Chestnut Comtois horses carrying a silver coat colour mutation have been found to have a greater prevalence of eye abnormalities, a study in France has revealed.
The Comtois is a light draft horse, of up to about 16hh, and are generally chestnut with flaxen manes and tails, but can also be bay. Researchers in France found that chestnuts of the breed had a higher prevalence of Multiple congenital ocular anomalies (MCOA) relating to the PMEL17 (Silver) coat colour mutation. The bay horses were negative for the mutation.
In the study, 74 purebred Comtois and one half-bred, 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 eight 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%).
Ultrasonography detected cysts in all Silver carriers apart from one, some of them being localised in the nasal part of the eye.
Only eight 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.
The researchers said that MCOA-syndrome related to PMEL17 mutation was overrepresented in the breed, and should be taken into consideration when breeding.
The Comtois is believed to have descended from horses brought by the Burgundians of northern Germany to France during the fourth century, and they have been bred at the Franche-Comté and in the Jura Mountains since the sixth century. In the Middle Ages they were used as war horses.
During the 16th century, the Comtois breed was used to improve the Burgandy Horse. In the 19th century, other draft horses such as the Norman, Boulonnais, and Percheron were bred into the Comtois, and more recently the Ardennes was used to produce a stronger horse with better legs. Today, the Comtois breed is second only to the Belgian draft horse in number in France.
In the16th century, they were used as a cavalry and artillery horse, and today they are used for hauling wood in the pine forests of the Jura in the mountainous regions of the Massif Central, and for working in the vineyards in the Arbois area. They are also bred for the French horsemeat industry.
Phenotypic description of multiple congenital ocular anomalies in Comtois horses: M. Depecker (Centre International de Santé du Cheval d’ONIRIS (CISCO), LUNAM Université, ONIRIS); E. Ségard and J-L. Cadoré (Université de Lyon, VetAgro-Sup, Campus Vétérinaire de Kyon). DOI: 10.1111/eve.12075