Late-Term Pregnancy Troubles in Broodmares

Learn about 5 complications veterinarians might encounter toward the end of a mare’s gestation.

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Late-Term Pregnancy Troubles in Broodmares
Monitor your broodmares regularly, and communicate any possible problems to your veterinarian without delay. | Photo: iStock

Five complications veterinarians might encounter toward the end of a mare’s gestation

You’ve invested a lot into your mare this past year—time, money, energy. You’ve hauled her to various reproductive exams and provided her with first-rate care. And after a long gestational wait, your hopes and expectations are high. A health scare or, worse, loss of mare and/or foal, would be devastating. Fortunately, serious late-term complications—that is, within the last 100 days of the 320-365-day gestational period—are relatively rare.

“The good news is that most of the issues we see in our late-term pregnant mares are relatively uncommon, so the average mare owner or breeding facility may not see problems every year,” says Ryan Ferris, DVM, MS, Dipl. ACT, owner of Summit Equine in Newberg, Oregon. “Some complications may even be once-in-a-lifetime type of events.”

Here we’ll describe how to recognize, avoid, and address five late-gestation emergencies broodmare owners and managers might encounter.

Equine Herpesvirus Infection

David Scofield, DVM, MS, Dipl. ACT, of Select Breeders Services, in Chesapeake City, Maryland, says one of the simplest means of heading off disease is to keep your mare up to date on her core vaccinations and equine herpesvirus-1 (EHV-1, also known as equine rhinopneumonitis) inoculation. The EHV-1 virus infects the mare through the respiratory tract, after exposure to secretions or airborne viral particles from infected horses, and enters the bloodstream, whereby the fetus can also become infected; this infection has an affinity for causing abortion in the last trimester.

Veterinarians administer an inactivated EHV-1 vaccine (approved for use during pregnancy) at five, seven, and nine months of gestation. Some veterinarians recommend an earlier vaccination for EHV-1 at three months of gestation.

Vaccination alone, however, is not foolproof. “Along with the equine herpesvirus vaccination, your broodmare population should be isolated from horses that travel to shows and other events where they can be exposed to the virus and come back and infect other horses on the property,” says Scofield. “This includes general practices such as having no shared water, nose-to-nose contact, and neighboring stalls or fencelines. Even if a horse returning to the farm has immunity to EHV, it may be harboring the live virus.”

And, frustratingly, vaccinated mares sometimes do still abort due to EHV, says Patricia Sertich, VMD, MS, Dipl. ACT, associate professor-clinician educator at the University of Pennsylvania School of Veterinary Medicine’s New Bolton Center, in Kennett Square. “The vaccination seems to control outbreaks of equine herpes abortion, but it’s not 100% effective at prevention,” she says. “Even in mares that have been naturally infected with equine herpesvirus, the immunity from that natural infection is short-lived, so it’s not surprising that the vaccinated immunity is also short-lived.”

Veterinarians make a definitive diagnosis of EHV-1 via a PCR (polymerase chain reaction) analysis of a nasal swab, which detects viral DNA. If a pregnant mare has contracted EHV-1, she might abort without signs of imminent parturition (birth) or clinical signs of infection. In the event of a live birth, the foal is usually infected, compromised, and typically dies shortly thereafter. Direct contact with the mare; aborted fetus and fetal tissues and fluids; and contaminated bedding can cause this highly contagious disease to spread, so use good biosecurity practices in these circumstances. 

This placenta shows the classic sign of ascending placentitis.
This placenta shows the classic sign of ascending placentitis: a distinct line of demarcation between the normal (red) and abnormal, inflamed (pale) chorion, or outermost placental membrane. | Photo: Courtesy Dr. Patricia Sertich


If you notice vaginal discharge or your mare’s udder “bagging up” prematurely, she might be suffering from placentitis, an inflammation of the placenta resulting from bacterial and/or fungal infection, says Ferris. Ascending placentitis, the most common form, occurs when microorganisms, either already present in the reproductive tract or that have ­contaminated it, ascend into the uterus during pregnancy. Other forms of placentitis include diffuse, with bacteria circulating systemically, and nocardioform, with localized uterine lesions.

“In late pregnancy, one to two months before the mare’s due date, the cervix normally starts to soften and relax,” Sertich says. “All mares have normal bacterial flora in the vestibule of the reproductive tract (the opening just within the vulvar lips); most of the bacteria maintain a nice balance and prevent any pathogens from overgrowing.   

