Monorchidism in two horses.

This research article discusses monorchidism in horses, a rare condition presenting a complete absence of one testicle. It outlines two case studies, involving a 2-year-old Thoroughbred colt and a 3-year-old Thoroughbred colt, and describes diagnosis and treatment processes.

Case Study One

• The 2-year-old Thoroughbred colt was presented for castration. During the physical examination, the left testis was not palpable, despite no previous surgical history.
• An approach was made through the left inguinal canal via a 10 cm scrotal incision, and no testis could be identified. The epididymis, ductus deferens, and mesorchium were crushed, resected, and submitted for histopathology.
• The right testis was removed by routine open castration. After the operation, a human chorionic gonadotropin (hCG) stimulation test was performed. Testosterone levels at different sampling times were undetectable, indicating castration.
• The absence of stallion-like behaviour post-surgery and the absence of testicular tissue confirmed the diagnosis of monorchidism.

Case Study Two

• A 3-year-old Thoroughbred colt was presented for castration. Normal vital signs were reported with the left testis nonpalpable.
• When surgical exploration failed to reveal the left testicle, the epididymis, the ductus deferens, and the mesorchium were crushed and resected. The right testis was removed by open castration.
• Testosterone levels were undetectable at different sampling times when hCG was administered before and after surgery.
• The horse showing no signs of stallion-like behaviour post-surgery confirmed the diagnosis of monorchidism.

Discussion

• Both horses were diagnosed with monorchidism after considering their history, revealing no previous castration, physical examination, surgical exploration, and hormonal testing.
• While external palpation revealed testicular structures only on the right side, the approach to transrectal examination was avoided due to potentially fractious behaviour and the risk of injury.
• When no testicular tissue was found using the inguinal approach, the parainguinal approach was necessary to explore the caudal abdomen.
• Given the peculiar behavioural differences between castrated and semi-castrated horses, it was necessary to confirm the absence of the left testicle in both horses.
• Basal testosterone concentration can vary. To avoid conclusions based on a single blood sample, the hCG stimulation test was used. The low basal testosterone levels, the lack of testicular tissue, and normal gelding behaviour following surgery affirmed the diagnoses of monorchidism in both horses.