An XY agonadal Oldenburg warmblood horse exhibiting a male phenotype.

The research investigated an unusual case of a 1.5-year-old horse exhibiting male characteristics despite having an ‘XY’ sex chromosome, missing gonads, and a clitoris-like structure instead of a penis.

Phenotype and Symptoms Observed

• The horse was observed to have a substitutive clitoris-like structure in place of a penis. Despite the anomaly, no other visible irregularities were noted in the external genital organs.
• The owner noted a continuous dripping of urine and a propensity for mild colics, beginning two weeks after the birth of the horse.

Hormonal Levels and Genital composition

• The horse’s testosterone concentration was noted to be 0.01 ng/ml, a level considered below the threshold for castrated male horses (geldings).
• Despite administration of gonadotropin-releasing hormone (GnRH), the horse did not exhibit any response, indicating a lack of reproductive hormone response or production.
• Internal examination revealed a male urinary tract in the horse. However, both gonads and male accessory glands were absent, and no residual structures indicating a female genital tract were observed.

Mesentery Abnormalities

• The researchers detected abnormalities within the mesentery (a set of tissues supporting and connecting the horse’s intestines) which could potentially explain the recurring instances of mild colic in the horse.

Cytogenetic Examination

• A cytogenetic analysis of the horse revealed a karyotype – a chart of the horse’s chromosomes – of 2n = 64,XY. This implies that the horse has 64 chromosomes with an XY pair indicating a genetic male.
• The horse also tested positive for the SRY-PCR test, which detects the presence of the sex-determining region Y (SRY) gene. The SRY is a critical gene on the Y chromosome that normally initiates male sex differentiation.
• Despite the cytogenetic findings, the horse exhibits characteristics of a pseudohermaphrodite or presenting a phenotype that contradicts its genotype.

Significance of the Study

• This study reported the first known case of an agonadal horse exhibiting a male karyotype alongside a pseudohermaphrodite phenotype, presenting a unique example in equine genetics and biology.