“Mares that have inadequate competency of the caudal genital tract, due to poor perineal conformation, thin body condition, or just too many births,” she continues, “are at risk for some of these microorganisms to simply migrate up through the reproductive tract and eventually sneak through the relaxing cervix to invade the uterus and infect its placenta.”

If this infection sets in early enough, it can compromise the pregnancy. ­“Interestingly, in a lot of these cases, the fetus itself isn’t infected, but the inflammatory changes that take place in the placenta cause the fetal membranes to become thick and tough,” says Sertich. “Then at time of foaling, the membranes do not rupture properly and delivery of the foal is delayed, resulting in a hypoxic foal that hasn’t had enough oxygen during the delivery process. If the infection occurs early in pregnancy and progresses aggressively, the foal can also get infected, causing it to be too compromised to roll over and stretch out its neck and legs to participate in the delivery process. If the infection is extensive enough, the fetus may die and the mare will abort it.”

To diagnose placentitis, the veterinarian performs a transrectal ultrasound examination, looking for abnormal uteroplacental thickening (combined thickness of the uterus and placenta), areas of membrane separation, and infectious material in the placenta and uterus, says Scofield.

The veterinarian might also perform a bacterial culture. Most veterinarians are reluctant to procure a cervical or uterine sample for culture, as just performing the procedure will introduce bacteria from the mare’s vestibule further into the reproductive tract, says Sertich. However, identifying the causative bacteria allows the practitioner to choose the most effective antibiotics for treatment.

“To treat, we typically start the mare on a regimen of supplemental progestins, antibiotics, and non-steroidal anti-­inflammatories (NSAIDs),” says Scofield, adding that treatment can be performed on the farm and is typically very effective.

Ferris says his go-to placentitis treatment triad includes the antibiotic ­sulfamethoxazole/trimethoprim (SMZ/TMP), a double-dose of the synthetic progesterone Regu-Mate, and the anti-inflammatory pentoxifylline (PTX). He notes that the antibiotic ceftiofur has been shown ineffective as it cannot cross the placenta and reach the fetus.

Fescue Toxicosis

Late-term broodmares that consume endophyte-infected fescue hay or grass can develop fescue toxicosis, also known as ergot alkaloid toxicosis, which might lead to a variety of issues, including abortion or stillbirth; thickened placenta; prolonged gestation; limited colostrum and milk production; and weak or underdeveloped foals.

As with EHV-1, prevention is the name of the game. You can test forage for the presence of endophytes, the microscopic fungi that live within the fescue plant, as well as concentrations of toxic alkaloids. If you think your pregnant mare is at risk of ingesting endophyte-infected fescue, and you can’t remove her from that pasture, administer the dopamine antagonist domperidone, which blocks the toxic effects of the endophytic alkaloid, ergovaline, at the cellular level.

“This is something I see in my area (Maryland),” says Scofield. “Owners don’t realize the mare is on fescue or think they have managed it appropriately when they haven’t. The mare will then require therapy to mitigate the effects of ergot alkaloid toxicosis; however, (keep in mind that) once you begin domperidone therapy, you lose the ability to monitor mammary secretions for impending parturition.”

While tall fescue grows all throughout North America, it’s most prevalent in the southeastern states.

Abdominal Wall Disruption

An abdominal wall disruption in late gestation might be due to an abdominal hernia (when the body wall muscles tear) or a prepubic tendon rupture (when the tendon that supports the abdominal wall breaks, allowing the heavy pregnant uterus to drop), says Sertich. These scenarios might also disrupt the mammary gland support system, allowing the gland to drop and move forward.

Sertich says owners should suspect an abdominal wall disruption if the mare’s mammary secretions are bloody or edema (fluid swelling) is forming high in the flank area. Mares will appear quite painful and be reluctant to move. Keep these mares on stall rest with a body wall support bandage to allay some of the pressure on that area. She says it’s best to house them at a location where veterinary assistance is available for delivery.

