Uterine Infections

Uterine infections in mares represent one of the most common reproductive problems affecting horses, and understanding what causes them, how to recognize them, and what treatment options exist can make a real difference in a mare's fertility and overall health. The uterus is protected by several defense mechanisms that normally keep bacteria out, but when those defenses fail—whether from breeding injury, retained placenta, poor drainage, or other factors—infection can develop. Unlike some reproductive conditions that are purely cosmetic concerns, uterine infections directly threaten a mare's ability to conceive and maintain pregnancy, making them significant both for breeding programs and for general mare health.

Uterine infections fall into several categories depending on whether they're acute or chronic, whether they involve fluid accumulation, and what organisms are responsible. Endometritis refers to inflammation of the uterine lining, often with bacterial infection. Pyometra refers specifically to pus accumulation in the uterus, a more serious condition that requires prompt treatment. Some mares develop chronic low-grade infections that don't cause obvious symptoms but damage the uterine environment enough to prevent conception. Understanding which type of infection your mare has matters tremendously for determining the right treatment approach.

The reason these infections matter extends beyond whether your mare conceives during the current breeding season. Repeated or chronic uterine infections cause scarring and damage to the uterine lining, permanently reducing fertility even after the infection clears. A mare who has suffered multiple uterine infections or chronic infection might lose a significant portion of her reproductive value, even if treatment eventually eliminates the active infection. This makes early recognition and aggressive treatment of uterine infections important for preserving fertility long-term.

Mares of all ages can develop uterine infections, though older mares and those with a history of breeding complications are at higher risk. Young mares experiencing their first pregnancies sometimes develop infections from breeding-related injury. Mares that have had retained placentas, dystocia, or post-partum complications are at increased risk for uterine infection. Mares used heavily in breeding programs, particularly those bred during postpartum estrus, face higher risk from the accumulated trauma of frequent breeding. Understanding your individual mare's risk factors helps guide preventive care and early treatment.

This guide covers the causes of uterine infections, how they develop, what symptoms to watch for, how veterinarians diagnose them, what treatment approaches are available, and how to manage a mare's recovery and long-term reproductive health. Understanding these factors empowers you to recognize problems early and work with your veterinarian to preserve your mare's fertility.

Uterine infections develop when bacteria enter the uterus and overcome the mare's natural defenses. The uterus is normally protected by several mechanisms: the cervix acts as a physical barrier, the uterine lining produces immune factors that kill bacteria, and the uterus has some capacity to drain infected material through the cervix. When any of these mechanisms fail, infection can establish. Understanding what causes these defenses to fail helps prevent infection and guides treatment decisions.

Breeding trauma represents the most common cause of uterine infections, particularly in mares bred repeatedly during short intervals. The breeding process itself can cause small tears or irritation in the uterine lining and cervix, especially if breeding is rough or if the mare isn't properly prepared. A single instance of breeding trauma might be insignificant, but repeated breeding during short cycles accumulates damage that increases infection risk. This is why mares bred intensively during the breeding season, or bred during the postpartum estrus shortly after giving birth, have higher infection rates than those with longer intervals between breedings.

Retained placenta creates a perfect environment for uterine infection. When the placenta doesn't separate and pass normally within a few hours after foaling, remaining placental tissue and the wound area it creates become contaminated with bacteria. Mares with retained placenta develop uterine infections in a high percentage of cases unless treated aggressively. The longer the placenta remains, the greater the infection risk and the more damage occurs to the uterine lining. This is why retained placenta is considered a veterinary emergency requiring immediate treatment.

Poor uterine drainage affects mares whose uterus can't empty fluid and contaminated material normally. Some mares, particularly older ones or those with conformation issues, have a uterus that's positioned in a way that prevents complete emptying. Fluid accumulates, bacteria proliferate in that stagnant environment, and infection develops. Pneumovagina, or air aspiration into the vagina, is related to this problem—mares whose vulvar anatomy allows air entry also often have drainage problems, creating conditions favorable for infection.

