Although the disease is rare in general, within an affected line it is important to control the gene frequency so as to prevent producing puppies affected with the disease. The mutations causing CD in these two breeds occur in the same gene (CNGB3) but are distinct mutations and arose separately. |
CD disease causes day blindness due to degeneration of the retinal “cones” – cone-shaped cells in the retina that respond primarily to bright daylight. CD can be diagnosed in the early weeks of the affected dog’s life. Between 8 and 12 weeks of age, when retinal development is normally completed in dogs, signs of vision problems are noticeable. The pups become day-blind and are photophobic – meaning that exposure to bright light is irritating or even painful. The pup will shun brightly-lit areas. Vision in dim light remains normal. The retina of the affected dog initially appears normal when examined by an ophthalmologist and initially the ERG (electroretinogram) recording is normal. However, the ERG response from the degenerating cones declines with age and is non-recordable in the mature CD-affected dog.
In contrast to PRA (Progressive Retinal Atrophy), which is the more common type of retinal disease in many dog breeds, CD does not affect night vision. A second type of cell in the retina, the “rods” – rod-shaped cells that respond primarily to dim light and detect movement – are not involved in this disease. The CD-affected dog keeps the ability to see at night or in dimly-lit areas.
Just as PRA is the canine version of human RP (retinitis pigmentosa), CD is the canine version of the human genetic disease achromatopsia – total color blindness and day-blindness. Another name for day blindness is “hemerolopia.” Exciting gene therapy research aimed at correcting this condition (in dogs as a model for human disease) is currenty being conducted at the University of Pennsylvania.
Reliable identification of dogs that do not carry disease genes is the key to controlling autosomal recessive diseases. The OptiGen CD test enables 100% accurate identification of these dogs. Called "genetically clear," "noncarriers" or, more formally, "homozygous normals," such dogs can pass only the normal gene on to all their pups - which means that none of their pups can ever be affected with CD. These "clear" dogs can be bred to any mate, even to a CD-affected German Shorthaired Pointer, which may be a desirable breeding prospect for other reasons.