Eye problems in silver coloured Icelandic horses
A mutation in the premelanosome protein (PMEL) gene causes both the silver colour in horses as well as the Multiple Congenital Ocular Anomalies (MCOA) syndrome. A recent study showed that Comtois horses carrying the silver mutation had deeper anterior chambers of the eye compared to wild-type horses, which could potentially cause refractive errors.
Horses homozygous for the silver mutation (TT) are shown to be affected by multiple eye defects causing visual impairment or blindness. In horses heterozygous for the silver mutation (CT) the effect is less severe. The aim of the current study was to examine if the mutation also causes refractive errors.
The study was executed by examining 152 Icelandic horses, 71 CT horses and 5 TT horses. Each CT or TT horse was matched with a wild-type horse of around the same age. Skiascopy and an ophthalmic examination were performed on all horses. The associations between age, eye, genotype and sex were tested.
The results show that CT horses older than 16 years were on average more myopic than wild-type horses of the same age, while horses younger than 16 years showed no difference in refractive state. TT horses were myopic regardless of age. The results suggest that the silver mutation has a progressive effect on the eye. However, horses were not followed from foals up to old age. Further research is required to analyse if the MCOA syndrome is progressive.
> From: Johansson et al., BMC Veterinary Research 13 (2017) 153. All rights reserved to BioMed Central Ltd . Click here for the online summary.