Postpartum complications beyond retained placenta also increase infection risk. Dystocia—difficult birth—causes tissue trauma that can introduce infection. Red bag delivery, where the placenta ruptures before the foal delivers, exposes the uterus to contaminated material. Vaginal or cervical lacerations during foaling create entry points for bacteria. Any birth complication that damages or contaminates the reproductive tract increases subsequent infection risk. Mares with severe foaling trauma require particularly aggressive post-partum monitoring for signs of infection.

Age influences uterine infection risk in multiple ways. Older mares develop age-related changes in the uterus that impair drainage and immune function. The cervix becomes less competent, allowing bacteria entry more easily. The uterine lining loses some capacity to resist infection. Mares in their late teens or twenties have substantially higher infection rates than younger animals, even with otherwise similar management. This is why careful veterinary management of older mares during breeding is important for maintaining fertility.

Management factors also contribute to infection risk. Mares kept in dirty conditions, with poor sanitation in breeding areas or stalling, have higher infection rates. Mares managed in groups where they kick and injure each other's reproductive tracts face increased risk. Mares standing in urine-contaminated bedding or stalls have greater exposure to bacteria. General health status matters too—mares with poor nutrition, chronic illness, or immune compromise have less capacity to fight infection. Even factors like extreme stress can reduce immune function and increase infection susceptibility.

Recognizing uterine infection in mares requires understanding that symptoms can range from obvious to subtle, and some infected mares show remarkably few external signs despite having active infection. The most common symptom is vaginal discharge—a thick, purulent discharge that often appears worse during estrus when the cervix is open. This discharge might smell bad, might be streaked with blood, or might be obvious only when the mare is in estrus. Some mares have discharge that's noticeable between estrous cycles, while others only show obvious discharge during heat. The character and timing of discharge provides important clues to your veterinarian about what's happening uterinally.

Estrous abnormalities frequently accompany uterine infection. A mare might have prolonged estrus, cycling for weeks rather than the normal five to seven days. Some mares have shortened estrous cycles, returning to heat every few days instead of normally every 21 days. Others fail to develop normal estrous signs, showing minimal external indication that they're cycling. Still others cycle normally, deceiving owners into thinking everything is fine when in fact infection is present. Changes in your mare's normal cycling pattern warrant investigation, particularly if she's been bred recently or has postpartum history.

Behavioral changes sometimes accompany uterine infection, though these can be subtle. A mare might be slightly less willing to be ridden or handled. She might seem irritable or sensitive to the touch around the flanks and abdomen. Some mares show mild colic-like discomfort with low-grade abdominal pain. These signs are easy to attribute to other causes, which is why mares with vague behavioral changes should have veterinary evaluation, particularly if they've had breeding-related incidents or postpartum complications.

Fertility problems are often the first clue that uterine infection is present. A mare that's been bred but fails to conceive might have uterine infection preventing pregnancy establishment. A mare that gets pregnant initially but loses the pregnancy might have infection damaging placental development. A mare with repeated breeding losses might have chronic uterine infection that prevents normal fetal support. When mares fail to conceive despite being bred during appropriate estrous phases, uterine infection becomes an important diagnostic consideration.

Some mares with uterine infection show no obvious external signs whatsoever. The infection is present and causing damage, but the mare seems completely normal—normal estrus, normal discharge, normal behavior. These subclinical infections are discovered only through veterinary investigation, often when a breeding program isn't producing expected results. This is why mares with a history of reproductive complications benefit from preventive veterinary evaluation, even if they seem perfectly fine.

Emergency symptoms develop when infection progresses to severe pyometra or systemic infection. Severe abdominal pain, high fever, depression, anorexia, or signs of sepsis indicate serious infection requiring immediate veterinary attention. Mares with emergency signs need hospitalization and intensive treatment. These presentations are less common than low-grade chronic infection, but they represent true medical emergencies.

Diagnosis of uterine infection begins with veterinary examination and evaluation of discharge if present. Your veterinarian will take a complete reproductive history, ask about breeding frequency, any dystocia or pregnancy loss, and postpartum complications. Observation of estrous behavior and examination of any vaginal discharge provides initial clues. However, definitive diagnosis requires investigation of the uterus itself, which can't be adequately evaluated by external examination alone.

