Equine Reproductive Disorders: Diagnosis and Management

At a Glance

Equine reproductive disorders affect breeding success in both mares and stallions. Early recognition of clinical signs, accurate diagnostic testing, and appropriate management decisions improve outcomes. The table below summarizes common disorders, primary diagnostic methods, and key management considerations.

Understanding Equine Reproductive Disorders

Reproductive disorders in horses encompass conditions that interfere with conception, pregnancy maintenance, or fertility in both mares and stallions. These disorders may be infectious, genetic, structural, or hormonal in origin. The Merck Veterinary Manual provides general reference information on horse health conditions, including reproductive system diseases (www.merckvetmanual.com/horse-owners). Breeders and veterinarians must recognize clinical signs early and pursue appropriate diagnostic testing to implement effective management strategies.

Infectious causes of reproductive failure include bacterial, viral, and protozoal agents. Contagious equine metritis (CEM) is caused by Taylorella equigenitalis and is a reportable disease in many jurisdictions. The World Organisation for Animal Health (WOAH) provides international standards for animal health and welfare, including guidelines for managing contagious reproductive diseases (www.woah.org/en/what-we-do/animal-health-and-welfare). Non-infectious causes include genetic disorders, hormonal imbalances, and anatomical abnormalities.

Endometritis in Mares

Endometritis is inflammation of the endometrial lining of the uterus and is a leading cause of infertility in mares. The condition can be acute or chronic and may result from bacterial infection, fungal infection, or persistent mating-induced inflammation. A review of endometritis in domestic species discusses general and comparative aspects of this condition (Reproduction in Domestic Animals, 2023, PubMed ID 37191856). Mares with poor uterine clearance mechanisms are particularly susceptible.

Clinical Signs and Risk Factors

Mares with endometritis may exhibit vaginal discharge, shortened interestrous intervals, or failure to conceive. Some mares show no external signs. Risk factors include advanced age, poor perineal conformation, pneumovagina, and a history of retained placenta or dystocia. Mares that accumulate intrauterine fluid after breeding are at increased risk for persistent endometritis.

Diagnostic Approach

Diagnosis of endometritis requires a systematic approach. Uterine culture and cytology are the primary diagnostic tools. A guarded swab or double-guarded culture instrument is used to collect samples from the uterus. Cytology samples are evaluated for the presence of neutrophils, which indicate inflammation. Ultrasound examination of the uterus can detect intrauterine fluid accumulation and changes in endometrial thickness.

Uterine biopsy provides information about endometrial health and fibrosis. The biopsy is graded based on the degree of inflammation and fibrosis, which helps predict future fertility. Mares with severe endometrial fibrosis have a poor prognosis for carrying a pregnancy to term.

Management Considerations

Treatment of endometritis depends on the causative agent. Bacterial endometritis requires identification of the organism and determination of antimicrobial susceptibility. Uterine lavage with sterile fluids helps remove inflammatory debris and bacteria. Mares with poor uterine clearance may benefit from ecbolic agents that stimulate uterine contractions.

Fungal endometritis is more difficult to treat and often requires prolonged therapy. Mares with persistent post-breeding endometritis may benefit from breeding management strategies such as using minimal contamination techniques or performing uterine lavage after breeding.

Records and Measurements

Maintain the following records for mares with suspected or confirmed endometritis:

• Date of last breeding and breeding method
• Results of uterine culture and cytology with dates
• Ultrasound findings including volume and character of intrauterine fluid
• Endometrial biopsy grade if performed
• Treatment administered including lavage volume and frequency
• Response to treatment including follow-up culture results
• Subsequent breeding outcomes

Common Failure Patterns

Common errors in managing endometritis include treating without culture and sensitivity results, using inappropriate antimicrobial agents, failing to perform uterine lavage before antibiotic infusion, and not addressing underlying conformational or clearance issues. Mares that fail to respond to initial treatment should be re-evaluated with repeat culture and cytology.

Professional Escalation Criteria

Consult a veterinarian with advanced reproductive training when:

• Mares fail to conceive after three or more breeding cycles despite treatment
• Endometrial biopsy reveals grade III fibrosis
• Fungal endometritis is confirmed or suspected
• Mares develop recurrent post-breeding endometritis
• Systemic signs such as fever or depression accompany uterine infection

Cryptorchidism in Stallions

Cryptorchidism is the failure of one or both testicles to descend into the scrotum. It is a common congenital condition in stallions and can affect fertility and behavior. A diagnostic approach to equine testicular disorders provides guidance on evaluating cryptorchid stallions (Veterinary Sciences, 2024, PubMed ID 38921990). The retained testicle may be located in the inguinal canal or within the abdomen.

