Hormone Healthcare

Hormone imbalances in horses are not uncommon—especially when it comes to mare and older horses.

Hormones are the body’s messengers, traveling through the bloodstream to communicate between various glands and organs. They affect and regulate body functions and metabolism and have an influence on behavior. Too much or too little of any hormone can result in imbalances that affect a horse’s health and behavior—most commonly, in heat behavior of mares, and Cushings in older horses.

The several endocrine glands, from ovaries and testes to pituitary and thyroid, produce most of the hormones that drive body processes. The pituitary (called the master gland) at the base of the brain secretes a wide variety of hormones that control and affect growth and influence secretion of hormones from other glands.

Here’s a look at how things go wrong, and what you can do about it.


Some mares are excessively cranky, hard to handle or unfocused in their work while in heat. Owners often try to manipulate the hormone balance of estrogen and progesterone in these mares so it won’t adversely affect performance during show season or an athletic career. Estrogen brings the mare into heat and prepares her reproductive tract for breeding, while progesterone keeps her out of heat.

In the non-pregnant mare, interaction of estrogen and progesterone are the main factors that determine her behavior, personality, and pain threshold. This is why, for instance, some mares become more sensitive to touch when in heat, and may bite, kick, flinch or squeal when being groomed or saddled. During the transition from one phase to the next, when both estrogen and progesterone are often present in fairly high levels, some mares may be moody or even aggressive. Some become irrational when separated from other horses, or more focused on other horses than on paying attention to the person riding or handling them.

Some mares seem to be in heat all the time, due to hormonal imbalance. A long transitional heat (as many mares experience in early spring when coming out of winter anestrus) may last 30 days or longer. She will usually come into a normal heat at the end of this transitional period, and cycle normally through the rest of the breeding season. She may also go through a long transitional heat at the end of the breeding season. Mares exposed to continuous light in barns (to keep hair coat short for showing) sometimes lose their ability to cycle normally and may show persistent or long heat periods.

Another cause of abnormal mare behavior is a granulosa cell tumor, which can destroy or change the ovary, interfering with proper hormone production—producing a strange mix of hormones. Some mares cease having heat cycles, while others have continuous or frequent intermittent estrus, and others show stallion-like behavior. If the tumor is removed, these mares gradually return to normal.

Treating Ill-Tempered Mares

Peter Sheerin, DVM (reproductive specialist at Rood and Riddle Equine Hospital, Lexington, Kentucky) says the first step is to determine whether the unwanted behavior truly stems from a hormonal problem. “I tell owners to keep track—write it on the calendar—when she has the problems. A mare usually comes into heat every 21 days or so. If the problem is related to her being in heat, it should appear in a cyclical fashion,” says Sheerin.

“If the mare is fine when she’s out of heat (under the influence of progesterone), the treatment we’ve used for years is Regu-Mate, a liquid oral progestin given daily by syringe into the mouth, or dribbled over her grain,” he says. People often use this during the months they’re showing the mare, or for a while before an important show.

Tim Bartlett, DVM (a veterinarian in Indiana who trains and shows reining horses) says many people use hormonal implants, but he’s had the most success with Regu-Mate, also. Other products, he notes, include various combinations of progesterone and estrogen.

Progesterone injections (intramuscular) can be effective within 24 hours. “There are two types of injectable products,” says Bartlett. “Progesterone in oil (peanut oil or sesame oil) is usually safer, if you find a product that works for a particular mare, though this involves giving an injection every other day. The compounded Repositol products, though they may last longer, are less reliable and also have more risk for reactions,” he says. Long term effects on fertility are not known, so compounded drugs should not be used if you plan to breed the mare later. And if injections must be repeated too often, it may be better to use the daily oral Regu-Mate.

Sheerin says some people use cattle implants that contain progesterone, since they are cheaper than Regu-Mate. “There are anecdotal reports that this works; clients report a decreased incidence of problem behavior, but in studies that were done to see if these products could maintain pregnancy, the implants did not release enough progesterone for that purpose,” he says. The same is true of Depoprovera, a synthetic progesterone. There is no scientific data to show it works, but there are anecdotal reports that it is effective.

