Cryptorchidism
Basic Information 
Epidemiology
Species, Age, Sex
Horses are one of domestic animal species with the highest prevalence rate of cryptorchidism. The overall prevalence is about 3% to 4%, with a much higher percentage in certain breeds.
Genetics and Breed Predisposition
• Percherons, American Saddlebreds, and American Quarter Horses have a higher risk than Thoroughbreds, Standardbred, Morgans, Tennessee Walking Horses, and Arabians.
• Unilateral testis retention occurs more often than bilateral. The right testis is more often retained in the inguinal canal and the left testis is more commonly retained in the abdomen cavity.
Risk Factors
• Familial: Cryptorchidism recurs in families and displays variability in the degree of manifestation (inguinal, abdominal, ectopic testes) and in the pattern of occurrence (breed differences).
• Drugs: The vulnerability of testicular descent to estrogenic and antiandrogenic influence in certain species (eg, pigs) should raise questions when potential hormones or drugs with hormone-like effects (eg, estradiol, diethylstilbestrol, progestagens, cimetidine, flutamide, finasteride) are considered during pregnancy.
Clinical Presentation
Disease Forms/Subtypes
Depending on the precise location of the retained testicle, different nomenclature may be used.
• Inguinal cryptorchid (high flanker) is defined as a stallion with one testis that did not pass the superficial inguinal ring.
• Complete abdominal cryptorchid has both the testis and the epididymis in the abdominal cavity.
• In partial abdominal cryptorchid, a portion of the epididymis lies in the inguinal canal.
• Ectopic retained testicles are located subcutaneously and cannot move toward the scrotum.
Physical Exam Findings
The conformation of the scrotum is meticulously assessed for the presence of testicles and for signs of castration scars. If no evidence of a testicle is noticed, the animal should be sedated (xylazine) to ensure good relaxation of the cremaster muscle revealing the testicle. In addition to the visual examination, palpation of the scrotum and the inguinal area is essential.
Etiology and Pathophysiology
• Sex differential events are generally influenced by multigenetic and multi-environmental factors.
• Failure of the testis to regress in size, overstretching of the gubernaculum, or insufficient abdominal pressure could mechanically prevent the engagement of the testis in the vaginal ring and further stop the testis’ advance through the inguinal canal.
• Defective hypothalamic-pituitary axis and deficiency of gonadotrophin hormone have been proposed.
• Factors such as Müllerian inhibiting substance (MIS), genitofemoral nerve and calcitonin gene-related peptides, and insulin-like factor-3 could play a role in cryptorchidism.
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