Clinical Presentation

Cryptorchid stallions may exhibit stallion-like behavior including aggression, mounting, and penile protrusion. They may have a small or absent scrotal testicle. Bilateral cryptorchid stallions are sterile because the retained testicles are unable to produce viable sperm due to elevated body temperature. Unilateral cryptorchid stallions may be fertile if the descended testicle is normal.

Diagnostic Methods

Diagnosis begins with careful palpation of the scrotum and inguinal region. Ultrasound examination can locate retained testicles in the inguinal canal or abdomen. Hormone testing is useful for confirming the presence of testicular tissue. Measurement of testosterone levels before and after administration of human chorionic gonadotropin (hCG) can help differentiate cryptorchid stallions from geldings.

Surgical Management

Surgical removal of the retained testicle is recommended for cryptorchid stallions. The approach depends on the location of the testicle. Inguinal testicles may be removed through an inguinal incision. Abdominal testicles require laparoscopy or laparotomy. Surgery should be performed by an experienced equine surgeon. After surgery, the horse is managed as a gelding.

Records and Measurements

Document the following for cryptorchid stallions:

• Age at diagnosis and breed
• Palpation findings including location of descended testicle if present
• Ultrasound findings describing location and size of retained testicle
• Hormone test results including baseline and post-hCG testosterone levels
• Surgical approach and findings
• Post-operative complications if any
• Behavior changes after surgery

Common Failure Patterns

Failure to correctly identify cryptorchid stallions can lead to unintended breeding. Mistaking a cryptorchid stallion for a gelding may result in continued stallion-like behavior and potential fertility. Incomplete removal of testicular tissue during surgery can result in continued testosterone production and behavior.

Professional Escalation Criteria

Refer to a specialist surgeon when:

• The retained testicle cannot be located by ultrasound
• The horse has a history of abdominal surgery that may complicate laparoscopy
• The retained testicle is suspected to be neoplastic
• The horse develops complications after initial surgical attempts

Contagious Equine Metritis

Contagious equine metritis (CEM) is a venereal disease caused by the bacterium Taylorella equigenitalis. It is a reportable disease in many countries. The disease was first recognized in the 1970s and remains a concern for horse breeding operations. Research on CEM epidemiology and control provides information on managing this condition (Journal of Equine Veterinary Science, 1988, DOI 10.1016/S0737-0806(88)80109-6). The disease is transmitted through breeding or artificial insemination with contaminated semen.

Clinical Signs in Mares

Mares infected with CEM develop a mucopurulent vaginal discharge within days of breeding. The discharge may persist for several weeks. Some mares show no clinical signs but remain carriers. Infected mares may have difficulty conceiving or may experience early embryonic death.

Clinical Signs in Stallions

Stallions infected with CEM typically show no clinical signs. They carry the bacteria on the external genitalia and transmit the infection to mares during breeding. Carrier stallions are a major source of disease spread.

Diagnosis and Reporting

Diagnosis of CEM requires bacterial culture of swabs collected from the clitoral sinuses and fossa in mares and from the urethra, urethral fossa, and penile sheath in stallions. The bacteria are fastidious and require specialized culture media. PCR testing is also available. Positive cases must be reported to regulatory authorities. The World Organisation for Animal Health provides guidelines for reporting and controlling CEM (www.woah.org/en/what-we-do/animal-health-and-welfare).

Control Measures

Control of CEM involves testing and treatment of infected animals. Mares and stallions are treated with topical and systemic antibiotics. Treated animals must be tested negative on multiple occasions before being considered free of infection. Breeding restrictions are imposed on infected premises. Imported horses should be tested for CEM before breeding.

Records and Measurements

Maintain the following records for CEM testing and management:

• Source of horse including country of origin and import date
• Dates and results of all CEM cultures and PCR tests
• Treatment protocols including drugs used and duration
• Post-treatment test dates and results
• Breeding restrictions imposed and dates of restriction
• Regulatory notifications and case numbers

Common Failure Patterns

Failure to test imported horses before breeding is a common route of CEM introduction. Inadequate sample collection from clitoral sinuses or urethral fossa can produce false negative results. Premature release from breeding restrictions before negative test confirmation can perpetuate disease spread.

Professional Escalation Criteria

Immediately contact regulatory veterinary authorities when:

• CEM is suspected based on clinical signs or breeding history
• A positive culture or PCR result is obtained
• An imported horse has not been tested before breeding
• Multiple mares bred to the same stallion develop vaginal discharge

Equine Herpesvirus-1 and Reproductive Loss

Equine herpesvirus-1 (EHV-1) is a significant cause of abortion, neonatal death, and respiratory disease in horses. An updated ACVIM consensus statement on equine herpesvirus-1 provides current recommendations for managing this virus (Journal of Veterinary Internal Medicine, 2024, PubMed ID 38497217). The virus can cause abortion storms in pregnant mares, with multiple abortions occurring within a short period.

