Sex reversal syndrome in a five year old Friesian horse

Intersexuality (hermaphroditism, pseudohermaphroditism, sex reversal syndrome) occurs due to disruptive sexual development. Organs of both genders are present in a single individual. External or internal sexual organs can be affected. Sexually intermediate forms occur in various mammal species, such as pigs, cattle, goat, or whales, but are rarely seen in horses. The most frequent form of intersexuality in the horse is pseudohermophroditism, i.e. gonads of only one sex are present. Female pseudohermaphroditism corresponds to a horse that has female gonads and internal sexual organs and an XX karyotype, but bears male external organs. Male pseudohermaphroditism, which corresponds to male gonads, female external organs and an XY karyotype has been described most frequently in horses. This syndrome is also referred to as testicular feminising syndrome and is related to an androgen receptor deficiency. In horses with a sex reversal syndrome, chromosomal and gonadal sex do not correspond to each other. An XX sex reversal syndrome is probably caused my mutation of autosomal genes resulting in disruptive development of the testes. In contrast, an XY sex reversal syndrome is related to a deletion of the SRY gene (sex determining region Y). Various chromosomal variations related to intersexuality such as 64,XX/64,XY, 63,X0/64,XY, or 63,X0/64,XXY genotypes have been described. Usually, these horses are presented for consultation because of infertility or erratic behaviour. In February 2010, a five year old Friesian horse was presented for consultation at the Tierklinik Wahlstedt because of its stallion-like behaviour. Its general condition and vital signs were good. The horse showed age appropriate development. Physical examination showed an abnormal combination of external genitalia: a hypoplastic, hypospadic penis perineally, and an udder, which was positioned slightly more cranially than usual. Findings of the rectal palpation included gonad-like structures in the dorsolateral aspcect of the abdomen in a position typical for the ovaries, which could also be seen in ultrasound examination. An inguinal canal was present on the right side. Laboratory findings included estradiol and progesterone levels of 15.6 ng/l or >0.2 ng/ml, respectively. This corresponds to levels seen in mares being in estrus. The level of testosterone was 0.025 ng/ml, which is lower than reference values given for both cryptorchid (0.1-0.3 ng/ml) or castrated (0.04 ng/ml) male, but higher than those of female horses (< 0,02 ng/ml). In contrast, testosterone concentration was 0.14 ng/ml in a blood sample taken by the transferring veterinarian in May 2009. A bilateral gonadectomy was carried out laparoscopically and yielded gonads closely resembling testes. Histopathological examination showed atrophied testes and epididymal tissue. Examination of chromosomes showed a 64,XX( genotype. SRY translocation could be ruled out by molecular genetics employing PCR. According to cytogenetic, histopathological, and adspectory findings, a sex reversal syndrome was diagnosed. This is assumed to be passed on by autosomal recessive genetics. The horse was treated antibiotically and antiphlogistically with penicillin/dihydrostreptomycine and flunixine, respectively. Development was without complications, and the horse was discharged after 6 days. A control of hormonal concentrations after six weeks yielded values of 22.27 pg/ml for estradiol and < 0.2 ng/ml for both progesterone and testosterone. The horse presented much calmer.