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Résumé :
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Multiple congenital ocular anomalies 5MCOA) and their relation to coat colour genotype have not yet been described in Comtois horses, unlike in Rocky Moutain 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 1 days to 18 years, were examinated by transillumation, direct ophtalmoscopy 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 horses) 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 geneteic 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 heterygous 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 appart from one, some of them being localised in the nasal part of eye.
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