NAVLE Reproductive

Equine Infertility Study Guide

📅 March 28, 2026 ⏱ 25 min read 👁 1 views

Equine infertility represents a significant challenge in both mare and stallion reproduction, with major economic implications for the breeding industry.

Overview and Clinical Importance

Equine infertility represents a significant challenge in both mare and stallion reproduction, with major economic implications for the breeding industry. Infertility in horses can result from anatomical abnormalities, endocrine disorders, infectious diseases, poor breeding management, or age-related changes. Understanding the diagnostic approach and management of reproductive disorders is essential for NAVLE success and clinical practice.

The NAVLE specifically tests on infertility in mare or stallion under the Equine Reproductive competency domain. This guide covers the major causes, diagnostic algorithms, treatment protocols, and prognosis for common reproductive disorders in both sexes.

Category	Common Causes
Failure to Cycle	Seasonal anestrus, transitional period, pregnancy, lactational anestrus, granulosa-theca cell tumor (GTCT), hypothalamic-pituitary dysfunction, poor body condition
Cycles but Fails to Conceive	Endometritis (infectious or persistent breeding-induced), poor breeding management, oviductal blockage, cervical abnormalities, semen quality issues
Early Embryonic Death	Endometrosis (chronic degenerative endometrial fibrosis), chromosomal abnormalities, twin pregnancy, luteal insufficiency, placentitis, stress

Grade	Histopathologic Findings	Foaling Rate
I	Normal endometrium OR mild, focal inflammation or fibrosis	Greater than 80%
IIA	Mild-moderate inflammation; 1-3 fibroblast layers around glands; less than 2 fibrotic nests per 5mm field	50-80%
IIB	Moderate inflammation; 4+ fibroblast layers; 2-4 fibrotic nests per 5mm field	10-50%
III	Severe inflammation; diffuse fibrosis; 5+ fibrotic nests per 5mm field	Less than 10%

Part 1: Mare Infertility

Mare infertility encompasses three main categories: mares that fail to cycle, mares that cycle normally but do not conceive, and mares that conceive but suffer early embryonic death. Endometritis is the most common cause of infertility in mares, affecting 25-60% of barren mares. Healthy mares under 16 years of age have approximately 70% conception rate per cycle when properly managed.

High-YieldThe four main defenses against uterine contamination are: (1) vulvar seal, (2) cervical barrier, (3) uterine contractions, and (4) lymphatic drainage. Failure of any defense mechanism predisposes to endometritis.

Categories of Mare Infertility

Endometritis

Persistent breeding-induced endometritis (PBIE) is the most common cause of mare infertility. Normal mares clear uterine inflammation within 24-48 hours post-breeding. Susceptible mares fail to clear debris and fluid, creating an inhospitable environment when the embryo arrives at day 5-6.

Pathophysiology

Sperm and seminal plasma components induce normal inflammatory response
Susceptible mares have impaired myometrial contractility and delayed uterine clearance
Older mares often have decreased uterine tone and dependent uterine position
Poor vulvar and cervical conformation allows ascending contamination

Common Bacterial Pathogens

Streptococcus equi subsp. zooepidemicus - most common pathogen
Escherichia coli - second most common
Klebsiella pneumoniae, Pseudomonas aeruginosa - often associated with venereal transmission or iatrogenic contamination
Fungal pathogens (Candida, Aspergillus) - usually secondary to overuse of antibiotics or immunocompromise

NAVLE TipIf you isolate two or more organisms from a uterine culture, suspect contamination. The exception is concurrent isolation of S. zooepidemicus AND E. coli, which can occur together in true uterine infections.

Diagnostic Algorithm for Mare Infertility

Complete history: Age, parity, previous breeding results, last foaling date, method of breeding
Physical examination: Body condition, perineal conformation, vulvar angle
Transrectal palpation: Uterine tone, ovarian structures, cervical assessment
Transrectal ultrasonography: Intrauterine fluid, endometrial edema, follicular dynamics, ovarian abnormalities
Uterine culture and cytology: Identify pathogens, assess inflammatory response (greater than 2% PMNs = positive)
Endometrial biopsy: Kenney-Doig grading for prognosis
Vaginal speculum and cervical examination: Assess cervical patency, lacerations, adhesions

Kenney-Doig Endometrial Biopsy Classification

The Kenney-Doig classification system is the gold standard for predicting a mare's ability to conceive and carry a foal to term. Biopsies are graded based on inflammation, periglandular fibrosis, and glandular nesting.