Ultrasound examination of the uterus is the primary diagnostic tool for identifying fluid accumulation, wall thickening, and other changes consistent with infection. A mare with endometritis often shows fluid in the uterine horn, visible as anechoic (dark) areas within the uterine tissue on ultrasound. Uterine wall edema might be visible. The extent and character of fluid provides information about the severity of infection and response to treatment. Serial ultrasounds during treatment help assess whether infection is resolving or persisting.

Uterine culture and cytology involve collecting a sample from the uterus and submitting it for bacterial culture and analysis of cells. Culture identifies what bacteria are present, allowing selection of antibiotics most likely to be effective. Cytology identifies the inflammatory response, helping characterize whether the infection is active and what type of inflammation is present. These samples are crucial for understanding the specific infection and selecting effective treatment. A sample showing normal bacteria with minimal inflammation might not warrant aggressive treatment, while one showing pathogenic organisms with heavy inflammatory response demands intensive intervention.

Cervical palpation and examination might show cervical competence issues or evidence of injury. Some mares have obvious cervical problems contributing to infection susceptibility. Identification of these issues helps guide both treatment and future management decisions, particularly regarding breeding practices and preventive care.

Treatment of uterine infection ranges from conservative observation to aggressive intervention depending on the severity and character of infection. Mild endometritis with minimal symptoms in a non-breeding mare might be observed closely without treatment, particularly if the infection appears to be self-limiting. However, most cases warrant active treatment, particularly in breeding mares where infection threatens fertility.

Intrauterine infusions represent the primary treatment for most uterine infections. Solutions containing antibiotics, immune factors, or dilute antiseptics are flushed into the uterus repeatedly, delivering high local concentrations of medication directly to the infected tissue. Treatment might occur daily for several days, then every few days, depending on response and the character of infection. Intrauterine infusions are more effective than systemic antibiotics alone because they achieve much higher drug concentrations in the uterus than can be achieved through injection or oral medication.

Systemic antibiotics are given concurrently with intrauterine treatment, usually selected based on culture results when available. Common choices include penicillin-based antibiotics, aminoglycosides, fluoroquinolones, or others depending on culture results and the severity of infection. Duration of systemic treatment varies but often continues for one to two weeks or longer for severe infections.

Immunostimulants and supportive treatment enhance the mare's own immune response to infection. Some practitioners use immune-stimulating treatments like interferon or plasma transfusions to boost the mare's ability to fight infection. General supportive care—good nutrition, appropriate exercise, stress reduction—supports overall health during treatment.

Prognosis for recovery from acute uterine infection is generally good if treatment begins promptly and is thorough. Most mares with acute endometritis recover completely with appropriate treatment and return to normal fertility. Chronic infections or infections that have caused significant uterine damage might have more guarded prognosis. The longer infection persists before treatment, the greater the risk of permanent fertility damage. This is why early recognition and aggressive treatment make such a difference in outcomes.

During treatment of uterine infection, close communication with your veterinarian helps ensure that therapy is achieving results and adjustments are made if the mare isn't responding as expected. Many mares require multiple intrauterine infusions over weeks, with ultrasound rechecks to confirm that infection is resolving. Some infections clear quickly with treatment, while others are stubborn and require extended therapy. Understanding that your mare might need weeks of treatment rather than days helps set realistic expectations.

Activity restriction during acute uterine infection generally means the mare shouldn't be ridden or engaged in strenuous exercise while being actively treated. Light exercise like hand-walking is usually acceptable and might be encouraged. As infection clears and treatment concludes, activity can gradually increase back to normal. The timeline for return to work or breeding depends on how completely the infection has resolved and whether any damage to the uterus has occurred.

Nutrition during and after infection treatment should support immune function and tissue healing. Quality hay, appropriate grain or concentrate, and good-quality proteins support recovery. Some mares benefit from additional supplements—vitamin E, selenium, probiotics—that support immune and digestive health. Avoid making major feeding changes while the mare is being treated, but ensure overall nutrition is adequate for supporting recovery.