Sheerin advises owners to first try something that’s been proven to work, like Regu-Mate. “If that works for the mare, then if a client wants to try different, less expensive options, they can do that—realizing they may or may not work. Whatever works for a particular mare, and causes the least problems, is the way to go,” he says.

In that vein, Bartlett recommends trying to “train through” the problem as much as possible before resorting to hormone therapy. Much of the bad behavior in young in-heat mares can be due to immaturity, lack of training, and inexperience, just as with a young stallion. Most mares will train out of obnoxious behavior. Use drugs as a last resort, he suggests.

The young in-heat mare should not be punished excessively if she’s obsessed with other horses or has a hard time concentrating on her work while under the powerful influence of estrogen. Punishment does little good, and may provoke a fight. It’s often best to ignore some of her infractions if she can’t help herself, and reinforce correct behavior at other phases of her cycle when she will pay attention. If a young mare becomes unmanageable, however, or her bad behavior lasts more of the month than not, have her checked by a veterinarian.

Other non-hormone methods are sometimes helpful. Acupuncture, which involves inserting surgical staples into an acupressure point in the mare’s ear tip, is usually quick and simple. The mare still has heat cycles, but with less violent behavior during heat. This seems to be safe, with no ill effects on fertility if a person decides to breed the mare, but it doesn’t work on all mares.

Another method that has come into use is insertion of a small glass ball (marble) into the uterus. “A study published a few years ago showed that about 65 percent of mares with marbles do not come into heat for several months. The practitioners I’ve talked with who use marbles have said that in their hands, this method works 75 percent of the time,” says Sheerin. The presence of the glass ball (similar to an embryo of that size) causes the CL on the ovary to remain, continuing to produce progesterone—the hormone that keeps the mare out of heat. To bring the mare back into heat, she can be given prostaglandin and the marble can be removed.

“A new method for keeping mares out of heat, reported at the last AAEP meeting, involves the use of oxytocin,” says Sheerin. “It hasn’t been done much in private practice, but this will probably change now that people are aware of this method. There will probably be more information available by the end of this year as to how well it works, after more people try it,” he says. It would be cheaper than keeping a mare on Regu-Mate.

Spaying (removing the mare’s ovaries) is another alternative, if the owner never plans to breed the mare. This eliminates the main source of hormone production and the mare has a more even temperament, like a gelding. When costs of hormone therapy are added up year after year, spaying is less expensive. “But I won’t do that unless I know the ovaries are actually causing the behavior,” says Sheerin.

Pregnancy is another option. “It may not be practical for some mares, but for one you eventually wanted a foal from anyway, it might work well—especially if you are only showing her during the summer and fall, in early to mid pregnancy,” says Bartlett. It doesn’t hurt to keep exercising mares during pregnancy. Plus, he adds, “Their attitudes are more stable while pregnant.”

But the effects can be long-lasting. “Once they’ve had a foal, many of these mares don’t have some of the behavioral problems they had as young mares. Their hormones are more in balance and you don’t have to use Regu-Mate to keep them out of heat,” Bartlett says.


Equine Cushings Disease, now called Pituitary Pars Intermedia Dysfunction (PPID) in veterinary literature, is a common though not well understood problem in horses in their late teens or older. Betta Breuhaus, DVM, PhD, associate professor of equine medicine at North Carolina State University, says there are misconceptions about this disease because horses with related endocrine problems and hormonal imbalances are often lumped together—horses with cresty necks, long hair coats that don’t shed, sluggish metabolism, insulin resistance, and tendency to develop laminitis.

“In the past, people thought fat horses with cresty necks were low on thyroid hormone,” says Breuhaus. “Veterinarians put them on thyroid medication. When I first started dealing with Cushings we were able to show that these horses had normal thyroid function,” she says.

