Part of the official UK Kennel Club testing scheme in Retriever (Labrador)
Macular Corneal Dystrophy is an inherited and progressive eye disease. Initial clinical signs are cloudy corneas and/or visual impairment, first observed between four and six years of age. In addition, white/grey opacities are also observed. The cloudiness and opacities are caused by an abnormal accumulation of glycosaminoglycans in the cornea.
MCD is progressive; as the density of the corneal haze increases, increased vascularization is observed and vision is increasingly impaired.
The single nucleotide polymorphism in the gene called CHST6 that causes Macular Corneal Dystrophy in Labrador Retriever is autosomal recessive. This means that dogs that carry two copies of the mutation (homozygotes) will almost certainly develop Macular Corneal Dystrophy during their lives. Dogs that carry a single copy of the mutation (also known as carriers or heterozygotes) will not develop Macular Corneal Dystrophy as a result of the CHST6 mutation, but they will pass the mutation onto about half of any offspring they have. Breeding dogs that will not develop Macular Corneal Dystrophy should be the breeder’s priority, with a reduction in mutation frequency within the whole breed being the secondary, longer-term target.
Carriers can be bred from safely, provided they are mated to a dog that has also been tested and is clear of the CHST6 mutation (i.e. carry no copies of the mutation). If a carrier is mated to a clear dog approximately half of the resulting puppies will also be carriers, so should be tested themselves prior to breeding. Breeding carriers to tested, clear dogs is safe, in terms of avoiding dogs affected with Macular Corneal Dystrophy, and will help to maintain the genetic diversity of a breed. It is therefore encouraged, particularly in the first few generations following the availability of a new genetic test, so that other desirable characteristics and traits can be preserved before the frequency of the disease mutation within the breed is gradually reduced.