Aniridia in Two Related Tennessee Walking Horses

Abstract

Aniridia in horses is rare and has previously been reported to be genetically transmitted in Belgian horses and Quarter horses. This paper describes the defect in 2 related Tennessee Walking horses, with special reference to new findings regarding the molecular genetics of ocular development and how they might relate to equine aniridia. In addition to aniridia, these 2 horses possessed additional ocular abnormalities including cataracts and dermoid lesions. Euthanasia was elected, and the eyes were examined histologically. Iris hypoplasia, atypical dermoids, and cataracts were confirmed in both horses. Due to the heritability of aniridia in horses, breeding of affected animals is not recommended.

Breed predispositions to congenital and juvenile cataracts in horses at two academic institutions

“…[6][7][8] Congenital and juvenile cataracts are known to have a genetic basis in many species, including horses. [9][10][11][12][13][14][15] However, aside from cataracts that occur as a part of multiple congenital ocular anomalies (MCOA), 16 specific mechanisms of inherited cataracts in horses have not yet been elucidated. Cataracts have been documented to have an autosomal dominant inheritance pattern in related Morgan horses (nuclear cataracts) 11,17 and also in Belgian draughts, American Quarter Horses and Tennessee Walking Horses in which cataracts are reported concurrently with ocular dermoids and iris hypoplasia or aniridia.…”

“…Cataracts have been documented to have an autosomal dominant inheritance pattern in related Morgan horses (nuclear cataracts) 11,17 and also in Belgian draughts, American Quarter Horses and Tennessee Walking Horses in which cataracts are reported concurrently with ocular dermoids and iris hypoplasia or aniridia. 10,12,13 A pedigree analysis of Exmoor ponies affected with varying stages of cataracts is suggestive of a sex-linked inheritance pattern in the breed, 18 and a prevalence of cataracts of 17.2% in Australian Thoroughbreds is suggestive that the condition may also be inherited in that breed. 19 To date, no genetic variants have been identified to cause nonsyndromic inherited cataracts in horses, while a single variant is reported in dogs and there are at least 71 mapped loci for inherited cataracts reported in humans.…”