Monorchidism in a Phenotypic Mare With a 64,XY, SRY-Positive Karyotype.

The research presents a unique case study of a mare that showed stallion-like behavior and was found to have only one testicle, a condition called monorchidism, with a chromosomal configuration that is typically found in males (64,XY, SRY-positive).

Clinical Assessment and Diagnosis

• The 4-year-old Quarter Horse mare showed no clear abnormalities in its external genitalia. However, the internal assessment using transrectal palpation did not detect any female reproductive organs such as the uterus or cervix.
• Further, through transrectal ultrasonography, a single testicle was observed in the right dorsal abdomen of the mare. This was a rare condition known as monorchidism. Supporting this diagnosis, hormonal evaluations revealed high concentrations of testosterone and anti-Müllerian hormone (AMH), generally associated with males.
• The testicle was ultimately removed through standing exploratory laparoscopy and subjected to histopathological examination for further confirmation.

Cytogenetic and Molecular Analysis

• A cytogenetic analysis was conducted to examine the chromosomal constitution of the mare. Against usual female (XX) genetics, the results revealed that the mare had a 64,XY, SRY-positive chromosomal profile, which is a pattern typically seen in males.
• This finding further corroborated the diagnosis of a disorder of sexual development given that the chromosomal configuration was incongruent with the phenotypic sex of the horse.

Hormonal Evaluation Post-Surgery

• Five weeks subsequent to the testicle removal, levels of testosterone and AMH in the serum diminished to basal levels. This was taken as another piece of evidence in favor of the diagnosis of monorchidism as these should be elevated in a typical male animal.

Conclusion

• In conclusion, this rare case of monorchidism in a phenotypic mare elaborates on the importance of combining clinical signs, endocrine evaluations, chromosomal and molecular analysis, and histopathology in diagnosing disorders of sexual development.
• It also highlights the possibility of underestimation of such conditions in the equine population and calls for increased attention to irregular behavioral signs in horses, as they might indicate rare but significant health issues.