Transmission and Risk Factors

EHV-1 is transmitted through respiratory secretions, aborted fetuses, placental tissues, and contaminated fomites. Latently infected horses can reactivate the virus during periods of stress. Pregnant mares are at highest risk for abortion during the last trimester. Outbreaks are more common in large breeding operations with high horse density.

Clinical Signs

Mares infected with EHV-1 may abort without prior warning signs. The abortion typically occurs between 7 and 11 months of gestation. Some mares show mild respiratory signs before abortion. Foals infected in utero may be born weak or die shortly after birth. Neurologic signs can occur in some infected horses.

Diagnosis

Diagnosis of EHV-1 abortion is confirmed by PCR testing of fetal tissues and placenta. Serology can detect antibodies in the mare. Virus isolation is also possible. Necropsy of the aborted fetus reveals characteristic lesions including pulmonary edema and hepatic necrosis.

Management and Prevention

Management of EHV-1 involves biosecurity measures to prevent introduction and spread of the virus. Pregnant mares should be isolated from other horses, particularly during the last trimester. Vaccination is available but does not provide complete protection. The ACVIM consensus statement provides detailed recommendations for outbreak management and vaccination protocols.

Records and Measurements

Document the following for EHV-1 management:

• Vaccination history for all horses on the premises
• Dates of any abortions with gestational age
• Results of PCR testing on fetal tissues and placenta
• Serology results from affected and exposed mares
• Biosecurity measures implemented including isolation protocols
• Movement records of horses onto and off the premises
• Dates of any neurologic signs in affected horses

Common Failure Patterns

Failure to isolate pregnant mares from incoming horses is a common risk factor for EHV-1 introduction. Delayed submission of fetal tissues for testing can compromise diagnosis. Inadequate biosecurity during an outbreak allows rapid spread through the herd. Assuming vaccination provides complete protection can lead to complacency.

Professional Escalation Criteria

Contact a veterinarian immediately when:

• Any mare aborts, particularly in the last trimester
• Multiple abortions occur within a short period
• A mare shows neurologic signs
• A foal is born weak or dies shortly after birth
• EHV-1 is confirmed by laboratory testing

Equine Viral Arteritis

Equine viral arteritis (EVA) is a viral disease that can cause abortion, respiratory disease, and persistent infection in stallions. Research on EVA reproductive risks and management strategies provides information on managing this disease in horse populations (Journal of Advanced Veterinary Research, 2026, Scopus ID 105035257271). The virus is transmitted through respiratory secretions, venereal routes, and contaminated fomites.

Clinical Signs

Infected horses may show fever, depression, conjunctivitis, and edema of the limbs and ventral abdomen. Pregnant mares may abort, typically during the later stages of gestation. Stallions can become persistently infected and shed the virus in semen for months or years.

Carrier Stallions

Carrier stallions are a major source of EVA transmission. They show no clinical signs but shed the virus in semen. Testing of stallions for EVA is recommended before breeding. Carrier stallions should be bred only to seropositive mares or mares that have been vaccinated.

Diagnosis and Control

Diagnosis of EVA involves virus isolation or PCR testing of blood, nasal swabs, or semen. Serology can detect antibodies. Control measures include vaccination of at-risk horses, testing of breeding stallions, and isolation of infected animals. Imported horses should be tested for EVA.

Records and Measurements

Maintain the following records for EVA management:

• Serology results for all breeding stallions
• Vaccination status of mares bred to carrier stallions
• Dates of any abortions with gestational age
• Results of virus isolation or PCR on aborted tissues
• Semen testing results for carrier stallions
• Movement records of horses onto and off the premises

Common Failure Patterns

Failure to test breeding stallions for EVA carrier status can lead to widespread venereal transmission. Breeding carrier stallions to seronegative mares without vaccination risks abortion. Inadequate biosecurity during outbreaks allows respiratory spread.

Professional Escalation Criteria

Contact a veterinarian when:

• A stallion is suspected to be an EVA carrier
• A mare aborts and EVA is suspected
• Multiple horses develop fever and edema
• Imported horses have not been tested for EVA

Equine Coital Exanthema

Equine coital exanthema is a venereal disease caused by equine herpesvirus-3 (EHV-3). The disease is characterized by the development of pustules and ulcers on the external genitalia of mares and stallions. An occurrence of equine coital exanthema in mares from an embryo transfer center has been documented (Journal of Equine Veterinary Science, 2010, DOI 10.1016/j.jevs.2010.01.059). The disease is self-limiting but causes temporary infertility.

