The initial onset of disease occurs at different ages in different breeds of dogs. Dogs with PRA gradually lose eyesight due to degeneration of the retina. The retina is a layer of cells in the back of the eye which absorbs light, and sends the appropriate electrical signals to the brain. Two types of cells are involved in light reception, rods and cones.
Rods absorb light in low light situations, and are therefore essential to night vision.
PRA is characterized by a loss of rod cells initially. The effect of this is that the dog may have, or appear to have, normal vision in daylight, but is cautious and disoriented at night. Eventually the cones are also affected, and the dog gradually loses day vision as well. Most PRA affected dogs eventually become blind entirely. Because of the dog's ability to move around well in familiar surroundings, vision loss is often quite advanced before the owners notice the dog has a problem.