Oxytocin can settle problem mares, but researcher urges prudent useYou have secured a season to a sought-after stallion that you know is the perfect match for your mare. You have had her under lights since the end of November. You have succeeded in booking her on exactly the day she signaled she was ready to be bred. You are thinking you have done everything right; the rest is up to the powers that be.

If your mare is susceptible to persistent endometritis and you want to improve your chances of not having to return her to the breeding shed, do not stop reading now.

All mares experience an inflammation in the uterus after breeding. In a perfect world, all mares would clear the inflammation within 24 hours, leaving the uterus healthy and able to receive a fertilized egg about five days later.

Debris from mating, such as seminal fluid and dust from the breeding shed, can interfere with pregnancy if allowed to remain in the uterus longer than 24 hours. Infection develops, so that even when mating does result in a fertilized egg, the uterus is not ready to receive and nourish it. This condition is classified as persistent endometritis (see box). Age, foal production, and the position of the uterus in the body are a few factors believed to contribute to persistent endometritis, or mating-induced infertility.

Uterine lavage with buggered saline administered several hours post-breeding is sound management for mares diagnosed as having persistent endometritis. Saline has been used for the past ten years to flush the uterus of debris introduced during breeding. The effectiveness of this procedure has been studied by Dr. Mats Troeddson, a veterinary researcher now at the University of Minnesota.

Beginning in 1993, an oxytocin injection administered in conjunction with uterine lavage has gained rapid acceptance as a method of increasing the effectiveness of lavage. Oxytocin is the hormone that induces parturition by causing the uterus to contract. It has been used by the medical community for years to induce labor in women.

When used on broodmares 4-to-8 hours after breeding, oxytocin dramatically hastens clearance of the uterus by inducing contractions that expel debris. Its effectiveness in treating mating-induced endometritis in mares has been studied at the University of Florida by Dr. Michelle LeBlanc, a veterinarian and professor of large animal clinical sciences.

"The question we're trying to answer is, 'Can we get away from the uterine lavage and just do oxytocin on all mares?' " said LeBlanc. "Definitely, there are some mares that don't need a lavage because the degree of their problem is not that bad. All they need is a little boost, and that's what the oxytocin does. But then there are some mares that are pretty bad."



Oxytocin bandwagon

Although 1996 is just the fourth breeding season in which oxytocin has been used to combat infertility, a trend has begun toward using this method alone.

"What people are tending to do is use the oxytocin without the lavage," LeBlanc said. "Lavage is the hard part. It takes time, you have to have a lot of gear, whereas with the oxytocin, all you do is walk up to the horse and give it a shot."

A shot of oxytocin is inexpensive. Temptation may be strong for breeders to view oxytocin as a magic bullet. For a couple of reasons, this temptation should be resisted.

First, more will be known about the effectiveness of oxytocin on persistent endometritis after data from a University of Florida study is released in June. At this early stage, LeBlanc believes research will show that oxytocin without lavage might be enough for some, but not all, mares.

"I think there will definitely be instances where you'll need both," she said.

Meanwhile, the only mares that should receive oxytocin after being bred are "those who need it," LeBlanc advised.

"The biggest problem I'm worried about now is that it is getting used for every infertility problem there is, and there are more causes of infertility than mating-induced endometritis. It's very important that a diagnosis is made."



Diagnosis critical

Scintigraphy can be used prior to breeding to determine if a mare is susceptible to persistent endometritis. The procedure involves using a radioactive liquid to evaluate a physiologic mechanism. Once inside the body, the liquid emits gamma rays, which are detected by a special camera that produces a picture similar to an X ray. The dose used in the uterus is one-tenth the dose used for a bone scan.

Cost varies from state to state, but diagnosing a type of infertility by this method is considerably higher than simply administering an injection of oxytocin. "It costs much more money to make a diagnosis than it does to treat the horse," said LeBlanc.

For this reason, researchers at the University of Florida are also exploring whether ultrasound might be employed to greater advantage as a method for determining which mares need uterine lavage because of a sagging uterus. An ultrasonic scan taken 24 hours after breeding can detect excess fluid in the uterus, but for problem mares, that is already too late.

"For some mares who have a bad problem, waiting 24 hours allows those inflammatory products to stay there long enough to cause irritation in the endometrial lining, so that the lining cannot recover by the time the egg comes down on day five," LeBlanc said. "We're trying to come up with a cheaper test, and we'll have that done by this summer."





Mares with foals may not breed

Some mares with foals at their side will not return to heat for rebreeding. A small but significant number of lactating mares do not come into estrus while they are nursing foals. This condition is called lactational/anestrus.

The use of hormone drugs can often induce estrus in mares after foaling, but mares who are in true lactational anestrus will not respond to this type of treatment because their ovaries are inactive; follicles are not developing. Usually mares who are not cycling at all during lactation cannot be rebred until their foals have been weaned. This is why some mares will produce a foal only every other year.

Some mares with foals are hard to breed because they are preoccupied with their babies and not showing signs of heat during estrus. If a mare is actually cycling but having "silent heats," she may be successfully bred through a careful and diligent teasing program or with veterinary examination to determine the status of her estrus cycle to pinpoint the time of ovulation.