The refractive state of the eye in Icelandic horses with the Silver mutation.
Authors: Johansson Maria K, Jäderkvist Fegraeus Kim, Lindgren Gabriella, Ekesten Björn
Journal: BMC veterinary research
Summary
# Editorial Summary The Silver coat colour mutation in horses stems from a PMEL gene variant that causes Multiple Congenital Ocular Anomalies (MCOA), with homozygous carriers (TT) experiencing severe visual impairment or blindness and heterozygous animals (CT) typically developing less severe ciliary body or retinal cysts. Researchers investigated whether the refractive errors previously documented in other breeds carrying this mutation also occur in Icelandic horses, using skiascopy and ophthalmic examination in 152 animals (71 CT horses and matched CC controls of equivalent age). Contrary to expectations based on earlier findings of deeper anterior chambers in Silver carriers, the CT Icelandic horses showed no significant refractive errors compared to wild-type controls when analysed by linear mixed-effect modelling. These findings suggest that whilst the ocular anatomy of heterozygous Silver horses may differ, functional refractive status—at least in this population—may not be substantially compromised, though the small cohort of homozygous (TT) horses precluded definitive conclusions about their refractive characteristics. For practitioners, this indicates that heterozygous Silver carriers may retain normal optical function despite documented structural eye changes, though comprehensive ophthalmological assessment remains essential given the breed-specific and individual variation in MCOA manifestation.
Read the full abstract on PubMed
Practical Takeaways
- •Icelandic horses carrying the Silver mutation should be screened for ocular abnormalities, as homozygous (TT) animals are at risk of visual impairment or blindness
- •Heterozygous (CT) carriers may have subtle refractive errors or cyst formation that could affect vision—clinical examination is warranted even if signs appear mild
- •Breeding decisions should consider the PMEL mutation status, particularly avoiding homozygous matings that cause severe congenital ocular disease
Key Findings
- •152 Icelandic horses were genotyped and examined for refractive errors associated with Silver mutation (71 CT heterozygous, 5 TT homozygous, matched with CC wild-type controls)
- •Skiascopy and ophthalmic examinations were performed to assess refractive state in relation to PMEL gene mutation status
- •TT homozygous horses showed severe multiple ocular defects; CT heterozygous horses showed milder signs including ciliary body cysts
- •Refractive errors were analysed using linear mixed-effect models controlling for age, eye, genotype and sex variables