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. 1. Introduction Aniridia is a rare condition marked by partial or complete absence of the iris. This condition has been reported in horses [6–11], cattle [1], laboratory animals [2, 3], and humans [4, 5]. In Belgian horses [6] and Quarter horses [7], the defect has been reported to be genetically transmitted as an autosomal dominant trait, but at least one case in a Swedish Warmblood was not dominantly inherited [8]. In humans, the anomaly either presents as a familial condition with autosomal-dominant inheritance or is sporadic [4, 5]. Affected animals are usually photophobic with absent direct and indirect pupillary light responses bilaterally, and they often have additional ocular abnormalities including dermoid lesions and cataracts. Dermoid lesions, like many instances of aniridia, form during foetal development, and our understanding of the molecular genetics of ocular development has improved since the last case report of aniridia in horses [8]. Therefore, the purpose of this report is to describe the clinical and histologic features of aniridia in 2 related Tennessee Walking horses, especially considering “new” information about the genetics of ocular involvement. 2. Case Presentation Two Tennessee Walking horses were presented to the University of Tennessee Equine Hospital for bilateral ocular abnormalities. Horse A was a 15-year-old mare, and horse B was her 12-month-old female offspring. Very limited history was available because both animals were rescued. Visual deficits had been recognized in both animals prior to presentation, and the new owners had noticed bilateral ocular opacities in both horses. On ophthalmic examination of horse A, both direct and indirect pupillary light responses were absent in both eyes. However, the horse did have positive menace responses bilaterally. The tips of the ciliary processes were visible in both eyes, and the
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