Irregular reproductive cyclicity in a mare with a mosaic karyotype: 64,XX/64,XY/63,XO

Summary A 5‐year‐old Oldenburg maiden mare presented for reproductive evaluation following several months of persistent oestrus‐like behaviour, as observed by the owner. At referral, examination revealed anatomical abnormalities of the caudal reproductive tract, including asymmetry of the ventral labium and multiple soft tissue horizontal bands located in the vulva and vestibulo‐vaginal fold. These findings raised the suspicion of an underlying disorder of sexual development (DSD). Cytogenetic evaluation via karyotyping was performed and revealed sex chromosome mosaicism. Chromosomal mosaicism is defined as the presence of two or more karyotypically different cell lines in the same individual. In this case, the mare was predominantly 64,XX, which is standard for an equine female. However, the mare also had individual cells with 63,XO (X monosomy) and 64,XY (male) karyotypes. While the presenting sign of persistent oestrus‐like behaviour may have been associated with this chromosomal abnormality, a definitive causal relationship could not be established. The body of literature documenting and explaining cases of sex chromosome mosaicism in horses is limited, but the recent innovations in genetic testing have allowed for DSD to be identified more frequently and with greater ease. Recent publications have shown that mosaicism may be underdiagnosed in the equine population and can occur in phenotypically normal and fertile individuals. Therefore, mosaicism should be considered as a differential diagnosis in cases of infertility and sub‐fertility. This report describes the diagnostic process leading to the identification of DSD in a mare presenting with persistent oestrus‐like behaviour. It illustrates how routine reproductive complaints may have a more complex underlying aetiologies with significant implications for the horse's future working and reproductive potential. Furthermore, it underscores the value of considering DSD when investigating ‘the problem mare’ and considering karyotype as an ancillary diagnostic test.