Fooling mother nature: DVMs being asked to control strong heats

Fooling mother nature: DVMs being asked to control strong heats

Apr 01, 2003

The weather is slowly getting warmer and that means that two of an equine veterinarian's busiest seasons are soon to be here - breeding and showing. Unfortunately these two activities do not go together well for some mares.

The physiological changes that occur in the spring as mares come into estrus and begin cycling in earnest often conflict with a busy training and show schedule. Veterinarians are being asked more frequently to assist in controlling what the owner and trainer feel are excessively strong heat cycles in some mares.

Control the cycle

Typically these horses are normally well behaved when not in season. Mares that are generally strong willed or slightly aggressive and difficult to deal with normally may become even more so during heat. When mares begin to cycle, they can become very vocal, calling to other horses constantly.

They may stop to "flag" (raising and averting the tail) and urinate excessively. Some mares will kick at a stall wall or repeatedly bump the walls with a side or flank. When being ridden, these horses can vary from slightly resistant to leg pressure and unfocused to being totally reluctant to respond to rider aids and acting aggressively - bucking, kicking and/or biting.

None of these behaviors is consistent with pleasurable riding or success in the show ring, so veterinary help is being sought to keep these mares calm and often, to prevent or delay normal estrus behavior.

Suppress estus

The most effective method of manipulating a mare's cycle to keep her from showing estrus is to maintain the corpus luteum (CL) as the mare would normally do in a pregnant state. This ovarian structure would produce endogenous progesterone at a level sufficient to suppress estrus for a period of roughly 150 days.

There are various sources and forms of progesterone available to the practitioner for this purpose. Not all of these products, however, are labeled for use in the horse and some have not shown any evidence of efficacy.

The most traditional method of estrus suppression is through the use of daily oral administration of a synthetic progestin (Regumate-alternogest). This method works well for most mares and, if the animals are taken off the drug and given a sufficient amount of time for the system to normalize post long-term use, these mares will return to normal cyclical activity and normal reproductive function.

The disadvantages with daily oral use relate to the difficulty of getting the drug in some mares with ease and confidence and the fact that this product can be absorbed through human skin and can adversely affect human hormone activity.


Intramuscular injections of progesterone are also currently available with the dosage and periodicity dependent on the type of formation used and its concentration.

Daily injections are often not well tolerated by the animal, especially when we consider that these mares are usually harder to handle and deal with than most mares. Since these injectable preparations, especially the ones designed to be given at weekly or longer intervals, are commonly prepared in a slowly dissolving oil they can cause soreness at the injection site and, over time, have been associated with firm nodules or lumps of fibrous tissue at the injection sites.

Again, this is unacceptable in sport horses since the resultant neck stiffness can be even more detrimental to performance than the initial estral behavior. Injectable depo-provera is a more recent product that has, as an advantage, a minimally reactive formulation and a dose interval for most mares of 60 to 90 days. Effectiveness can be variable though.

Some owners and trainers, however, have horses that do not respond to these drugs or they may be reluctant to use hormone therapy in their horses. A relatively new approach may be beneficial for these cases.

Mimic embryonic development

Drs. Nie, Johnson, Braden and Wenzel, from the College of Veterinary Medicine at Auburn University are responsible for research presented in a paper at the 2001 American Association of Equine Practitioners meeting in San Diego. Their research focused on the use of a glass ball inserted into the uterus of a mare in order to suppress estrus.

This relatively simple idea has caught on and has been working well for a number of practitioners throughout the country.

Nie, speaking on behalf of his research group, said that the exact mechanism as to how and why this approach works is unclear at this time. It is postulated that the glass ball mimics an early developing embryo and sends the same tactile stimulus to the uterine lining which, in turn, encourages maintenance of the CL. Nie's study showed that five of 12 mares (41.7 percent) had a prolongation of luteal function and these mares did not cycle for 76, 78, 86, 95 and 109 days respectively.

The other seven mares in this study cycled back in an average of 20 days. Other practitioners who have since tried this approach are reporting slightly better results with some listing success rates as high as 67 percent. In the mares where this glass ball implant works estrus has been suppressed for an average of three to four months.

Clearly there are individual differences and some, as to yet undetermined reasons, for the high success with some mares and the failure with others. It may be likely that internal uterine conditions, uterine-ovarian hormonal interaction or a combination of both may be the factors that affect the outcome.

Safe procedure

The actual procedure is relatively easy to accomplish and is safe. No complications from the placement of these glass balls have been noted and this has been confirmed through ultrasound and histological testing.

The mare should be prepared as for artificial insemination and attention to cleanliness is important since the primary concern is that these implants may be a source of infection.

The glass balls themselves can be obtained from any number of places. Glass Ball Studios in Oregon ( produces a dazzling array of what are really collector grade marbles.

Some practitioners have found marbles at antique stores and one veterinarian reports good results with marbles purchased at his local Toys-R-Us. Some practitioners have used silicon eye prosthetics available from Jardan EyeProsthetics Inc. of Southfield, Mich. All of these products should be autoclaved and Nie recommends using the autoclave cycle recommended for fluid, as the long, slow cool down in this particular cycle will avoid breaking the glass balls.

The size of the ball is important, and a 35-mm diameter ball seems optimal. After adequate preparation, a sterile palpation sleeve is used and the glass ball is passed vaginally through the cervical lumen and into the uterine body. The glass ball can then be manipulated via rectal palpation and moved further into the uterine body.

Correct placement of the ball at the horn-body junction can be easily confirmed with an ultrasound examination.

Some practitioners follow placement of the ball with infusion of antibiotic (ticarcillin at 1 gram is recommended by Nie) to help with any possible infection. Correct sterile technique will lessen the need for such use however.

The ball should be placed within 24 hours of ovulation. Some veterinarians have reported that certain mares will spontaneously expel the balls, but this is uncommon and was not observed in any of the mares in Nie's study.

The tag line from a 1980s advertising slogan was "It's not nice to fool Mother Nature." Perhaps, in this case, an exception can be made. The use of glass balls to suppress estrus provides the practitioner with a safe, simple method of keeping some mares out of heat.

This method does not have any significant disadvantages for immediate performance use or for long-term reproductive use. It does not require extra-label drug use and it offers the owner a more "natural" means of controlling a potentially difficult mare. So before you lose your marbles dealing with that "witchy" sporthorse, consider putting one in.