Clinical Signs

Mares develop pustules and ulcers on the vulva and perineal region. Stallions develop lesions on the penis and prepuce. The lesions are painful and may cause reluctance to breed. Affected animals may show signs of discomfort during urination.

Diagnosis and Management

Diagnosis is based on clinical signs and confirmed by PCR testing of lesion swabs. The disease is self-limiting and typically resolves within two weeks. Affected animals should not be bred until lesions have healed completely. Secondary bacterial infection may require treatment. The virus does not cause abortion.

Records and Measurements

Document the following for equine coital exanthema:

• Date of lesion onset and location
• PCR results from lesion swabs
• Breeding rest period and date of lesion resolution
• Any secondary infections requiring treatment
• Number of affected animals on the premises

Common Failure Patterns

Breeding affected animals before lesions heal can transmit the virus to susceptible mares or stallions. Confusing EHV-3 lesions with other genital conditions can delay appropriate management. Failure to rest affected animals allows continued spread.

Professional Escalation Criteria

Contact a veterinarian when:

• Lesions are severe or extensive
• Lesions do not heal within two weeks
• Secondary bacterial infection is suspected
• Multiple animals are affected

Testicular Disorders in Stallions

Testicular disorders in stallions include neoplasia, trauma, infection, and degeneration. A diagnostic approach to equine testicular disorders provides guidance on evaluating these conditions (Veterinary Sciences, 2024, PubMed ID 38921990). Testicular neoplasia is most common in older stallions, with seminoma and Leydig cell tumor being the most frequently reported types.

Clinical Signs

Stallions with testicular neoplasia may have a firm, enlarged testicle. The affected testicle may be painful on palpation. Some stallions show no clinical signs. Testicular degeneration results in reduced fertility and may be associated with age, heat stress, or systemic illness.

Diagnostic Methods

Diagnosis begins with palpation and ultrasound examination of the testicles. Ultrasound can identify masses, changes in echogenicity, and abnormalities in testicular architecture. Hormone testing may be useful. Biopsy of testicular tissue can confirm the diagnosis but carries risks of hemorrhage and infection.

Management

Surgical removal of the affected testicle is recommended for testicular neoplasia. The remaining testicle may be sufficient for breeding if it is normal. Testicular degeneration may be managed by addressing the underlying cause, but the prognosis for recovery of fertility is guarded.

Records and Measurements

Document the following for testicular disorders:

• Age and breed of stallion
• Palpation findings including size, consistency, and pain
• Ultrasound findings including mass location and echogenicity
• Hormone test results if performed
• Biopsy results if obtained
• Surgical findings and histopathology results
• Post-operative fertility if applicable

Common Failure Patterns

Delaying diagnosis of testicular neoplasia can allow tumor growth and metastasis. Assuming testicular enlargement is due to infection without ultrasound examination can miss neoplasia. Failure to evaluate the contralateral testicle after surgery can result in continued breeding with a compromised testicle.

Professional Escalation Criteria

Refer to a specialist when:

• A testicular mass is identified on ultrasound
• The stallion has unexplained testicular pain or swelling
• Testicular degeneration is suspected and fertility is important
• Biopsy is needed for diagnosis

Leptospirosis and Reproductive Loss

Leptospirosis is a bacterial disease that can cause abortion in mares. The disease is caused by infection with Leptospira species. Research on animal leptospirosis provides information on the epidemiology and pathogenesis of this disease (Current Topics in Microbiology and Immunology, 2015, PubMed ID 25388134). Leptospirosis is a zoonotic disease, meaning it can be transmitted from animals to humans.

Clinical Signs

Mares infected with Leptospira may abort without prior warning signs. Abortion typically occurs during the last trimester. Some mares show mild fever or depression before abortion. The aborted fetus may be autolyzed.

Diagnosis

Diagnosis of leptospirosis-associated abortion is confirmed by serology, PCR testing, or culture of fetal tissues. The microscopic agglutination test (MAT) is the standard serologic test. Positive results in the mare and detection of the organism in fetal tissues confirm the diagnosis.

Management and Prevention

Management of leptospirosis involves rodent control, vaccination, and biosecurity measures. Rodents are a major reservoir of Leptospira. Vaccination of mares may reduce the risk of abortion. Infected mares should be isolated, and personnel should use appropriate personal protective equipment when handling aborted tissues.

Records and Measurements

Document the following for leptospirosis management:

• Serology results including MAT titers
• PCR or culture results from fetal tissues
• Vaccination history of mares
• Rodent control measures implemented
• Dates of any abortions with gestational age
• Personal protective equipment used by personnel

Common Failure Patterns

Failure to implement rodent control measures allows continued exposure to Leptospira. Handling aborted tissues without appropriate protective equipment poses zoonotic risk. Delayed diagnosis can result in additional abortions in the herd.

