Mares: The Possible Problems They Deal With in Late-Term Pregnancy

There are a number of conditions that mares can go through during late-term gestation (seven months and beyond) and result in abortion – a situation even more traumatic by comparison, both physically for the horse and emotionally for the owner, than the early and relatively invisible “slipping” of an embryo.

Equine Placentitis

One of the most common causes of late-term abortion in horses is placentitis. An inflammation of the placenta often caused by an infection invading the uterus via the cervix, placentitis is responsible for up to 40% of late-term abortions in mares. According to a report by C. Scott Bailey, DVM, an assistant professor of theriogenology at the North Carolina State University College of Veterinary Medicine, at the 2013 Society for Theriogenology Conference in Kentucky, 60% of placentitis cases are caused by bacterial infection, usually by Streptococcus zooepidemicus, or less often Leptospira spp. or E. coli. Fungal pathogens are also responsible for a percentage of cases.

Placentitis symptoms are difficult to spot, and may be as subtle as a slight vaginal discharge or udder enlargement. Often abortion is the first sign there is a problem, or if the foal does make it to term, it is weak and sickly.

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Diagnostic options for placentitis are limited. Ultrasound screening of the uterus and placenta, while effective, can be expensive if frequent monitoring is required. Ultrasound images determine the combined thickness of uterus and placenta at various stages of pregnancy, and measurements exceeding normal ranges can suggest placentitis or other uterine abnormality. Other testing includes measuring serum progestagen (hormone) levels for elevation. In mares with placentitis or other compromised pregnancies, hormone levels will either elevate suddenly before the 310-day mark or drop suddenly, which often precedes abortion and at which point treatment is usually too late.

Recent research has found that inflammatory blood protein levels can help identify inflammation in the body. In controlled testing, serum amyloid A (SAA) levels increased significantly between two to six days after infection; it is hoped that with further research (as any infection in the body can cause levels to rise), SAA testing might be used as a screening tool for placentitis.

If placentitis is caught in time and treatment commences immediately, the chances of the mare delivering a live, healthy foal increases dramatically. Most often, treatment consists of broad-spectrum antibiotics, and anti-inflammatories such as flunixin, aspirin, pentoxyfylline or corticosteroids. According to Dr. Bailey’s research, a combination of the antibiotic trimethoprim sulfamethoxazole, pentoxyfilline, and the progestin altrenogest saw 83% of mares in a controlled clinical environment deliver viable foals. Delaying treatment resulted in only a 40% live foal delivery rate.

When dealing with equine disease, prevention is always the best option. Tracey Chenier, DVM, is assistant professor of equine reproduction at the Ontario Veterinary College at the University of Guelph. In her experience, “Most often, we see placentitis in older or thin mares that have poor perineal conformation. Make sure your mare has a Caslick’s [vulvar suture] if she needs one. Mares that have had placentitis previously are also at-risk. While I don’t recommend monthly ultrasound evaluations of all mares to screen for placentitis, I do believe that it is warranted in at-risk mares. When we find the disease early in its course we have the best chance of getting a live foal. By the time the mare has vaginal discharge and premature lactation, it has often been present for a while and the prognosis is poor.”

Equine herpesvirus

Ominously dubbed the “abortion virus,” a major cause of abortion in mares is equine herpes virus, most notably the EHV-1 and EHV-4 strains. If a pregnant mare becomes infected with EHV-1, or if she is carrying a latent infection that is activated by stress during pregnancy, the virus can cross the placenta and cause the foal to be aborted. This usually occurs in late pregnancy, but can happen as early as the fourth month. Infection with EHV-1 or EHV-4 very late in pregnancy may result in a stillborn foal, or one that dies within a few days of birth.

Says Dr. Chenier, “We currently recommend vaccinating at months five, seven, and nine of gestation. Some will add month three as well. There are many examples of abortion in vaccinated herds, and with any vaccine, it will not provide 100 per cent protection. Biosecurity is very important with EHV-1.”

Uterine torsion

Uterine torsion can occur during the last three months of pregnancy. It is an emergency situation where, as the name implies, the body of the uterus twists clockwise or counterclockwise 90°-360° on its long axis (180° is most common). Sometimes the small colon can become incorporated in the torsion as well, causing a life-threatening blockage. The condition may be caused by rolling, falling, extreme fetal movement, or a large fetus in a small amount of amniotic fluid.

“We think that mares are prone to uterine torsion in mid-gestation due to the position of the fetus, and the anatomic manner in which the uterus is ‘slung’ in the pelvis and abdomen,” says Dr. Chenier. “A pendulum effect is created when the fetus becomes highly active, and sometimes they fling themselves all the way around.”

