Cone Degeneration (CD) disease causes day blindness due to degeneration of the retinal cone 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.