Progressive Retinal Atrophy type rcd4 (rcd4-PRA) is a form of progressive retinal degeneration described in several dog breeds. It involves the gradual loss of photoreceptors (cones and rods), with initial night blindness and progression towards marked visual loss. It is distinguished from other PRAs by its later onset (usually 3-5 years) and relatively slow progression. The causal variant is in the C2orf71/PCARE gene (not in CNGB3, which corresponds to a different form of PRA).

Described in the Gordon Setter, Irish Setter and other breeds with late-onset PRA associated with C2orf71/PCARE; the population frequency in specific breeds is variable and with limited data. The commercial nomenclature 'rcd4' may group variants of the same gene in different breeds.

- Initial night blindness (nyctalopia), with difficulty orienting in low light
- Progressive deterioration of daytime vision over months or years
- Increased tapetal reflex and more dilated pupils on examination
- Retinal vascular attenuation and tapetal changes in the fundus
- Blindness in advanced stages; no pain

The C2orf71 (PCARE) gene was associated with late-onset PRA in the Gordon Setter and Irish Setter by sequencing (Downs et al. 2013). The same family of variants explains forms of PRA in other breeds, including the one known as rcd4. The nomenclature rcd4 is used commercially, but the causal gene is C2orf71/PCARE; older descriptions that attributed it to CNGB3 correspond to a different form of PRA and not to this one.

- Test breeding animals before mating with the laboratory's specific test
- Do not mate two carriers: 25 % risk of affected homozygotes
- Carrier x clear: acceptable (50 % healthy carriers), planning progressive replacement with clear offspring
- Complement with periodic ophthalmological examination, which helps detect the disease before signs appear at home
- Confirm with the laboratory which exact variant the test covers before advising on a mating

Differential diagnosis with other PRAs (prcd-PRA, rcd1, rcd3, Cord1) and with cataracts or acquired ocular diseases; the age of onset and progression guide, but the genetic test for the specific variant is the confirmation. There is no specific treatment: supportive management and environmental adaptation. Slow but generally inexorable progression.

1. Downs LM et al. 2013, PRA de inicio tardío en Gordon e Irish setter asociada a C2orf71 (PMID 22686255)
2. Miyadera K et al. 2012, variaciones genéticas y fenotípicas en enfermedades retinianas hereditarias del perro (PMID 22065099)
3. Genotipos y frecuencias alélicas de prcd-PRA y otras variantes de PRA (PMID 38028226)
4. OMIA:001575 PRA (rcd4/C2orf71)