“Then Dr. Philip Johnson suggested this [cresty neck] was a different disease syndrome, which he called Equine Metabolic Syndrome (EMS). He likened it to Type II diabetes in humans and suggested these horses are insulin resistant,” says Breuhaus. “The body puts fat in abnormal places: crest of the neck, rump, sheath, etc. Equine Cushings can also cause abnormal fat deposits and these horses can also be insulin resistant, so this can be confusing.

“The only way to tell the syndromes apart is by response to certain tests. One is a dexamethasone suppression test,” she says. Horses with PPID (Cushings) do not slow their production of cortisol after administration of dexamethasone (a similar steroid), whereas horses with EMS do. In addition, treating a Cushings horse with pergolide (a drug that stimulates production of dopamine) may relieve some of the clinical signs, whereas pergolide will not help a horse with EMS.

Last but not least, EMS can show up in younger horses, whereas Cushings generally does not become obvious until the late teens or older.

Cushings starts in the pituitary gland. “The intermediate lobe becomes larger and produces more of its hormones,” says Breuhaus. “The reason this happens is that the hypothalamus does not function properly. It ordinarily secretes dopamine, which inhibits or keeps that lobe of the pituitary under control, decreasing the output of hormones from that part of the pituitary,” she says. Without dopamine, there is no negative feedback to shut off the extra production of certain hormones.

Excessive levels of pituitary hormones cause the adrenal glands to overproduce cortisol. The excess cortisol (along with the other excess hormones) then causes the effects seen in the horse—changes in hair coat, feet, muscles, etc.

Pergolide, originally used for treating Parkinson’s disease in humans, replaces the missing dopamine to control the pituitary. However, this is not a “cure.” This condition is life-long. Treatment just allows the horse to have a better quality of life. “You have to decide, with each horse, whether this is something you can manage for awhile just with good care and then eventually go to drugs, or whether the horse is already to the point where it needs drugs,” says Breuhaus.

Laminitis is probably the most devastating sequel. Cushings is the main reason a lot of older ponies and horses suffer from chronic laminitis. Some horses may be genetically predisposed to this condition; Cushings is found in all breeds but is most common in ponies and Morgans, according to Breuhaus.

Signs of Cushings may be similar to EMS at first (extra fat deposits), but as the disease progresses, the typical Cushings symptoms include weight loss (rather than fat and cresty neck), loss of muscle mass, muscle wasting over the topline, pot belly, recurrent infections, delay in shedding winter coat in the spring, and eventually a long, curly hair coat that doesn’t shed at all. The long hair may be retained just on the legs, in patches on the body, or all over. Hairs at the base of the tail are bushy. The long hair makes these horses more susceptible to skin problems like rain rot, and predisposes them to excess sweating. Some secrete a greasy sweat, while others lose their ability to sweat.

In advanced cases, some horses drink more water than normal and urinate excessively. They may be lethargic. Mares are subfertile and hard to breed. It can be hard to differentiate between the metabolic problem and general signs of old age.

Horses that don’t yet have serious complications such as laminitis or infections can often be managed with a diet that provides the necessary nutrients yet is low in starch and sugar, to help avoid laminitis. Horses that tend to become fat should have dietary management to help control their weight. These horses tend to develop laminitis when allowed free access to pasture because of the fructans and high calorie content of green grass. They should be limited in grazing time or wear a grazing muzzle. The same is true for horses with EMS.

Several steps can help keep affected horses comfortable and healthy. Horses that don’t shed can be helped by clipping. Blanketing in cold weather, regular dental and hoof care, deworming and vaccinations become especially important. Any kind of infection should be treated medically. “It’s very important to keep up good parasite control, since PPID horses are much more susceptible to parasites. With old retired horses, however, basic care is often minimal and no one looks at their feet or teeth. If a problem isn’t taken care of early, and a tooth abscesses, the horse is less able to fight off the infection,” says Breuhaus.






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