Autosomic 27 Trisomy in a Standardbred Colt

A 19-month-old Standardbred colt was donated to the University of Pennsylvania School of Veterinary Medicine with a suspicion of intersexuality. The anal-genital distance and penis were normal, and there was no evidence of intersexuality, but the colt was bilaterally cryptorchid. Several aspects of the colt's behavior appeared unusual, including general temperament and behavior described as sympathetically dull and affable. With herd mates, the colt appeared slow to perceive or to learn the usual intraspecies social cues. An atypical gait characterized by intermittent unnatural shuffle of the hind limbs, sliding them along in short rhythmic strides for 3 to 10 seconds at a time was noted at times when a horse might normally transition from a slow walk to a fast walk or a slow trot. Occasionally the colt exhibited slight protrusion of the tongue through the teeth and lips with jaw movements and smacking of the tongue against the teeth as if struggling to retract the tongue to the normal position. Evaluation of the karyotype combined with fluorescent in situ hybridization (FISH) revealed an abnormal male karyotype showing trisomy of chromosome 27 (65, XY + 27). The colt was euthanized at 24 months of age, and a necropsy revealed no significant abnormalities. This case of trisomy was not associated with developmental abnormalities described in other rare reports of trisomy in horses; however, some features were strikingly similar to that of humans with trisomy 21. FISH was demonstrated to be an excellent method for correct identification of equine chromosomes.