Signs of uterine torsion – responsible for 5-10% of all obstetrical emergencies in horses – include abdominal pain that may be mistaken for colic. The mare may be off her feed, anxious, and restless, which may also be confused with signs of early labour. She may also have an elevated heart rate (tachycardia). If the uterus ruptures, fever and severe hypovolemic shock will result. “Any pregnant mare showing colic signs should be examined by the veterinarian to determine whether uterine torsion is present,” warns Dr. Chenier.

Treatment must commence immediately upon diagnosis (usually by transrectal palpation) to provide the optimum chances of survival for the mare and foal. A common nonsurgical procedure involves putting the mare under general anesthesia and rolling her while applying pressure to her abdomen with a long wooden plank, holding the fetus and uterus in position until the uterine torsion has been corrected. If this method fails, or if the mare is near full term, surgical intervention (cesarean section) or manual derotation of the fetus and uterus through the dilated cervix is an option. All procedures come with some risk, including uterine rupture or death of the mare and/or foal.

“Virtually all of the cases referred here opt for surgery,” says Dr. Chenier, “which is done through the flank, with the mare standing, heavily sedated, with local anesthetic. Surgical correction success rates are very high. The likelihood that a mare will carry the foal to term successfully depends on the severity (degree) of the torsion and how long-standing it was. So the further torsed, the higher likelihood there has been significant compromise to the uterine blood supply, which results in death of the fetus and abortion.

“Rolling [procedure] carries a significant risk of uterine rupture, and the risk of making the torsion worse if you have not diagnosed the direction of the torsion accurately. In addition, sometimes a piece of bowel becomes trapped within the uterine torsion. This cannot be corrected if you opt for the rolling technique (and may worsen things significantly). At surgery, we can evaluate whether there is any bowel trapped as well and fix it then.”

Prepubic tendon rupture

Although more common in older draft mares than sport horses and other “athletic” breeds, a “prepubic tendon” rupture is actually a separation of ventral abdominal musculature from where it is anchored to the pelvis. It is characterized by ventral edema (accumulation of fluid in the abdomen), followed by a visibly “dropped” abdomen, bloody mammary secretions, displacement of the udder and a raised tail head. The mare will be reluctant to walk or lie down and may assume a ‘saw-horse’ stance.

Prepubic tendon ruptures can be caused by trauma, hydrops (abnormal fluid production) of the placental membranes, and a heavy uterus in a petite mare, such as from carrying a large foal or twins.

The condition can be confirmed through physical examination, rectal palpation, or ultrasonography. Treatment consists of supportive abdominal wraps and stall rest. Performing a C-section is the traditional treatment, although Dr. Chenier says that economics usually dictates the choice of treatment. “Mares with PPT rupture are often euthanized due to a very poor prognosis, so most owners will elect to induce parturition when she is ready and avoid the expense of C-section. You must be ready to assist, as they can have difficulty foaling with the lack of abdominal muscle contribution. PPT rupture is very painful for the mare, so it goes beyond just her future reproductive potential.” The mare may die during delivery due to evisceration, internal hemorrhage, uterine artery or bowel rupture, or port-parturition from colic. The foal may die during or following delivery.

An abdominal wall rupture will look similar, but rectal palpation may reveal a tear in the body wall. To relieve the discomfort, the mare may adopt an unusual stance or lie down more often than usual, but unlike a prepubic rupture, can usually walk and her udder placement is not affected. “If the mare is not ready to foal just yet, we will keep her comfortable, limit her to stall rest, and try a belly wrap for additional support. Once her milk calcium increases, we can go ahead and induce her to foal,” says Dr. Chenier. “Abdominal wall ruptures can have surgical repair attempted, but success rates are low. They are not as painful, however, and the mare can live out life as a pasture ornament.” A mare who survives either type of rupture should not be used for breeding again, except for embryo transfer.

Other conditions can also cause premature parturition or abortion, including torsion (strangulation) of the umbilical cord, where excessive twisting or wrapping around a limb shuts off the flow of blood, resulting in the death of the fetus; twinning, which can be easily diagnosed and prevented with early ultrasound; and gastro-intestinal colic (esp. hypoxia occurring during colic surgery). But before you lose any more sleep worrying about late-pregnancy complications, be assured that the vast majority of foals hit the ground safely without any help from humans at all. And even if the unthinkable happens, with swift and thorough veterinary care your mare should be able to successfully carry a future foal to term.