Equine Genital Abnormalities of the Stallion – NAVLE Study Guide

Overview and Clinical Importance

Genital abnormalities of the stallion encompass a spectrum of congenital and acquired conditions affecting reproductive function. Cryptorchidism is the most common developmental disorder of the male reproductive tract, affecting 2-8% of male horses. Understanding these conditions is essential for the NAVLE, as they frequently appear in clinical scenarios involving breeding soundness examinations, behavioral problems in supposed geldings, and surgical decision-making.

This guide covers the major categories of stallion genital abnormalities including cryptorchidism, penile and preputial disorders, testicular conditions, and inguinal hernias. Each condition is reviewed with emphasis on etiology, clinical presentation, diagnostic approach, and treatment options relevant to NAVLE preparation.

Cryptorchidism

Cryptorchidism (from Greek: kryptos = hidden, orchis = testis) refers to the failure of one or both testes to descend into the scrotum. It is the most common disorder of sexual development in male horses, with a prevalence of 2-8% in the general population.

Normal Testicular Descent

In the normal foal, both testes typically descend into the scrotum between 30 days prior to birth and 2 weeks after birth. The process of testicular descent involves two distinct phases:

Transabdominal Phase: The fetal gonad begins in the dorsal abdomen near the kidney at approximately 5.5 weeks of gestation. Under hormonal influence (primarily insulin-like peptide 3 and anti-Mullerian hormone), the testis migrates toward the internal inguinal ring.

Inguinoscrotal Phase: The gubernaculum guides the testis through the inguinal canal into the scrotum. This phase is primarily regulated by testosterone and requires a patent vaginal process.

Etiology and Breed Predispositions

The etiology of cryptorchidism is multifactorial, involving genetic, hormonal, and mechanical factors. The condition is considered heritable, and many breed associations prohibit registration of cryptorchid stallions.

Breed Predispositions for Cryptorchidism

Classification of Cryptorchidism

Clinical Presentation

Cryptorchid stallions may present in several clinical scenarios:

• Known failure of testicular descent: Young colts with only one palpable scrotal testis
• Stallion-like behavior in a supposed gelding: This is the classic 'rig' presentation
• Pre-castration examination: Discovery during routine breeding soundness or pre-purchase exam
• Unknown history: Purchased horse with no surgical records

Stallion-like behaviors include: sexual excitement, erection, mounting behavior, aggression toward other horses, interest in mares, vocalization, arched neck, and territorial marking. These behaviors persist because the retained testis continues to produce testosterone.

Diagnostic Approach

Physical Examination

External palpation: Should be performed with the horse sedated to allow relaxation of the cremaster muscle. Palpate the scrotum, inguinal region, and superficial inguinal ring carefully.

Rectal palpation: In adult horses, examination of the vaginal rings can help determine if testicular tissue is present. A larger ring with evidence of the ductus deferens indicates retained tissue. Reported accuracy of 88% for determining side of retention.

Ultrasonography

Ultrasonography is highly valuable for localizing retained testes:

• Transinguinal: For inguinal and incomplete abdominal testes
• Transabdominal: For complete abdominal retention; scan caudoventral abdomen
• Transrectal: Provides excellent visualization of abdominally retained testes

Retained abdominal testes appear smaller (up to 20 times smaller than normal) with lower and less heterogeneous echogenicity than scrotal testes.

Endocrine Testing

Treatment: Cryptorchidectomy

Surgical removal of all testicular tissue is the definitive treatment. Medical (hormonal) treatment is not recommended in horses due to the heritable nature of the condition.

Complications and Prognosis

Potential complications include hemorrhage, infection, incomplete removal (resulting in persistent stallion behavior), and hernia formation. Retained abdominal testes are at increased risk of neoplasia (seminoma, teratoma, Sertoli cell tumor). Prognosis is excellent following complete cryptorchidectomy, though learned stallion behaviors may persist for varying periods.

Monorchidism and Testicular Hypoplasia

Monorchidism

True monorchidism (complete agenesis of one testis) is rare but should be differentiated from cryptorchidism. In these cases, a degenerative cryptorchid testis has undergone complete atrophy, often due to torsion of the retained testis resulting in ischemic necrosis. During surgery, remnants of the epididymis, ductus deferens, and testicular vessels may be found without identifiable testicular tissue.

Testicular Hypoplasia

Testicular hypoplasia is a congenital condition characterized by abnormally small testes due to incomplete development of seminiferous tubules. It may be unilateral or bilateral and must be distinguished from acquired testicular degeneration.

Clinical features: Small, soft testes; poor semen quality; oligospermia or azoospermia; may have chromosomal abnormalities.

Differentiation from degeneration: Hypoplasia is congenital (present from birth) while degeneration is acquired (history of declining testicular size or semen quality). A thorough breeding history is essential for accurate diagnosis.

Penile and Preputial Disorders

Paraphimosis

Paraphimosis is the inability to retract a protruded penis into the prepuce. It is a common condition requiring prompt treatment to prevent permanent damage.

Phimosis

Phimosis is the inability to protrude the penis from the prepuce. It is less common than paraphimosis in horses.

Causes: Stenosis of the preputial orifice (congenital or acquired from scarring), space-occupying masses, prior trauma or surgery.

