Monorchidism in a Phenotypic Mare With a 64,XY, SRY-Positive Karyotype.
Authors: Middlebrooks Brittany, McCue Patrick, Nelson Brad, May Emily, Divine Christina, Barton Charlie, Conley Alan
Journal: Journal of equine veterinary science
Summary
# Editorial Summary Disorders of sexual development (DSD) remain underrecognised in equine practice, with monorchidism—the presence of a single testicle—being exceptionally rare in horses. This case describes a 4-year-old Quarter Horse presenting the unusual combination of female external genitalia and behaviour with a 64,XY, SRY-positive karyotype (the chromosomal signature of a genetic male), ultimately diagnosed with a single functional testicle in the right dorsal abdomen. Diagnostic integration proved essential: transrectal ultrasonography and standing laparoscopy localised the gonad, histopathology confirmed testicular tissue, whilst elevated serum testosterone and anti-Müllerian hormone (AMH) concentrations pre-surgery and their subsequent decline to basal levels post-removal provided endocrine validation of the diagnosis. The case illustrates how a systematic approach combining physical examination, imaging, hormonal profiling, cytogenetics, and histology can unravel complex presentations that might otherwise be overlooked or mismanaged. For equine practitioners, this work underscores the importance of investigating unusual behavioural presentations in mares with normal external genitalia—particularly unexplained stallion-like conduct—as underlying DSD may benefit from surgical intervention and hormone management.
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Practical Takeaways
- •Monorchidism is rare in horses; suspect DSD in mares presenting with stallion-like behavior and abnormal palpation findings despite normal external genitalia
- •Elevated testosterone and AMH in a phenotypic mare should trigger further investigation including imaging and hormonal confirmation before and after gonadectomy
- •Standing laparoscopy is an effective diagnostic and therapeutic tool for locating and removing cryptorchid/monorchid gonads in horses
Key Findings
- •A phenotypic mare with stallion-like behavior was diagnosed with monorchidism and a 64,XY, SRY-positive karyotype
- •Presurgical hormonal evaluation revealed elevated serum testosterone and anti-Müllerian hormone (AMH) concentrations that decreased to basal levels 5 weeks postsurgery
- •Transrectal ultrasonography, laparoscopic exploration, histopathology, and cytogenetic analysis were combined to diagnose this disorder of sexual development