Androgen Insensitivity

History. You are called to examine a mare that has recently been brought to a broodmare farm after a successful racing career. It is late spring, but the mare has shown estrous behavior only on an intermittent basis.

Clinicsl Examination. As you approach the mare, you notice that she is large. The genital examination reveals a normal vulva, but when you introduce the speculum, it can be inserted only about 5 to 6 inches. Digital examination of the genital tract through the vulva results in a finding of complete blockage at the level of the Vestibulovaginal conjunction, with no evidence of the external os of the cervix. On examination per rectum, you find the vagina, cervix, uterus, and oviducts to be absent; the gonads are symmetric in shape without the usual indentation caused by the ovulation fossa that is characteristic of equine ovaries.

Comment. You tell the shocked owner that you are sus­picious that the animal is not really a mare, but a male mas­querading as a female. One of the easiest ways to confirm the diagnosis is to have a testosterone analysis done on plasma. If the gonads are testes, they still retain the ability to secrete significant amounts of testosterone, even though they are retained (cryptorchid, in a sense) within the abdominal cavity. You could also have a chromosomal analysis to verify that the animal has an XY sex chromosome complement. In this case, it is likely that the testes were able to secrete the nuilleriαn- Inhibitingfactor₉ which resulted in regression of the tubular system of the genital tract that forms the female system (oviducts, uterus, cervix, vagina). But why, asks the owner, did the external genitalia not turn out to be male? There is evi­dence, in cases such as this, that the tissues of the external genitalia lacked critical receptors for androgens; thus the external genitalia were female in type. The rule of sexual development is that the female state develops in the absence of testicular input, the latter including mullerian-inhibiting factor and testosterone. In this case the lack of sexual differ­entiation also appeared to involve the hypothalamus, because the “mare” did not exhibit male behavior despite relatively high testosterone concentrations.

Treatment. There is obviously no treatment for this syndrome. It would be unethical to take her back to the track and race her once again as a female, when the owner knows that “she” is a male. The horse could be used as a performance horse (i.e., hunter/jumper, dressage, eventing) or for pleasure riding.