Professional Escalation Criteria

Contact a veterinarian when:

• A mare aborts and leptospirosis is suspected
• Serology confirms Leptospira infection
• Multiple abortions occur on the premises
• Personnel develop flu-like symptoms after handling aborted tissues

Genetics of Equine Reproductive Disorders

Genetic factors contribute to some equine reproductive disorders. Research on the genetics of equine reproductive diseases provides information on inherited conditions (Veterinary Clinics of North America: Equine Practice, 2020, PubMed ID 32534849). Genetic testing is available for some conditions and can inform breeding decisions.

Inherited Conditions

Cryptorchidism has a genetic component, with certain breeds having a higher prevalence. Other reproductive disorders with genetic influences include some forms of infertility and congenital abnormalities. Breeders should be aware of the genetic risks in their breeding stock.

Genetic Testing

Genetic testing can identify carriers of some inherited reproductive disorders. Results can be used to make informed breeding decisions. Testing should be performed by accredited laboratories. Breeders should consult with veterinarians to interpret test results and develop breeding strategies.

Diagnostic Techniques for Reproductive Disorders

Accurate diagnosis of reproductive disorders requires a combination of clinical examination, laboratory testing, and imaging. The Merck Veterinary Manual provides general reference information on diagnostic techniques for horse health conditions (www.merckvetmanual.com). The American College of Veterinary Internal Medicine (ACVIM) provides consensus statements and guidelines for veterinary practice (www.acvim.org).

Ultrasound Examination

Ultrasound is a valuable tool for evaluating the reproductive tract in both mares and stallions. In mares, ultrasound can detect intrauterine fluid, endometrial changes, and ovarian abnormalities. In stallions, ultrasound can evaluate testicular structure and identify retained testicles.

Bacterial Culture and Sensitivity

Bacterial culture of uterine swabs or semen samples is essential for diagnosing infectious causes of reproductive failure. Samples should be collected using aseptic technique and transported to the laboratory promptly. Sensitivity testing guides antimicrobial selection.

PCR Testing

PCR testing is used to detect the genetic material of infectious agents. It is available for many reproductive pathogens including EHV-1, EHV-3, Taylorella equigenitalis, and Leptospira species. PCR is highly sensitive and can detect low levels of infection.

Hormone Testing

Hormone testing is used to evaluate reproductive function. Measurement of progesterone, estrogen, and testosterone can provide information about ovarian function, pregnancy status, and testicular function. Dynamic testing, such as the hCG stimulation test, can help differentiate cryptorchid stallions from geldings.

Practical Implementation Steps for Breeding Operations

Implement the following steps to manage reproductive health in breeding operations:

1. Establish a pre-breeding examination protocol for all mares including uterine culture, cytology, and ultrasound
2. Test all breeding stallions for CEM and EVA before the breeding season
3. Isolate pregnant mares from incoming horses during the last trimester
4. Maintain vaccination protocols for EHV-1 and EVA based on veterinary recommendations
5. Implement rodent control measures to reduce leptospirosis risk
6. Maintain accurate records of all breeding activities, test results, and treatments
7. Develop a biosecurity plan for the premises including isolation protocols for sick horses
8. Train personnel on proper sample collection techniques and biosecurity procedures

Welfare and Safety Context

Reproductive disorders can cause pain and distress in affected horses. Endometritis causes inflammation and discomfort. Cryptorchidism may be associated with testicular torsion or neoplasia. Infectious diseases can cause systemic illness. Prompt diagnosis and appropriate management improve welfare outcomes.

Personnel handling horses with reproductive disorders should use appropriate biosecurity measures. Zoonotic diseases such as leptospirosis pose a risk to human health. The World Organisation for Animal Health provides guidelines for managing zoonotic diseases in animal populations (www.woah.org/en/what-we-do/animal-health-and-welfare).

Professional Escalation Criteria

Veterinarians should be consulted for any suspected reproductive disorder. The following situations require immediate veterinary attention:

• Abortion in a pregnant mare
• Vaginal discharge in a mare
• Difficulty breeding or reluctance to breed in a stallion
• Swelling or pain in the testicles of a stallion
• Suspected contagious disease requiring regulatory reporting
• Failure to conceive after multiple breeding attempts

Decision Framework for Selecting Diagnostic Tests in Equine Reproductive Cases

Selecting the appropriate diagnostic test for equine reproductive disorders requires a structured approach that considers the clinical presentation, timing relative to breeding, and the specific pathogen or condition suspected. A systematic decision framework helps avoid unnecessary testing, reduces diagnostic delays, and improves the accuracy of treatment decisions. The Merck Veterinary Manual provides general reference information on diagnostic approaches for horse health conditions (www.merckvetmanual.com). The American College of Veterinary Internal Medicine (ACVIM) publishes consensus statements that guide diagnostic decision-making in equine practice (www.acvim.org).