Abdominal hernia
While abdominal hernias can be life-threatening, this mare with a bilateral (in both flanks) abdominal hernia survived, as did her foal, thanks to veterinary assistance during delivery. | Photo Courtesy Dr. Patricia Sertich

“In late gestation, most of the cases I’ve seen have been abdominal hernia, but both prepubic tendon rupture and abdominal hernia end up causing the same problem,” a visible failure of the abdominal wall, says Sertich. “The outcome is quite variable. Some mares will foal and be okay, while others will herniate the abdominal organs, which is quite devastating.”

Hydrops, an excessive accumulation of fetal fluid in the uterus, can also disrupt the abdominal wall. It’s characterized by a greatly distended uterus and an enlarged abdomen. Veterinarians diagnose it by ­transrectal palpation and ultrasound.

“We don’t understand why hydrops occurs, but the excessive fluid produced within the fetal membranes puts the mare at risk of her body wall not being able to support the enlarged uterus and … ­rupturing before the foal is mature enough to be born,” says Sertich. “If the mare is near term, you can do a ­slow-­controlled drainage of the uterine fluid while administering intravenous fluids to the mare to prevent shock, and then manually deliver the foal. If the foal is mature enough, it may be fine. If it’s too early in gestation, you may have to sacrifice the fetus to save the mare’s life.”

Horse laying down
If a mare shows signs of colic, such as pawing at the ground or rolling, your veterinarian might suspect a uterine torsion. | Photo: iStock

Uterine Torsion

Uterine torsion, or twisting of the ­uterus, typically occurs between five and 11 months of gestation. “If we have a mare that’s showing signs of colic—pawing at the ground, rolling—a uterine torsion should be suspected,” Ferris says.

The severity of the clinical signs depends on how much the uterus is twisted and if other abdominal organs are involved. “If it’s just the uterus twisted, it may not be as compromised,” Sertich says. “If the gastrointestinal tract is involved, it can be violently painful. It’s seen both in mares that are pregnant for the first time and mares that have had a number of foals. ”

Veterinarians diagnose torsion via rectal palpation and ultrasound to evaluate the position and character of the broad ligaments supporting the uterus. “If caught early, there is often a good resolution for both mare and foal,” says Ferris. “The most common management is surgery—either a standing surgery or under general anesthesia.”

Uterine Torsion
Here we can see a mare’s twisted uterus upon post-mortem exam. The top and bottom arrows point to the left and right uterine horns, respectively. In the normal mare, you would not see dark red areas on the horns, as shown here, and they would not be twisted. | Photo: Courtesy Dr. Patricia Sertich

“It’s one of the few abdominal surgeries that’s preferred to perform on a standing horse, because it is easier to flip the uterus back into proper position if the mare is standing,” Sertich says. “The mare is heavily sedated, the flanks are blocked (meaning anesthetic agents are used to numb the area), and the uterus is repositioned. Once you get the uterus repositioned, the mares usually go on to term and foal without incident.”

Take-Home Message

Monitor your broodmares regularly, and communicate any possible problems to your veterinarian without delay. “Owners know their mares well,” says Ferris. “If they notice something is off, I encourage them to have the mare evaluated. With early intervention a good, successful outcome for mare and foal is possible.”

Simple daily horse management goes a long way toward keeping mares healthy. Mare owners and managers should keep an eye on their mares’ body condition, check the vulva and mammary glands daily—particularly as mares near term—and watch for signs of edema, says Sertich.

While every case is different, the overall outlook for mares experiencing late-term gestational problems is positive. “For a lot of these conditions, the mare will be initially evaluated in the field,” she says. “For example, uterine torsion may be diagnosed in the field, but will then need to be referred in for a surgical procedure. Conditions like uterine torsions are typically recognized early now, giving good success rates. Early referral for treatment is key to a good outcome.”

When things do go wrong and a mare loses her foal, Ferris encourages veterinary follow-up. “Have your veterinarian check that the mare is systemically and reproductively healthy,” he says. “Collect (tissue) samples to try to determine why abortion occurred. This is important for both future management and to assess risk factors for other pregnant mares on the property.”


Written by:

Freelance journalist Natalie DeFee Mendik is a multiple American Horse Publications editorial and graphics awards winner specializing in equestrian media. She holds an MA in English from Colorado State University and an International Federation of Journalists’ International press card, and is a member of the International Alliance of Equestrian Journalists. With over three decades of horse experience, Natalie’s main equine interests are dressage and vaulting. Having lived and ridden in England, Switzerland, and various parts of the United States, Natalie currently resides in Colorado with her husband and two girls.

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