High-YieldNeutrophils on cytology/biopsy indicate acute inflammation (likely bacterial). Lymphocytes indicate chronic inflammation. Eosinophils suggest pneumovagina or urovagina. The presence of eosinophils should prompt evaluation of vulvar conformation!

Treatment of Endometritis

Granulosa-Theca Cell Tumor (GTCT)

Granulosa-theca cell tumors are the most common ovarian tumor in mares, representing approximately 2.5% of all equine tumors. They are almost always unilateral, benign, and hormonally active. The contralateral ovary becomes small and inactive due to inhibin-mediated suppression of FSH secretion.

Clinical Signs

Behavioral changes: Stallion-like behavior (aggression, mounting, herding other mares), cresty neck
Reproductive abnormalities: Anestrus, continuous estrus (nymphomania), or erratic cycling
Physical findings: One enlarged ovary (can be massive), one small inactive ovary
Ultrasound appearance: Honeycomb or multicystic appearance of affected ovary

Hormonal Diagnosis of GTCT

High-YieldAMH (Anti-Mullerian Hormone) is the BEST diagnostic test for GTCT with 98% sensitivity. Treatment is unilateral ovariectomy, typically performed laparoscopically in a standing sedated mare. Normal reproductive function returns in 6-8 months. GTCTs do NOT metastasize.

Treatment	Protocol	Notes
Uterine Lavage	1-2 L warm sterile saline or LRS; repeat until effluent clear	First-line treatment; removes debris, bacteria, inflammatory products
Oxytocin	10-20 IU IV or IM; repeat every 4-6 hours during estrus	Promotes uterine contractions; most effective during estrus when cervix is open
Cloprostenol (PGF2alpha)	250 mcg IM	Longer-acting ecbolic; promotes uterine clearance
Intrauterine Antibiotics	Based on culture/sensitivity; given during estrus for 3-7 days	Always perform culture before treatment; avoid aminoglycosides (inactivated by debris)
Caslick's Procedure	Vulvoplasty - suturing upper vulvar lips	Indicated for poor vulvar conformation (pneumovagina); must be opened before foaling

Hormone	Finding in GTCT	Sensitivity
Anti-Mullerian Hormone (AMH)	Markedly elevated (greater than 10 ng/mL)	98% - BEST TEST
Inhibin	Elevated	80-90%
Testosterone	Elevated (greater than 100 pg/mL = diagnostic)	48-50%

Part 2: Stallion Infertility

Stallion infertility can result from congenital abnormalities, testicular disorders, poor semen quality, behavioral issues, or management problems. The breeding soundness examination (BSE) is the cornerstone of stallion fertility assessment. Remember: a BSE is NOT a direct measure of fertility - the only true measure is successful pregnancy in mares.

High-YieldSpermatogenesis in the stallion takes 57 days. Therefore, any insult (fever, medication, trauma) will affect semen quality for approximately 60 days afterward. Always ask about events 2 months prior when evaluating subfertility!

Breeding Soundness Examination (BSE) of the Stallion

Components of the BSE

Breeding history: Previous pregnancy rates, number of mares bred, breeding method
General physical examination: Body condition, musculoskeletal soundness, neurologic function
External genitalia examination: Penis (lesions, masses, habronemiasis), prepuce, scrotum, testes
Testicular palpation and measurement: Size, symmetry, consistency; minimum total scrotal width of 8 cm to pass
Internal genitalia examination (per rectum): Accessory sex glands, ampullae
Libido and mating ability assessment: Sexual interest, erection, mounting, intromission, ejaculation
Semen collection and evaluation: Volume, concentration, motility, morphology

Normal Stallion Semen Parameters

NAVLE TipTo be classified as a SATISFACTORY PROSPECTIVE BREEDER, a stallion must produce at least 1 BILLION progressively motile, morphologically normal sperm in the second (or third) ejaculate collected. Stallions with congenital defects (cryptorchidism) should be classified as UNSATISFACTORY regardless of semen quality.