Step 1: Classify the Clinical Presentation

Begin by categorizing the presenting problem into one of five clinical scenarios. This classification determines the initial diagnostic pathway.

Scenario A: Mare with vaginal discharge or poor conception. This presentation suggests endometritis or venereal infection. Initial testing should include uterine culture with sensitivity, uterine cytology, and transrectal ultrasound to assess intrauterine fluid and endometrial thickness. If the mare has been bred within the previous 72 hours, consider post-breeding endometritis as a primary differential. If the discharge is mucopurulent and occurs within days of breeding, include testing for contagious equine metritis (CEM) using bacterial culture of clitoral sinus and fossa swabs. Research on CEM epidemiology and control provides information on diagnostic sampling protocols (Journal of Equine Veterinary Science, 1988, DOI 10.1016/S0737-0806(88)80109-6).

Scenario B: Mare with abortion or pregnancy loss. This presentation requires immediate investigation for infectious causes. Collect fetal tissues and placenta for PCR testing for equine herpesvirus-1 (EHV-1), leptospirosis, and equine viral arteritis (EVA). Collect maternal blood for serology. The updated ACVIM consensus statement on equine herpesvirus-1 provides current recommendations for diagnostic testing in abortion cases (Journal of Veterinary Internal Medicine, 2024, PubMed ID 38497217). Submit samples to a diagnostic laboratory within 24 hours of the abortion. If the fetus is autolyzed, leptospirosis becomes a more likely differential, and PCR testing of fetal fluids may be more sensitive than culture.

Scenario C: Stallion with abnormal external genitalia or breeding difficulty. This presentation includes testicular enlargement, pain, swelling, or lesions on the penis or prepuce. Initial testing includes palpation and ultrasound of the testicles and external genitalia. If pustules or ulcers are present on the penis or prepuce, collect swabs for PCR testing for equine herpesvirus-3 (EHV-3), the cause of equine coital exanthema. If the stallion has a firm, enlarged testicle, ultrasound-guided biopsy may be indicated to rule out testicular neoplasia. A diagnostic approach to equine testicular disorders provides guidance on evaluating these cases (Veterinary Sciences, 2024, PubMed ID 38921990).

Scenario D: Stallion with suspected cryptorchidism. This presentation occurs when a horse exhibits stallion-like behavior but has no palpable scrotal testicles or has only one descended testicle. Initial testing includes careful palpation of the scrotum and inguinal region, followed by ultrasound of the inguinal canal and abdomen. If ultrasound does not locate the retained testicle, perform hormone testing with baseline testosterone measurement followed by human chorionic gonadotropin (hCG) stimulation. A rise in testosterone after hCG administration confirms the presence of testicular tissue.

Scenario E: Pre-breeding screening of breeding stock. This presentation involves apparently healthy horses that require testing before the breeding season. For stallions, test for CEM and EVA carrier status using bacterial culture of genital swabs and serology or virus isolation from semen. For mares, perform a pre-breeding examination including uterine culture, cytology, and ultrasound. Imported horses require additional testing based on country of origin and regulatory requirements. The World Organisation for Animal Health provides international standards for pre-movement testing of breeding horses (www.woah.org/en/what-we-do/animal-health-and-welfare).

Step 2: Select Diagnostic Tests Based on Timing and Sensitivity

After classifying the clinical scenario, select specific tests considering the timing of sample collection relative to the clinical event and the sensitivity and specificity of each test.

Culture versus PCR. Bacterial culture is the gold standard for diagnosing CEM and bacterial endometritis because it allows antimicrobial sensitivity testing. However, culture requires specialized media and takes several days for results. PCR testing is faster and more sensitive for detecting low levels of bacteria or viruses but does not provide sensitivity data. For CEM, both culture and PCR are recommended, with culture required for regulatory confirmation. For EHV-1 and EHV-3, PCR is the preferred diagnostic test due to its high sensitivity and rapid turnaround time. Research on the microbial quality of equine semen provides context for interpreting culture results from breeding samples (Veterinary Practitioner, 2012, Scopus ID 84874339917).

Serology versus direct detection. Serology detects antibodies and is useful for identifying past exposure or vaccination status. It is less useful for diagnosing acute infection because antibodies take time to develop. Direct detection methods such as PCR, virus isolation, or culture identify the pathogen itself and are preferred for diagnosing active infections. For EVA, serology is used for screening, while virus isolation or PCR from semen confirms carrier status in stallions. Research on EVA reproductive risks and management strategies provides information on interpreting serologic results (Journal of Advanced Veterinary Research, 2026, Scopus ID 105035257271).

