Endometritis Diagnosis and Management

When a mare is bred by natural cover, the uterine lining, or endometrium, becomes inflamed. The stallion deposits semen in the uterus, as well as bacteria, bits of debris, and seminal fluid. The normally sterile uterus becomes irritated and inflamed, resulting in acute endometritis (inflammation of the endometrium). This breeding- associated endometritis typically lasts only one to two days because the uterine muscles respond to the inflammation and begin to contract, pushing the excess fluids and debris out through the cervix. About 48 hours after breeding, the uterus begins to return to normal and prepare to accept the newly formed embryo.

Breeding-associated endometritis doesn't involve infection, but rather inflammation. When this inflammation progresses to infection, however, it is still considered endometritis. Confusing? Here's more. Infectious endometritis can involve a variety of bacteria, including Streptococcus, Escherichia coli, and Pseudomonas, or it can be fungal, involving organisms such as Aspergillus or Candida. Fungal endometritis is often a chronic problem. In some mares, inflammatory endometritis is a chronic problem, but there might not be any infectious organisms involved.

Endometritis, to say the least, is a complex disease.

The first step in understanding endometritis is learning the situations that put a mare at risk. Endometritis can develop at other times besides the post-breeding period, and there are predisposing factors that can lead to recurring endometritis in susceptible mares.

Predisposing Factors

Anatomy In the mare, the area between the anus and the vulva is known as the perineum. There is bony ridge, called the pelvic brim, which sits behind the perineum and extends from left to right. The anus sits above the pelvic brim, and the folds of the vulva sit below. In a normal, healthy mare, the anus and the vulva are vertically aligned.

The vulva's large external folds normally remain closed and prevent feces, dirt, air, and other contaminants from entering the vestibule (which sits just inside the vulva and houses the opening of the urethra) and vagina. The vestibule leads into the vagina, which ends at the cervix. The cervix is usually closed--every place that the tract is closed, there is a barrier to prevent the entrance of outside organisms or debris. However, the sterile portion of the tract is beyond the cervix.

All points external to the cervix (vulva, vestibule, and vagina) contain natural populations of organisms that live in the reproductive tract, while all points beyond the cervix (uterus and oviducts) are sterile and do not contain these organisms.

Problems can arise when there are conformation faults in the perineal area and the mare lacks the important vertical orientation of the anus and vulva. Some mares are born with poor conformation. The usual fault is characterized by a recessed anus, which then pulls the vulva up toward the pelvic brim. In this position, the vulva tends to open, which increases the chances of fecal contamination inside the vagina and resulting endometritis.

Training Young Thoroughbred fillies must achieve and maintain a very high level of fitness in order to race. This degree of fitness significantly reduces body fat, including fat in the perineum. These fillies often have the classic changes in conformation described above, with a recessed anus and displaced vulva. With exercise and fatigue, air can be drawn in through the vulva and into the vagina. Air carries many contaminants. This "windsucking," or pneumovagina, can lead to repeated bouts of endometritis. To complicate things, racing Thoroughbred fillies are often placed on progesterone-type medications to prevent normal reproductive cycling. These progesterone analogues reduce the ability of the uterus to fight infection.

Previous Foals Mares that have carried several foals to term have physical changes within the reproductive tract that can increase their likelihood of developing endometritis. These changes are related to breakdowns in the animals' natural anatomical barriers to infection.

As a broodmare ages, there is an increased risk of complications during foaling and postpartum that can make her more susceptible to endometritis. These include abnormal deliveries, retained placentas, and delayed uterine involution (return to normal size).

Anytime a foaling requires assistance, there is a significant risk of internal injury to the mare. Lacerations to the cervix, vestibule, and vulva can occur and can lead to adhesions and scarring. Because these organs are physical barriers to infection, the broodmare is then less able to prevent contamination of the reproductive tract.

Cervical lacerations can be the most serious because of the importance of maintaining sterility in the uterus.

Once a barrier structure is damaged, there is a chance of repeated problems with infection, and the mare might suffer chronic bouts of endometritis.

Once the foal is delivered and the placenta is expelled, healthy mares undergo the process of uterine involution over a period of seven to 12 days. If a mare foaled while in poor body condition or a poor nutritional state, had a very difficult delivery, or foaled in filthy conditions, etc., uterine involution can be delayed. This is a problem because the process of involution is important for cleaning out the afterproducts of delivery and preventing the onset of postpartum endometritis.

The cervix must tighten and close in order to restore its important barrier function. Uterine and cervical involution can be delayed if the placenta is retained after foaling. The longer the placenta stays attached to the uterine wall and extends through the cervix to the exterior environment, the greater the chances that organisms will travel into the uterus and acute endometritis will develop.

Diagnosing Endometritis

Endometritis is often diagnosed prior to breeding, when broodmares are routinely examined for uterine infection. A veterinarian performs an ultrasound examination of the uterus to locate any free fluid that might be present, which usually indicates endometritis. It's important to note, however, that the absence of fluid on ultrasound examination doesn't rule out endometritis.