BSE Classification Categories

Satisfactory Prospective Breeder: Passes all components, produces adequate sperm
Questionable Prospective Breeder: Temporary or correctable issue; retest in 6-12 months
Unsatisfactory Prospective Breeder: Permanent defect, heritable condition, or irreversible infertility

Testicular Disorders

Cryptorchidism

Cryptorchidism (undescended testis) is the most common disorder of sexual development in stallions, affecting 5-8% of male foals. The condition is considered heritable. Normal testicular descent occurs between 30 days before birth to 2 weeks after birth.

Locations: Inguinal (partial abdominal) or complete abdominal retention
Left side more commonly affected (due to longer gubernaculum)
Diagnosis: Physical exam, transrectal/transinguinal ultrasound, hormone testing

Hormone Testing for Cryptorchidism

High-YieldTreatment for cryptorchidism is SURGICAL REMOVAL (cryptorchidectomy). Cryptorchid stallions should NOT be used for breeding due to the heritable nature of the condition. Retained abdominal testes have increased risk of neoplasia (seminoma, teratoma).

Testicular Degeneration

Testicular degeneration is a common cause of subfertility and infertility in stallions. It can be divided into two categories: degeneration from known insult (potentially reversible) and idiopathic/age-related degeneration (usually irreversible).

Causes of Testicular Degeneration

Heat stress: Fever, hot weather, scrotal insulation
Drugs: Anabolic steroids (most common), corticosteroids
Trauma/orchitis: Scrotal injury, bacterial infection
Vascular compromise: Testicular torsion, varicocele
Age-related (senile): Common in middle-aged and older stallions; irreversible

Clinical Findings

Decreased testicular size (atrophy)
Soft, mushy testicular consistency
Decreased sperm count and motility
Increased proportion of morphologically abnormal sperm

NAVLE TipStallions recently retired from performance careers may have received anabolic steroids. If a stallion fails initial BSE, retest in 3-6 months to allow recovery from drug-induced testicular suppression.

Sperm Morphology Abnormalities

High-YieldA high percentage of PROXIMAL cytoplasmic droplets indicates immature sperm that have not completed epididymal transit. This is common in stallions with sexual overuse or insufficient sexual rest before collection.

Mare Infertility "E.F.F.I.C.I.E.N.T.":

Endometritis (most common cause)

Fibrosis assessed by biopsy

Fluid accumulation on ultrasound

Inhibin elevated in GTCT

Caslick's for pneumovagina

Inactive contralateral ovary with GTCT

Ecbolic agents (oxytocin) for uterine clearance

Neutrophils indicate acute inflammation

Testosterone elevated in GTCT

Stallion BSE "57 DAYS": Spermatogenesis takes 57 days. Any event (fever, drugs, trauma) affects semen quality for ~60 days. Always ask about the previous 2 months!

Satisfactory BSE "1 BILLION": Stallion must produce at least 1 billion progressively motile, morphologically normal sperm in the second ejaculate to be classified as Satisfactory Prospective Breeder.

Parameter	Normal Value	Clinical Significance
Gel-free Volume	30-150 mL	Highly variable; depends on sexual rest
Concentration	100-400 million/mL	Lower than bulls; measure with hemocytometer or photometer
Total Motility	65% or greater	Assess at 37 degrees C; circular pattern normal initially
Progressive Motility	50% or greater	Forward progressive movement
Morphologically Normal	50% or greater	Use phase-contrast microscopy at 1000x
Total Sperm Number	8-10 billion (1st ejaculate); 4+ billion (2nd ejaculate)	After 5+ days sexual rest

Test	Intact Stallion/Cryptorchid	Gelding
Serum AMH	Detectable (secreted by Sertoli cells)	Undetectable
Estrone Sulfate	Greater than 400 pg/mL	Less than 100 pg/mL
hCG Stimulation Test	Testosterone increases post-hCG	No response

Defect Category	Examples	Clinical Significance
Head Defects	Microcephalic, macrocephalic, pyriform, detached heads, nuclear vacuoles	Often associated with spermatogenic defects; may be heritable
Acrosome Defects	Knobbed acrosome, detached acrosome	Knobbed acrosome associated with chromatin defects; may be heritable
Midpiece Defects	Bent, thickened, swollen, or disrupted midpiece	May indicate epididymal dysfunction
Tail Defects	Coiled, bent, broken tails	Affects motility; may result from cold shock
Proximal Droplets	Cytoplasmic droplet near head	Indicates IMMATURITY; should migrate distally during epididymal transit

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