Progressive retinal atrophy – Rod-cone dysplasia type 2 (PRA-RCD2)
Progressive retinal atrophy (PRA) comprises autosomal recessively inherited diseases that lead to degeneration of retinal photoreceptor cells in dogs and other pets. In general, these diseases are characterized by disturbance of dark vision, visual field defects, and abnormalities in the electroretinogram, which can progress to blindness. Some forms of PRA are common to multiple dog breeds, while others are recognized in just a single breed.
Rod-cone dysplasias are group of recessively inherited diseases with early onset where photoreceptors are disturbed in their normal development and never manage to develop properly. The progression of rod-cone dystrophy is caused by sequential degeneration of rod and cone photoreceptors.
PRA-RCD2 is an autosomal recessive disorder that segregates in collie dogs. As far as is known, PRA-RCD2 segregates exclusively in rough and smooth collies. Night blindness is the earliest clinical sign detectable in 6-week-old affected dogs. Retinal dysfunction can be detected by electroretinography (ERG) as early as 16 days of age. At 6 weeks of age, when the photoreceptors of normal dogs are fully developed, only a few underdeveloped segments are visible in PRA-RCD2 dogs, by 2–2.5 months age, they completely disappear in the affected retina. Both types of photoreceptors subsequently degenerate, cones more slowly than rods. Ophthalmoscopic abnormalities can be detected at 3.5 to 4 months of age, including tapetal hyperreflectivity, retinal vascular attenuation and optic nerve pallor. By 6 to 8 months of age rcd2 dogs become functionally blind.
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