Ultrasound as a primary screening tool. Transrectal ultrasound should be performed in all mares presenting with reproductive problems. It provides immediate information about intrauterine fluid, endometrial cysts, ovarian abnormalities, and pregnancy status. In stallions, scrotal ultrasound evaluates testicular parenchyma, identifies masses, and assesses blood flow. Ultrasound findings guide the selection of additional diagnostic tests. For example, a mare with intrauterine fluid on ultrasound should have uterine culture and cytology performed, while a mare with a normal ultrasound may not require immediate sampling.

Step 3: Interpret Results Using a Diagnostic Algorithm

Use the following algorithm to interpret test results and guide management decisions.

For mares with vaginal discharge or poor conception:

• If uterine culture is positive and cytology shows neutrophils: Diagnose bacterial endometritis. Treat based on sensitivity results. Perform uterine lavage before antibiotic infusion.
• If uterine culture is negative but cytology shows neutrophils: Consider persistent post-breeding endometritis or fungal infection. Repeat culture using enriched media for fungi. Evaluate uterine clearance mechanisms.
• If uterine culture is positive but cytology is normal: Consider contamination during sample collection. Repeat sampling using strict aseptic technique.
• If both culture and cytology are negative: Evaluate for non-infectious causes such as endometrial fibrosis, hormonal imbalance, or stallion factors.

For mares with abortion:

• If fetal PCR is positive for EHV-1: Diagnose EHV-1 abortion. Implement biosecurity measures. Review vaccination history. Isolate pregnant mares.
• If fetal PCR is positive for Leptospira: Diagnose leptospirosis-associated abortion. Implement rodent control. Review vaccination protocol. Use personal protective equipment when handling tissues.
• If fetal PCR is positive for EVA: Diagnose EVA abortion. Test the mare and any in-contact horses. Identify potential carrier stallions.
• If all PCR tests are negative: Consider non-infectious causes such as placental insufficiency, twinning, or genetic abnormalities. Submit fetal tissues for histopathology.

For stallions with abnormal genitalia:

• If PCR for EHV-3 is positive: Diagnose equine coital exanthema. Rest from breeding until lesions heal. No specific antiviral treatment is needed.
• If ultrasound reveals a testicular mass: Diagnose suspected testicular neoplasia. Recommend surgical removal and histopathology. Evaluate the contralateral testicle.
• If bacterial culture of genital swabs is positive for Taylorella equigenitalis: Diagnose CEM. Report to regulatory authorities. Implement treatment and breeding restrictions.

Step 4: Document Results and Track Outcomes

Maintain a diagnostic decision log for each case that includes:

• Date of initial presentation and clinical scenario classification
• Diagnostic tests selected and rationale for each choice
• Sample collection dates, methods, and laboratory submission details
• Test results with reference ranges or interpretation criteria
• Diagnosis based on test results
• Treatment or management decisions made
• Follow-up testing dates and results
• Final outcome including pregnancy status or breeding success

This log allows retrospective analysis of diagnostic accuracy and helps refine future decision-making. For breeding operations with multiple horses, aggregate data can identify patterns such as recurrent infections in certain mares or stallions, seasonal variations in disease incidence, or emerging antimicrobial resistance patterns.

Common Failure Patterns in Diagnostic Decision-Making

Testing without a clear clinical question. Submitting samples without first classifying the clinical presentation often results in inappropriate test selection. For example, testing a mare for EHV-1 when she has vaginal discharge but no history of abortion wastes resources and delays the correct diagnosis.

Using the wrong sample type. Collecting a vaginal swab instead of a guarded uterine swab for endometritis diagnosis introduces contamination from the lower reproductive tract. For CEM testing, failing to sample the clitoral sinuses and fossa in mares or the urethral fossa in stallions produces false negative results.

Delaying sample submission. Bacterial culture and PCR results are most reliable when samples are submitted to the laboratory within 24 hours of collection. Delayed submission allows bacterial overgrowth or degradation of viral genetic material, leading to false negative results.

Ignoring negative results. A negative culture or PCR result does not rule out infection if the sample was collected improperly, at the wrong time, or from the wrong site. Repeat testing should be considered when clinical signs persist despite negative results.

Over-relying on a single test. No diagnostic test has 100% sensitivity and specificity. Combining multiple tests, such as culture and PCR for CEM or ultrasound and hormone testing for cryptorchidism, improves diagnostic accuracy.

