Phenotypic description of multiple congenital ocular anomalies in Comtois horses

Multiple congenital ocular anomalies (MCOA) and their relation to coat colour genotype have not yet been described in Comtois horses, unlike in Rocky Mountain Horses. The objectives of the study were to describe prevalence, nature and severity of congenital ocular anomalies relating to the PMEL17 (Silver) mutation in Comtois horses. Seventy‐four purebred Comtois and one half‐cross Comtois horses, aged 10 days to 18 years, were examined by transillumination, direct ophthalmoscopy and ultrasonography. Hair samples were collected from 34 horses for coat colour genotyping. Sixty‐six horses (88%) revealed cysts (65 horses) or abnormal thickness (one horse) of the ciliary bodies, most of them only diagnosed by ultrasonography. Cysts were localised in the nasal part of the eye in 8 horses. All these horses presented the silver phenotype with mane and tail being white or flaxen, or were chestnut with genetic testing confirming PMEL17 mutation. Of these, 39 (58%) showed MCOA‐syndrome with iridal hypoplasia (100%), cataract (85%), cornea globosa (56%) and lens luxation (8%). Only 8 bay mature horses (11%) were classified as being disease‐free. Genetic testing confirmed that cyst‐phenotype horses were heterozygous carriers for the Silver mutation, MCOA‐phenotype horses were homozygous carriers, and bay horses were noncarriers. Bay homozygous carriers had significantly lighter coat colour than heterozygous carriers. One foal with heterozygous mutation had normal eyes. Thus, MCOA‐syndrome related to PMEL17 mutation is overrepresented in Comtois horses, and should be taken into consideration for breeding purposes. Ultrasonography permitted detection of cysts in all Silver carriers apart from one, some of them being localised in the nasal part of the eye.     

Multiple congenital ocular anomalies syndrome in a family of Shetland and Deutsches Classic ponies in Belgium

The multiple congenital ocular anomalies (MCOA) syndrome has been associated with the Silver phenotype only in a few equine breeds. This report describes the phenotypic and genotypic characteristics of MCOA in a family of Silver‐coated Shetland ponies including a 20‐year‐old stallion, 17‐year‐old mare and their 1.5‐year‐old female offspring. Another 7‐year‐old Silver female Deutsches Classic Pony descending from the same dam but from a different sire, was also examined. Each pony underwent a complete ophthalmic examination, tonometry, ocular ultrasonography and genotyping for the silver coat colour. The stallion had a thickened iris, temporal retinal atrophy and bilateral iridociliary and peripheral retinal cysts. All females presented more severe anomalies: cornea globosa, iridocorneal adhesions, miosis, hypoplastic granula iridica and poorly responsive pupils to light and to pharmacological mydriasis. Iris hypoplasia, anterior cortical cataracts and temporal retinal atrophy were detected in 2 mares. One female presented bilateral lens subluxation. Supero‐temporal cystic structures were confirmed ultrasonographically in all ponies. The stallion was heterozygous for the Silver mutation, whereas all females were homozygous. This is the first report of the MCOA syndrome in a family of Shetland ponies and a Deutsches Classic Pony in association with the Silver phenotype, in Europe.