XY Sex Reversal in a Quarter Horse Mare

Abnormalities of sexual differentiation in horses are commonly reported, including monosomy X and sex reversal cases. These manifest as irresolvable infertility issues with the animal having either ambiguous or female external genitalia and gonads that may not match the karyotype or are hypoplastic. A ten year-old Quarter Horse mare presented for breeding management. The client reported that the mare had never been bred and had erratic heat cycles. The external genitalia appeared normal. Transrectal palpation detected a cervix with low tone, small uterine horns and small, firm structures in the areas of the ovaries, which measured 1.2 by 1.6 cm and 1.3 by 1.8 cm on the left and right respectively, using transrectal ultrasonographic evaluation. An intersex condition with gonadal dysgenesis or a prior ovariectomy were considered as potential differential diagnoses. [...]