A 4-year-old Quarter Horse mare was referred to the Equine Reproduction Laboratory for evaluation of aggressive, stallion-like behavior when around other mares. On initial physical examination, the horse had the appearance of a normal mare. However, ultrasound examination failed to identify any female structures – no ovaries, no uterus, and no cervix. In contrast, ultrasound also revealed a structure in the right abdominal cavity that had the appearance of a testis. There was not a matching structure noted on the left side. A vaginal speculum exam revealed a shortened vaginal cavity with no cervix at the front end.
Blood samples were subsequently collected for hormone analysis and genetic testing. Testosterone and other hormones were markedly elevated over values present in normal mares and were in the range associated with stallions. The genetic test results revealed a normal number of chromosomes for a horse (64), but with the sex chromosomes consistent with that of a male (XY). The genetic code for differentiation of the embryonic gonads into the male form (testes) is located on the Y chromosome.
Disorder of sexual development
Based on the ultrasound exam, vaginal speculum exam, hormone analysis and genetic test, a diagnosis was made of a disorder of sexual development (DSD), or the physical appearance of a mare, but the genetics and (single) gonad of a male.
The potential causes of aggressive stallion-like behavior in a mare include an alpha (boss) mare, an ovarian tumor, pregnancy, the estrus stage of the cycle, administration of an anabolic steroid, and a disorder of sexual development. The challenges are to determine which of the potential causes is responsible for the abnormal behavior and if there are any treatments. In this case, a cause was identified and a treatment for the abnormal behavior was available.
A laparoscopic (“keyhole”-type) surgical procedure was performed with the mare sedated and standing in examination stocks. A laparoscope camera was inserted through a small incision in the flank which enabled the surgical team to view the abdominal area of the mare. A cryptorchid testis was identified and removed from the right side of the abdomen. A search of the left side confirmed the absence of any gonadal structures. Follow-up evaluation 5 weeks after surgery indicated that the abnormal behaviors were absent and the hormone values were now in the range of a gelding.
What initially appeared to be just a “mare with an attitude” turned out to be a very rare case of a horse with the outward appearance of a mare, but the genetic makeup and gonad of a cryptorchid male, albeit one testis short of a full pair. This unique case provided an excellent learning opportunity for faculty, staff, residents, and veterinary students to solve a clinical mystery using a combination of patient history, clinical signs, hormonal analysis, laparoscopy, histopathology, cytogenetic analysis, and molecular analysis.