Professional Escalation Criteria for Diagnostic Challenges

Consult a veterinary specialist with advanced reproductive training when:

• Test results are inconsistent with clinical findings
• Multiple diagnostic tests have been negative but clinical signs persist
• A rare or reportable disease such as CEM is suspected
• Advanced imaging such as laparoscopy is needed to locate a retained testicle
• Interpretation of hormone testing results is unclear
• Antimicrobial sensitivity results show resistance to commonly used drugs
• An outbreak of abortion occurs and the cause is not identified after initial testing

Specialists can recommend additional testing such as endometrial biopsy, advanced imaging, or referral to a diagnostic laboratory with specialized capabilities. The ACVIM provides a directory of board-certified specialists in equine internal medicine who can assist with complex reproductive cases (www.acvim.org).

Frequently Asked Questions

What are the most common reproductive disorders in mares?

Endometritis is the most common reproductive disorder in mares. Other common conditions include persistent post-breeding endometritis, uterine infections, and hormonal imbalances. Mares with poor perineal conformation or advanced age are at higher risk for reproductive disorders.

How is endometritis diagnosed in mares?

Endometritis is diagnosed using uterine culture, cytology, and ultrasound. A guarded swab is used to collect samples from the uterus. Cytology reveals the presence of neutrophils. Ultrasound detects intrauterine fluid accumulation. Uterine biopsy provides information about endometrial health and fibrosis.

What causes cryptorchidism in stallions?

Cryptorchidism is a congenital condition with a genetic component. The testicle fails to descend into the scrotum during development. The retained testicle may be located in the inguinal canal or within the abdomen. Certain breeds have a higher prevalence of cryptorchidism.

How is contagious equine metritis transmitted?

Contagious equine metritis is transmitted through breeding or artificial insemination with contaminated semen. Infected stallions carry the bacteria on their external genitalia and transmit the infection to mares. The disease can also be spread through contaminated equipment or personnel.

What is the difference between EHV-1 abortion and EVA abortion?

EHV-1 abortion is caused by equine herpesvirus-1 and typically occurs during the last trimester. EVA abortion is caused by equine viral arteritis and can also occur during later gestation. Both viruses can cause abortion storms. Diagnosis requires laboratory testing to differentiate the two viruses.

Can equine coital exanthema be treated?

Equine coital exanthema is self-limiting and typically resolves within two weeks. No specific antiviral treatment is available. Affected animals should not be bred until lesions have healed completely. Secondary bacterial infection may require treatment with antibiotics.

What are the zoonotic risks associated with equine reproductive disorders?

Leptospirosis is a zoonotic disease that can be transmitted from horses to humans. Personnel handling aborted tissues or infected animals should use appropriate personal protective equipment. Other reproductive diseases such as CEM and EVA are not considered zoonotic.

How can reproductive disorders be prevented in breeding operations?

Prevention of reproductive disorders involves biosecurity measures, routine testing, and vaccination. Imported horses should be tested for contagious diseases. Breeding stallions should be tested for CEM and EVA. Pregnant mares should be isolated from other horses. Vaccination against EHV-1 and EVA is available.

Related Veterinary Guides

• History Of Diseases
• Swine Respiratory Disease Observation And Diagnostics
• Alpaca Llama Reproduction Breeding Soundness Estrus Parturition
• Camel Reproduction Management Estrus Detection Ai Calving
• Working Equid Dental Care Routine Problem Management

References and Further Reading

• www.merckvetmanual.com
• www.acvim.org
• Merck Veterinary Manual. Merck Veterinary Manual.
• Animal Health and Welfare. World Organisation for Animal Health.
• Updated ACVIM consensus statement on equine herpesvirus-1.. Journal of veterinary internal medicine, 2024.
• Animal leptospirosis.. Current topics in microbiology and immunology, 2015.
• Genetics of Equine Reproductive Diseases.. The Veterinary clinics of North America. Equine practice, 2020.
• General and comparative aspects of endometritis in domestic species: A review.. Reproduction in domestic animals = Zuchthygiene, 2023.
• Genetics of equine bleeding disorders.. Equine veterinary journal, 2021.
• Diagnostic Approach to Equine Testicular Disorders.. Veterinary sciences, 2024.
• Contagious equine metritis. Comparative Immunology Microbiology and Infectious Diseases, 1996.
• Contagious equine metritis-epidemiology and control. Journal of Equine Veterinary Science, 1988.
• Equine Viral Arteritis: Reproductive risks and management strategies in horse populations. Journal of Advanced Veterinary Research, 2026.
• Microbial quality of fresh and frozen equine semen of indian horses. Veterinary Practitioner, 2012.
• Occurrence of Equine Coital Exanthema in Mares from an Embryo Transfer Center. Journal of Equine Veterinary Science, 2010.

This article is educational and is not a substitute for veterinary diagnosis or treatment. Contact a veterinarian for advice about an individual animal.