Page 563 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

538 CHAPTER 2

VetBooks.ir target organs may be responsible for the phenotypic 2.142
female appearance. Recently, a family of Quarter
horses was identified in which 64XY-SRY positive
male pseudohermaphroditism was inherited due to
androgen insensitivity caused by a mutation on the
start codon of the androgen receptor gene.

Clinical presentation
A variety of clinical presentations have been described
with DSD in horses, ranging from infertility in
otherwise normal appearing animals to stallion-
like behaviour with ambiguous external genitalia.
Affected animals are often presented for veterinary
evaluation due to inappropriate male-like behaviour
in an apparent female or abnormal appearing external
genitalia. Rarely, an explanation is sought for a high
testosterone level following regulatory testing in a
performance filly. In monosomy X (64X0; Turner’s
syndrome), a mare intended for breeding may pres-
ent for small stature and failure to cycle. Transrectal
ultrasound examination reveals small, inactive ova-
ries and a small, flaccid, thin-walled uterus and cer- Fig. 2.142 View of the external genitalia of a true
vix. In trisomy (65XXY; Kleinfelter’s), animals are hermaphrodite. The animal presented as a filly with
male in appearance with either scrotal or cryptorchid stallion-like behaviour and ambiguous genitalia. Note
testes; however, they are sterile as ejaculates are azo- the presence of clitoromegaly. Chromosomal analysis
ospermic. On examination the testes and penis are revealed a 64XX karyotype. A blind-ending vagina was
found to be hypoplastic. True hermaphroditism, present, but no tubular reproductive structures were
defined as an individual with both ovaries and testes, identifiable on transrectal palpation and ultrasound.
either in the same or separate gonads, is rare in the
horse (Figs. 2.142, 2.143). Typically, these animals
exhibit stallion-like behaviour and have ambiguous 2.143
external genitalia, including a fused vulva, clito-
romegaly and a poorly developed or absent uterus.
Retained abdominal ovotestes are found on palpation
per rectum with ultrasonography.
Female pseudohermaphroditism is very rare in
the horse. In these cases the gonads are ovaries, the
genotype is female (64XX) and the external genita-
lia are male, although often poorly developed and
ambiguous.
Sex reversal occurs when the genetic sex and
gonadal sex disagree. Both XX and XY sex reversal
have been reported in the horse. In horses, XX indi-
viduals with testes are SRY negative and demonstrate Fig. 2.143 Two abdominally retained gonads were
variable degrees of virilisation of female-appearing present in this filly, which were found histologically
external genitalia. XY individuals with ovaries have to be ovotestes. The filly underwent gonadectomy and
exhibited severe gonadal dysgenesis. clitoridectomy and was retained as a pleasure mount.