The 5-year-old Quarter Horse looked like a mare but acted like a stallion. Indeed, an ultrasound examination of the reproductive tract revealed testes instead of ovaries. Results from hormone and chromosome analyses showed elevated anti-Müllerian hormone and testosterone and a karotype of 64,XY and SRY-positive; the horse had a disorder of sexual development.

The Quarter Horse’s case was one of several Patrick M. McCue, DVM, PhD, Dipl. ACT, Iron Rose Ranch Professor of Equine Theriogenology at Colorado State University’s Veterinary Medicine and Biomedical Sciences, presented during the 2022 American Association of Equine Practitioners Convention, held Nov. 18-22 in San Antonio, Texas.

Veterinarians might find themselves evaluating a horse for a sexual development disorder under the following clinical circumstances:

• During a routine breeding exam.
• If the horse is abnormal in size or stature.
• If a phenotypical (based on physical characteristics) female is behaving like a male.
• If a mare fails to show estrus.
• If a mare fails to cycle.
• If the horse has reduced fertility or infertility.
• After embryonic loss.
• If the horse has ambiguous external genitalia.

Understanding Reproductive Development

To understand a disorder of sexual development, McCue said one must first understand the normal order of reproductive development. He explained that the process starts with the establishment of genetic sex, which is determined by the type of sperm that fertilizes the egg and whether that sperm contained an X or Y sex chromosome. The egg or oocyte always has an X sex chromosome. Consequently, if the sperm that fertilized the egg had an X chromosome, the resulting embryo would be female, with two X chromosomes. In contrast, if the sperm had a Y sex chromosome, the embryo would be male, with X and Y sex chromosomes.

Gonad development and establishment of gonadal sex follows the genetic sex. Testes will develop if a Y chromosome is present, and ovaries will develop in the absence of a Y chromosome. More specifically, the presence of the sex determining region of the Y chromosome (SRY region) carries the genetic code for development of the male gonad.

All individuals carry both male and female internal genitalia. The male internal genitalia, called the Wolffian duct, will develop if stimulated by testosterone produced by Leydig cells from the testes. Also in the male, the female internal genitalia, called the Müllerian duct, will regress following production of anti-Müllerian hormone (AMH) produced by Sertoli cells from the testes. In the absence of testes, testosterone, and AMH, the Wolffian duct will not develop and the Müllerian duct will develop and form the oviduct, uterus, and cervix.

The external genitalia develop into the male form (i.e., the penis and scrotum) under the influence of dihydrotestosterone (DHT) produced in the tissues by conversion from testosterone originating from the testes. In the absence of DHT or a functional androgen receptor, the external genitalia will develop into the female form (i.e., the vulva and clitoris).

Overall, the development of the gonads, internal genitalia, and external genitalia depend on the presence or absence of the SRY region normally found on the Y chromosome.

Diagnosing Disorders of Sexual Development

To work up a possible clinical case of disorder of sexual development in a horse, McCue said, veterinarians should perform the following:

• A complete medical and reproductive history
• Behavior assessment
• General physical exam
• External genitalia exam
• Transrectal ultrasound exam
• Vaginal speculum exam
• Endocrine analysis
• Exploratory laparoscopy (if indicated)
• Chromosomal analysis (karotype), including PCR analysis for the SRY gene
• Histopathology of excised gonadal tissue (if indicated)

“It’s been estimated that somewhere between 1 and 5% of horses have some type of chromosome disorder,” he said. “In a more focused study, the incidence of chromosomal abnormalities in problem mares was 4%. In sterile mares, the incidence was 12.8%. Chromosomal abnormalities are more common than we realize.”

The most common chromosomal abnormalities are 63X monosomy (63,XO) and its mosaic forms, said McCue. Other less common chromosomal abnormalities include 64,XY SRY-negative and 64,XY SRY-positive disorders of sexual development. Other types of chromosomal abnormalities are rare in horses.