Why Spay Isn’t Always The Way For Mares – And How To Tell When It’s Right

Every weekday, Bob Barker would remind viewers to spay and neuter their pets. Of course, he was talking about dogs and cats. Spaying (the surgical removal of the ovaries and uterus in female animals) is a common practice for controlling the population of cats and dogs. However, this is not a premise that is not as prevalent for horses. There are several reasons for this difference in approach.

“The reason that spaying is not that common in horses is that intact horses don't show heat signs similar to dogs and cats,” said Dr. Pouya Dini, an assistant professor at the University of California, Davis, who has studied mare reproduction and behavior. “There are still some behaviors that owners and trainers don't like (with mares in heat), but it's not like dogs or cats. Anyone who has a female cat that is not spayed knows that when they come into heat, they make noise (caterwaul), and they completely change their behavior. When a dog is in heat, she attracts other dogs, and they sniff each other or have blood-tinged vaginal discharges.

“Of course, we see stallion-like behavior in males, and that's why we geld a horse that we don't want for breeding. But in a female, there are not that many undesired behaviors related to their reproductive cycles.”

There are some instances that an owner will complain about a mare’s behavior during her heat cycle. However, Dini stresses to owners that they need to realize that there could be other reasons for the change in attitude or behavior than the normal cycle. One of these reasons could be due to medical issues such as an ovarian tumor which would create a medical reason to remove the ovary.

Dini and his fellow researchers at the Clinical Endocrinology Laboratory at the University of California-Davis have seen a few thousand cases where the owner believes the behavior problem with their mare is related to their ovaries.

“We have hormones that correspond to ovarian ‘health,’ and in 90% of these abnormal behavior cases, there was nothing wrong with the ovary based on these hormones,” said Dini.

If an abnormal behavior is not related to the ovary or the reproductive system, owners really start looking for other reasons: Why is my mare suddenly acting aggressive? Why is she starting to be so bossy? The reasoning behind this change in temperament could be a social issue – what’s been changed around the barn recently? Or, it could be a training issue or even pain in other body parts, such as musculoskeletal issues.

Timelines are flexible with mares
With cats and dogs, there are certain age targets veterinarians prefer for a spay surgery – usually that the animal is mature enough and large enough to safely undergo anesthesia, but still not a fully-grown adult. That’s not the case with horses.

The routine clinical presentation that Dini and his colleagues see is mares that suddenly change their behavior and performance. The mares suddenly start underperforming under saddle or show a decline in their jumping form. Or, in their paddock, they suddenly start bossing around other mares.

“Out of 2,800 cases of abnormal behavior, less than 10% of them were associated with abnormal hormonal levels, indicating an ovarian problem, and the majority of these 10% had one thing in common: they had stallion-like behavior,” said Dini. “These are mares that are teasing other mares -- sniffing them, rearing up, and starting to have a cresty neck. So these are the ones that, okay, there was something wrong with their ovaries.”

In those 10% of cases, the cause of the stallion-like behavior was usually a granulosa cell tumor, or GCT. This is the most common type of tumor found on the reproductive tract of a mare. There is no certain age or breeding that can pinpoint when a mare might develop one – it can happen to a young filly in training or an older retired broodmare.

The Clinical Endocrinology laboratory at UC-Davis has a great diagnostic method for discovering if a mare has a GCT or not. They measure several different hormones, including anti-mullerian, inhibin and testosterone.

“So we measure these three hormones to see if there is potentially a tumor in the ovary or not. We also ultrasound and feel their ovaries to evaluate how they look and measure their size” said Dini. “But the majority of cases do not have a tumor. Maybe there is a new mare in that paddock; that's why our mares start changing their behavior. Maybe it's a new trainer, or maybe something changed in the lifestyle of the mare. But owners tend to ignore those and go directly to the medical route.”

Side effects of spay

Mares’ ovaries produce two main steroid hormones: estrogen and progesterone. When a mare is in heat, there is estrogen but no progesterone. That is why she shows signs of heat, such as teasing into a stallion, squatting, and urinating for a few days. Once out of heat, ovaries produce progesterone, which suppresses the teasing behavior. The squatting and urinating are all suppressed. Once you remove the ovary, you're removing that source of progesterone, while the adrenal glands can produce some estrogens, and there will be no progesterone to block its effect and the balance is out of whack. This now leaves you with what is called a “teasing mare.”

“We need to make sure that the ovary is the problem before we remove them otherwise you'll make a new problem by removing the ovary because you're removing that source of progesterone,” said Dini.

Specialists like Dr. Dini work with owners to scrutinize every facet of the mare, from her daily routine and habits to her eating in regard to her cycle. If the hormonal test comes back normal, they will still palpate the ovary to make sure that, anatomically, the ovary looks normal.

The next step will be to have the owner mark on a calendar whenever the horse behaves abnormally, and make notes as to what they feel could be the cause. In th spring and summer, mares cycle every 20-21 days, so if the issue is due to the ovary, the change can start to happen when a follicle starts to grow on the ovary, becoming too big and causing the mare to be uncomfortable. Again, this cause has been rare, but Dini has seen it happen and he won’t completely rule it out without testing.

“If you see a pattern on the calendar, then it probably can be the ovary,” he said. “That’s when we start to monitor them more closely to see when she becomes colicky or starts to act aggressive.”

There are medications that can induce ovulation to get rid of a big follicle, which gets rid of the pain. This should clear the behavior as well.

“If the behavior goes away, then we know that's an issue with the ovary,” said Dini. “That means that the ovary is a candidate to be removed; even though it’s not a tumor, there is some other issue there.”

If surgery is needed
In days past, to spay a mare you needed to put her under full anesthesia, place her on the operating table, and open up her belly. Now, surgeons are able to keep the mare standing while under anesthesia and go in from the side (paralumbar) to remove the ovary. It's a small incision, so there isn’t a big scar. While this is a big advantage, you have to remember that you are still going inside the abdominal cavity of the mare. This is not a procedure that can be performed on the farm, like castrating a colt.

“However, the procedure is very fast and also safe, at this point, for most of the horses,” said Dini. “We’ve been able to do this for several years now.”

Unlike a gelding procedure where it’s safe to ride the horse almost immediately after, a mare will need at least a week or two to recover from the surgery. Mares typically need at least a few days of stall rest and then a week of very low exercise and then they can typically go back to their normal routine.

Meanwhile, in Europe
European horse owners are tackling certain behavioral issues of mares not with spay surgery but with vaccines. There is a similar vaccine that was once used to reduce boar taint in for pigs, according to Dini. This vaccine works against Gonadotropin-releasing hormone (GnRH). This hormone is made by the hypothalamus to eventually induce the ovary or testicle to produce steroids.

Currently, this vaccine is not available in the United States, but it has been used in Australia and Europe. The vaccine will shut down ovarian hormone production from six months to a few years. A booster is sometimes required to keep the shutdown going. There are studies showing that this vaccine improves abnormal behavior in some mares.

In the United States, without this option, some people may learn towards surgical removal of the ovaries. Dini advises caution with this approach.

“Some owners might like the change they see in their mare after the ovaries are removed, but at what cost?” he says. “We are spending a few thousand dollars and we will be satisfied because of the money that we spend. But does it really help the mare? Without really testing and looking into the problem, that's questionable. Those ovaries could have been normal.”