Bilateral, progressive stromal corneal dystrophy of the Labrador retriever, caused by the accumulation of glycosaminoglycans in the corneal stroma. It appears between 4.5 and 6 years of age as diffuse corneal opacity with white-yellowish punctate opacities, without inflammation or pain, and causes progressive visual impairment. It is inherited in an autosomal recessive manner and is associated with a missense mutation in CHST6.

Affected breed: Labrador retriever. In the published British cohort the estimated allele frequency was 0.017, corresponding to a theoretical carrier rate of 3.3% and an affected rate of 0.028%. Limited data outside the United Kingdom.

- Bilateral corneal opacity with onset between 4.5 and 6 years of age
- Central white-yellowish punctate opacities, heterogeneous in size
- Diffuse stromal haze that progresses in density and extent
- Corneal vascularisation in some cases as the disease progresses
- Progressive decrease in vision
- Absence of pain and inflammation

MCD in the Labrador retriever was clinically characterised as a bilateral entity presenting in adulthood. Tetas Pont and colleagues (2016) sequenced the single coding exon of CHST6 in one affected dog and one control, and identified the missense variant c.814C>A (p.Arg272Ser). Of 151 controls, 11 were heterozygous and all affected dogs were mutant homozygotes. Later studies described the complete multimodal phenotype, with glycosaminoglycan deposits in the stroma and keratocytes with vacuolated rough endoplasmic reticulum, similar to human MCD (Busse et al., 2019; PMID 30701649).

- Test breeding dogs with the CHST6 test before mating
- Do not mate two carriers: 25% risk of affected homozygotes
- A carrier may be mated to a clear dog; test the offspring intended for breeding
- Progressively replace carriers with clear offspring without narrowing the gene pool
- Do not regard a CHST6-clear homozygote as free of other corneal dystrophies

Differential diagnosis with other corneal dystrophies (superficial, endothelial) and with chronic non-hereditary keratitis. Optical coherence tomography and in vivo confocal microscopy show multifocal hyperreflective regions and alterations in stromal reflectivity. Genetic testing identifies carriers before signs appear, which is key because onset is in adulthood.

1. Tetas Pont R, et al. A Carbohydrate Sulfotransferase-6 (CHST6) gene mutation is associated with Macular Corneal Dystrophy in Labrador Retrievers. Vet Ophthalmol 2016;19(6):488-92. PMID: 26585178
2. Busse C, et al. Phenotype of macular corneal dystrophy in Labrador Retrievers: a multicenter study. Vet Ophthalmol 2019;22(3):304-15. PMID: 30701649
3. OMIA:002071-9615 (LOC489707/CHST6). https://omia.org/OMIA002071/9615/