Michael Whitacre, DVM, MS, Dipl. ACT, associate professor of theriogenology at North Carolina State University's College of Veterinary Medicine, says he notes an absence of fluid in the uterus quite frequently, especially in the early stages of an infection.

"It usually depends on the organism that is causing the endometritis," he says, "the duration of the infection, and the state of the mare's uterine defense system."

With or without fluid present, when endometritis is suspected, additional steps are needed to determine if infection is present.

The next step in the diagnostic process is gathering samples for uterine cytology and culture. The veterinarian uses cytology to take a look at the types of cells that are present within the endometrium. The presence of different blood cell types indicates different kinds of inflammation or infection. A culture of endometrial fluid samples is used to help specifically identify infectious organisms, if they are present.

It is very important that the veterinarian consider both cytology and culture when evaluating endometritis.

Cytology alone can miss a low level of infection and provide a false negative result, leading the veterinarian to decide not to treat a mare. When culture is used alone, it can also be misleading. False positive results can occur if the culture swab is contaminated as it is introduced through the vagina and cervix.

But used together, cytology and culture provide valuable information about what is happening in the uterus.

Managing Endometritis

Whenever possible, the veterinarian and horse owner should take steps to prevent endometritis. Using procedures to clean the mare and stallion prior to breeding, providing a clean environment for foaling, and planning for routine pre-breeding and postpartum veterinary examinations are all tactics for endometritis prevention.

Predisposing factors such as conformation faults can't be prevented, but some of the specific problems related to conformation faults can be managed.

For example, there is a specific procedure available to address the common problem of a recessed anus and displaced vulva that makes the vulva prone to opening.

The procedure, known as a Caslick's surgery, involves temporarily suturing the vulvar lips closed to prevent feces, debris, and air from entering the vagina and contaminating the uterus. The Caslick's procedure, also called vulvoplasty, is a common procedure performed in racing Thoroughbreds and other performance mares affected with pneumovagina. The procedure has proven to be effective in preventing repeat cases of infectious endometritis.

When reproductive barrier structures such as the vulva, vestibule, or cervix are damaged, and particularly when any sustain lacerations, it is vital that a veterinarian repair the structure as soon as possible so that it can heal with minimal scarring.

Anytime a mare has a difficult delivery, a veterinarian should be called to examine the mare for lacerations.

When endometritis can't be prevented, it must be properly treated to give the mare the best chance of complete recovery.

As mentioned previously, there are two general types of organisms that cause infectious endometritis--bacteria and fungi. Once the exact organism is identified, the proper antimicrobial drug can be chosen for treatment based upon additional laboratory testing.

There are a variety of antibiotics available for the treatment of bacterial infections in the uterus. Fungal infections, which are more likely to cause chronic endometritis, are treated with antifungal agents.

Uterine lavage is the first choice for delivering both kinds of medications, as this procedure delivers the medication directly to the endometrium. Typically, the drugs are dissolved in sterile fluid, then passed into the uterus through a sterile tube.

Several hours later, after the medication has had enough time to take effect, oxytocin is administered to stimulate the uterus to contract and expel any remaining fluid that has not been absorbed. The lavage treatment is repeated once a day for several days to eliminate the infection.

In mares that are prone to persistent post-breeding endometritis, the veterinarian might prescribe systemic antibiotics each time the mare is bred. Research has shown that about 15% of these mares have defects in the uterine muscles and can't expel fluid and debris the way normal mares do after breeding.

On occasion, veterinarians also give systemic antibiotics to mares that have had endometrial biopsy results that indicate deep tissue inflammation.

The use of systemic antibiotics is not without controversy, however.

"It's difficult to get adequate amounts of antibiotic at the level of the endometrium," explains Whitacre, who does not advocate using systemic antibiotics to treat endometritis. "It's also difficult to get adequate levels in the uterus in the face of a significant amount of fluid that may contain a lot of cellular debris."

Take-Home Message

Endometritis is a complicated disease that can significantly impact the health and reproductive career of a broodmare.

Whether it's a one-time infection, a recurring post-breeding inflammation, or a chronic fungal problem, endometritis is a formidable disease that is best avoided whenever possible. Steps should be taken to minimize predisposing factors and optimize diagnostic and treatment plans.


  1. Hurtgen, J.P. Pathogenesis and treatment of endometritis in the mare: A review. Theriogenology, 66, 560-566, 2006.
  2. Rigby, S.L.; Barhoumi, R.; Burghardt, R.C.; et al. Mares with delayed uterine involution have an intrinsic defect in myometrial function. Biol Reprod, 65(3), 740-747, 2001.

3. N. Edward Robinson, ed. Current Therapy in Equine Medicine 5. Philadelphia: WB Saunders, 2003.

About the Author

Susan Piscopo, DVM, PhD

Susan Piscopo, DVM, PhD, is a free-lance writer in the biomedical sciences. She practiced veterinary medicine in North Carolina before accepting a fellowship to pursue a PhD in physiology at North Carolina State University. She lives in northern New Jersey with her husband and two sons.

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