Phenotypic description of multiple congenital ocular anomalies in C omtois horses

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Summary Multiple congenital ocular anomalies ( MCOA ) and their relation to coat colour genotype have not yet been described in C omtois horses, unlike in R ocky M ountain H orses. The objectives of the study were to describe prevalence, nature and severity of congenital ocular anomalies relating to the PMEL 17 ( S ilver ) mutation in C omtois horses. Seventy‐four purebred C omtois and one half‐cross C omtois 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 PMEL 17 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 S ilver 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 PMEL 17 mutation is overrepresented in C omtois horses, and should be taken into consideration for breeding purposes. Ultrasonography permitted detection of cysts in all S ilver carriers apart from one, some of them being localised in the nasal part of the eye.

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