Estrual management during and after treatment requires coordination with your veterinarian. Treatment decisions might be timed relative to estrous cycle—some practitioners prefer treating early in the cycle, others during different phases. Once infection clears, most mares return to normal cycling within a few cycles, though some need longer. Your veterinarian will advise on when it's safe to breed again, which might not be during the first estrus after infection clears if the uterus still shows signs of inflammation or if the mare isn't cycling normally.

Monitoring for recurrence is important during the weeks and months following successful treatment. Some mares have repeated infections, particularly if the underlying cause (like poor drainage or cervical incompetence) hasn't been addressed. Watching for return of discharge, abnormal estrous cycles, or fertility problems helps catch recurrence early if it develops. If a mare has another infection shortly after treatment of the first, more aggressive investigation of underlying causes is warranted.

Long-term reproductive management of a mare with history of uterine infection should consider what factors contributed to the infection and how to prevent recurrence. Mares with poor uterine drainage might benefit from careful positioning during breeding or might need repeated preventive infusions during breeding season. Mares with cervical incompetence might need to be bred less frequently or might benefit from cervical treatments. Postpartum mares need careful monitoring for signs of complications. Each mare's management should be individualized based on what caused her infection and her subsequent response to breeding and pregnancy.

Prevention of uterine infection in breeding mares starts with recognizing risk factors and implementing management strategies that reduce infection likelihood. For mares being used in breeding programs, limiting the frequency of breeding and allowing adequate recovery time between cycles reduces the cumulative trauma that predisposes to infection. Mares bred no more than every other estrous cycle have lower infection rates than those bred during consecutive cycles. Avoiding breeding during postpartum estrus, or at minimum limiting breeding during this period, reduces the risk in recently foaled mares.

Breeding hygiene matters tremendously for infection prevention. Cleanliness at breeding time, thorough washing of the mare's perineum before breeding, and use of clean breeding equipment reduce bacterial contamination. Some mares benefit from prophylactic intrauterine infusions after breeding—washing out the uterus or infusing protective solutions reduces infection risk in high-risk mares or those with previous infection history. Discussing preventive approaches with your veterinarian helps develop a strategy appropriate for your individual mare.

Prompt treatment of breeding complications prevents progression to serious infection. Retained placenta requires immediate attention within hours, not days. Any bleeding, fever, or signs of distress in the postpartum period warrant emergency evaluation. Dystocia and difficult births should be managed by experienced personnel and followed by close monitoring for postpartum complications. Mares with conformation issues like pneumovagina that predispose to infection might benefit from surgical correction to restore normal anatomy.

Early recognition and treatment of infection prevents chronic disease and permanent fertility damage. A mare with abnormal discharge or estrous irregularities warrants investigation before she's bred again. If infection is present, identifying and treating it before pregnancy is attempted prevents losses from infected pregnancies. Some veterinarians recommend pre-breeding evaluations for mares with any history of reproductive complications, using ultrasound and culture to identify and treat infection before breeding season begins.

Prognosis for uterine infection depends primarily on how severe the infection was, how promptly treatment was started, and whether any permanent damage to the uterus occurred. Most acute infections have excellent prognosis for recovery and return to fertility if treated appropriately. Chronic infections or those that caused severe scarring might permanently reduce fertility despite successful treatment of the active infection. Older mares or those with multiple previous infections might have guarded prognosis because the cumulative damage from repeated infections gradually destroys the uterus's capacity to support pregnancy.

Recurrence risk is significant for some mares, particularly those with underlying causes like poor drainage that haven't been corrected. Mares that have had uterine infection have higher risks for repeat infection than those with no history. Careful management and preventive strategies reduce recurrence risk significantly. Some mares successfully breed and carry pregnancies to term after uterine infection treatment, while others have repeated problems. The individual mare's anatomy, age, and specific circumstances determine her long-term fertility outlook.