Ocular anomalies in a herd of Exmoor ponies in Canada

To characterize the ocular anomalies, determine their recurrence in families and assess the mode of inheritance of the most prevalent anomalies through pedigree analysis in a herd of Exmoor ponies.

Thirty Exmoor ponies residing in south-western Ontario, Canada.

Ophthalmic examination was performed using direct ophthalmoscopy, slit-lamp biomicroscopy and indirect ophthalmoscopy, pre- and post-pupillary dilation. Following compilation of ocular findings, pedigree analysis was conducted to assess the recurrence pattern of specific anomalies in families and the data were tested for significance for breed prevalence and gender dependence, using Fisher's exact test.

Twenty-eight purebred ponies and two cross-breds, ranging from 14 days to 31 years were examined. A prominent brow above the eyes characteristic of this breed was a consistent periocular feature of adult ponies. In nine ponies, eyelid sarcoid, iris nevus, Mittendorf's dot, hyaloid artery remnant, and hyper-reflective vitreal strands were detected while the lesions recorded in the remaining 21 included macroblepharon, persistent pupillary membranes, iris hypoplasia, cataracts, focal choroidal or retinal pigment epithelium hypoplasia and optic nerve head coloboma. The most common ocular finding was cataracts seen mainly in females. Statistical tests on prevalence data confirmed a significant breed and sex association. Pedigree analysis favored a sex-linked mode of inheritance for cataracts in this line of Exmoor ponies.

Although the ocular anomalies detected in this group have been previously reported in other breeds of horses, this is the first report of equine cataracts showing a familial trend suggestive of a sex-linked genetic defect.