Treatment: Surgical correction of stenosis; remove masses; segmental posthectomy if preputial scarring is severe.

Priapism

Priapism is persistent penile erection lasting more than 4 hours. In stallions, it is almost invariably of the 'low-flow' (ischemic) type due to decreased venous outflow.

Causes: Phenothiazine tranquilizers (blocking alpha-adrenergic impulses that mediate detumescence), general anesthesia, nematodiasis, pelvic neoplasia.

Treatment: Early cases may respond to benzatropine mesylate (cholinergic blocker) or phenylephrine injection into the corpus cavernosum. Prolonged cases often progress to paraphimosis and may require phallectomy.

Penile Squamous Cell Carcinoma

Squamous cell carcinoma (SCC) is the most common tumor of the equine penis and prepuce. It primarily affects older geldings (mean age 16 years) and is rare in stallions.

Habronemiasis (Cutaneous)

Cutaneous habronemiasis ('summer sores') affects the external genitalia due to moisture that attracts flies carrying Habronema larvae. Lesions are granulomatous with yellow caseous granules ('sulfur granules') that can be expressed from the tissue.

Differentiation from SCC: Habronemiasis lesions typically appear seasonally (summer), contain eosinophils on cytology, and respond to ivermectin treatment. Biopsy is required for definitive diagnosis.

Treatment: Ivermectin (systemic), corticosteroids, surgical debridement if necessary. Urethral process amputation may be required if affected.

Inguinal and Scrotal Hernias

Inguinal hernia refers to passage of intestine through the vaginal ring into the inguinal canal. Scrotal hernia indicates extension of herniated contents into the scrotum.

Congenital Inguinal Hernia

Common in foals, particularly Standardbreds, draft breeds, and Tennessee Walking Horses. Usually involves small intestine or omentum protruding through a congenitally large vaginal ring.

Clinical features: Easily reducible scrotal enlargement in foals; non-painful unless incarcerated; often resolves spontaneously by 6 months of age.

Treatment: Conservative management with manual reduction and truss application; surgical repair if non-reducible or strangulated; laparoscopic ring closure can preserve testis.

Acquired Inguinal Hernia

Occurs almost exclusively in adult stallions. Two-thirds involve the left side. Standardbreds, Andalusians, and draft breeds are predisposed.

Precipitating factors: Breeding activity, racing, rising after anesthesia, increased intra-abdominal pressure.

Clinical signs: Acute severe colic (strangulating small intestine); enlarged, painful scrotum; scrotal and preputial edema; thickened spermatic cord palpable externally; intestine entering vaginal ring detectable on rectal examination.

Treatment: Surgical emergency requiring prompt intervention. General anesthesia, dorsal recumbency, inguinal exploration with possible celiotomy. Testis usually sacrificed. Attempt manual reduction before surgery under GA in some cases.

Spermatic Cord Torsion

Spermatic cord torsion involves rotation of the spermatic cord around its longitudinal axis, resulting in vascular compromise. Horses with a long gubernaculum or mesorchium are predisposed.

Classification: Torsion less than 180 degrees is often incidental. Torsion greater than 180 degrees causes venous congestion, edema, and eventually arterial occlusion with ischemic necrosis. Torsion greater than 360 degrees is a surgical emergency.

Clinical signs: Acute colic (may mimic GI disease), enlarged painful scrotum, scrotal and preputial edema, thickened spermatic cord. The epididymal tail may be displaced cranially.

Diagnosis: Ultrasound shows decreased or absent blood flow (Doppler), heterogeneous testicular parenchyma if hemorrhage/necrosis present, whirlpool appearance at torsion site, thickened spermatic cord.

Treatment: Unilateral castration to remove affected testis and prevent damage to contralateral testis (sympathetically mediated reflex vasospasm and autoimmune response with antisperm antibodies can affect the normal testis).

Testicular Degeneration

Testicular degeneration (TD) is an acquired condition characterized by progressive decline in testicular function, size, and semen quality. It is a common cause of subfertility and infertility in stallions.

Clinical and Diagnostic Features: Declining semen quality (decreased motility, increased abnormal morphology), decreasing testicular size, soft testicular consistency, increased premature round germ cells in ejaculate. Testicular biopsy shows seminiferous tubule degeneration with Sertoli cell-only tubules in advanced cases.

Treatment: No proven treatment for TD. If a cause is identified, remove or treat it to prevent progression. Manage affected stallions intensively to maximize remaining fertility. Unilateral orchiectomy if one testis is more severely affected.

Memory Aids for Board Preparation

CRYPTORCHID Mnemonic - 'CRYPTO':

• C - Castration is the treatment (surgical removal)
• R - Right side more often inguinal
• Y - Young horses need to be at least 2 years before diagnosis confirmed
• P - Produced by Sertoli cells = AMH (best test)
• T - Testosterone elevated (but 14% inconclusive)
• O - One testis = unilateral (90-95%), usually fertile

Penile Conditions - 'PPP':

• Paraphimosis = cannot retract (penis out)
• Phimosis = cannot protrude (penis stuck in)
• Priapism = persistent erection (Phenothiazines are the problem!)

Penile SCC Risk Factors - 'SUPP':

• Smegma accumulation
• UV exposure
• Pigment lacking (unpigmented skin)
• Papillomavirus (EcPV-2)