Progressive retinal atrophy (PRA) is a leading hereditary cause of blindness in pedigree dogs as is its counterpart retinitis pigmentosa (RP) in humans. PRA shows genetic heterogeneity, as does RP, with several distinct forms already recognized and several more remaining to be investigated.
The retina is a thin layer of neural cells that lines the back of the eyeball. The vertebrate retina contains photoreceptor cells (rods and cones) that respond to light. The cones mediate high-resolution vision and colour vision. The rods mediate lower-resolution, black-and-white, night vision. The degeneration of the retina results in loss of vision, often leading to blindness. One can distinguish between late onset forms of PRA and early onset (whelp-age) dysplastic changes. The clinical and ophthalmologic signs of both forms are similar.
Affected dogs suffer from bilateral Mydriasis, the reflection of the Tapetum lucidum is increased and the retinal vascular network appears atrophic. Currently, there is no treatment for the disease.
The “Collie-PRA”, also termed rod-cone dysplasia type 2 (rcd 2), is one form of progressive retina degeneration which is known for a long time as major health problem in the collie breed.
An abnormal development of the cones and rods of the retina leads to night blindness with early onset. First symptoms appear around whelp-age of about 6 weeks. In the majority of cases, a complete loss of vision occur by the age of 1 in rcd2-affected dogs.