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Endometritis is a condition involving inflammation of the uterine lining, and is a very common cause of infertility in broodmares. Mares with endometritis develop fibrosis and inflammation that prevent embryonic development, resulting in failure to conceive or early embryonic loss.
Endometritis comes in two major types: infectious endometritis, caused by bacterial or fungal infections, and persistent breeding-induced endometritis, in which the uterine lining reacts to seminal fluid within the uterus. The conditions have similar treatment approaches, and mares are sometimes affected by both forms of the condition.
Symptoms of endometritis are subtle, typically manifesting as an inability to conceive despite multiple breeding attempts. Some mares may have uterine fluid accumulation prior to or after breeding, vaginal discharge, or early embryonic loss after successfully conceiving.
Diagnostic testing aims to identify the type of endometritis and its severity, to determine an appropriate treatment protocol. Common diagnostics include bacterial or fungal culture, uterine lining biopsies, and ultrasound examination of the uterus.
Endometritis treatment may include antibiotics, repeated uterine lavages, surgical procedures, medications to treat underlying problems within the uterus, and changing breeding strategies. With appropriate management, many mares can successfully conceive despite having endometritis.
Endometritis in Horses
Endometritis refers to inflammation of the uterine lining, also known as the endometrium. It is one of the most common causes of infertility in broodmares.
Infectious endometritis affecting approximately 25-60% of barren mares, while persistent breeding-induced endometritis (PBIE) affects around 15% of all broodmares. [1]
Some mares have components of both types of endometritis, requiring extensive diagnostic investigation and treatment to improve their chances of conceiving.
Infectious Endometritis
Infectious endometritis occurs when there is an abnormality in the uterus’ ability to clear contamination. Without being able to remove contaminants from the uterus, bacterial or fungal pathogens proliferate, causing inflammation.
The most common bacteria involved in infectious endometritis are: [2][3][4]
- Streptococcus equi zooepidemicus
- Staphylococcus
- Escherichia coli
These pathogens reside in the environment or are part of the normal reproductive flora in horses, and gain access to the uterus through the cervix.
The most common fungal organisms present in the uterus of mares with endometritis are: [4]
- Yeast (Candida spp.)
- Aspergillus spp.
- Mucor spp.
Fungal organisms are detected in 1-5% of mares with infectious endometritis. Certain physical conformations or the use of antibiotics to treat or prevent endometritis can allow fungi to colonize the reproductive tract. [4]
Pathophysiology
Under normal conditions, the uterine environment is relatively sterile due to a firmly closed cervix preventing bacterial entry, and uterine contractions to remove any contaminants that do accumulate. [4]
For infectious endometritis to occur, there must be a failure in the protective mechanisms of the uterus, or the amount of bacteria present must overwhelm the ability of the uterus to clear the infection.
Major predisposing factors include:
- Poor vulvar conformation, allowing feces or urine to contaminate the reproductive tract
- Abnormalities in closure of the cervix, allowing bacterial entry
- Inability to contract the uterus to clear contaminants, usually associated with increased mare age or having multiple previous foals
- Immunosuppression reducing the efficacy of the local immune system. [1][3]
After the bacteria enter the reproductive tract, they often form a biofilm – a layer of bacteria adherent to the uterine lining. Within a biofilm, bacteria are resistant to antibodies produced by the immune system and to antibiotics, making treatment challenging. [4]
Over time, inflammation caused by the bacteria results in fibrosis, or scarring, of the uterine lining. [4] Fibrosis prevents embryos from implanting in the uterine lining, and impairs nutritional support of the embryo if it does implant.
Persistent Breeding-Induced Endometritis
Persistent breeding-induced endometritis (PBIE) is similar to infectious endometritis, however inflammation develops in response to sperm within the uterus rather than bacteria. [4][5] In some cases, bacteria are also introduced during breeding, resulting in PBIE and infectious endometritis occurring simultaneously.
In normal mares, sperm within the reproductive tract stimulates inflammation which triggers uterine contractions and an immune response to remove any potential contaminants introduced into the reproductive tract by the sperm. These mechanisms restore the sterile environment of the uterus within 24 to 36 hours, in preparation for the embryo to implant in the uterine wall. [4][6][7]
In mares with poor clearing mechanisms, such as inability to contract the uterus or immunosuppression, sperm and other contaminants remain within the uterus for longer than 36 hours. [4][5] The sperm continue to stimulate inflammation, which ultimately results in fibrosis of the uterine lining.
Risk Factors
Older mares are most at risk for endometritis for several reasons. Aging is associated with weakening of the uterine muscles, reducing contractile ability during the post-breeding period or if contaminants enter the uterus.
Similar changes can occur in the cervix, resulting in incomplete closure of the cervix allowing for bacterial entry. [4]
Older mares can also develop degeneration of the uterine lining due to aging, resulting in fibrosis without infectious endometritis or a PBIE event. [3]
Clinical Signs
The major symptom of endometritis is infertility, usually noticed as failure to conceive after multiple breeding attempts. Other symptoms of endometritis include: [3][13]
- Uterine fluid build-up before and/or after breeding
- Early embryonic loss after successful breeding
- Vaginal discharge
- Vaginitis (inflammation of the vagina)
- Shorts intervals between estrus (heat) cycles
- Excessive endometrial edema (swelling) after mating
Some mares have subclinical endometritis, meaning they exhibit few or no overt symptoms. In such cases, the condition is often diagnosed during reproductive evaluations or when the mare repeatedly fails to conceive.
Diagnosis
Mares bred for several cycles in a row that fail to become pregnant likely have undiagnosed endometritis. Diagnostic testing focuses on identifying the type of endometritis and the severity.
Your veterinarian may conduct several of the following to confirm a diagnosis of endometritis: [1][3][4][8]
- Breeding history
- Assessment of vulvar conformation
- Vaginal speculum examination of the cervix
- Ultrasound examination of the uterus
- Cytology (sampling of cells) of the endometrium
- Biopsy (sampling of tissue) of the endometrium
- Bacterial or fungal culture
- Endoscopy examination of the uterus
Endometrial Biopsies
The gold standard for the diagnosis of endometritis is the endometrial biopsy, where a sample of the uterine lining is submitted for evaluation.
Under the microscope, pathologists can evaluate inflammation, the presence of fibrosis, lymphatic and vascular changes, and whether the uterine lining is appropriately developed for the mare’s seasonal cycle. [3][9]
In cases of PBIE, taking samples before and after breeding can identify i