarding oxytocin's function in the estrous cycle has led to our work 20 to 30 years later, identifying this clinical application of oxytocin to suppress estrus," Vanderwall says.

"Our plan as we move to the future is determining how to simplify and optimize the oxytocin treatment to make it even more practical," he continues. "We need to refine the protocol, so perhaps we wouldn't need to know the exact day of ovulation. Or maybe we look at developing a longer-acting oxytocin that doesn't require daily treatment. These are the focus points of our continued research efforts."

Ed Kane, PhD, is a researcher and consultant in animal nutrition. He is an author and editor on nutrition, physiology and veterinary medicine with a background in horses, pets and livestock. Kane is based in Seattle.

References

1. Vanderwall, DK. Prolonging function of the corpus luteum to suppress estrus in mares, in AAEP Proceedings, 2013;59:342-349.

2. Klabnik-Bradford, J, Ferrer, MS, Blevins, et al. Marble-induced pyometra in an Appaloosa mare. Clinical Theriogenology 2013;5:410.

3. Vanderwall, DK, Rasmussen, DM, Woods GL. Effect of repeated administration of oxytocin during diestrus on duration of function of corpora lutea in mares. J Am Vet Med Assoc 2007;231:1864-1867.

4. Vanderwall, DK, Rasmussen, MS, Carnahan, KG, et al. Effect of administration of oxytocin during diestrus on corpus luteum function and endometrial oxytocin receptor concentration in cycling mares. J Equine Vet Sci 2012;32:536-541.

5. Keith, L, Ball, BA, Scoggin, K, et al. Diestrus administration of oxytocin prolongs luteal maintenance and reduces plasma PGFM concentrations and endometrial COX-2 expression in mares. Theriogenology 2013;79:616-624.