Inherited degeneration of the retinal photoreceptors. It begins by affecting the rods (night blindness, difficulty in low light) and progresses toward total vision loss in the medium to long term. It is the most common form of progressive retinal atrophy in dogs and is painless, so it is usually detected late, when the breeder or owner notices stumbling, insecurity in new environments or very dilated pupils with increased bright reflex.

Historically high in Poodle, American and English Cocker Spaniel, Labrador, Golden Retriever, Australian Cattle Dog, Papillon, Long-haired Chihuahua, among others (>20 breeds share exactly the same PRCD mutation). With testing and breeding programmes the frequency of the affected allele has fallen greatly in organised breeds; it remains relevant in untested breeding lines. Important: in some breeds other forms of PRA coexist (e.g. crd4/MAP9, GM2, Golden-type PRA) that this test does NOT cover.

- Initial night blindness (rod phase): the animal hesitates when lighting changes and bumps into things at home when the light is turned off.
- Progressive loss of daytime vision months to years later.
- Fundus: tapetal hyperreflectivity, retinal vascular attenuation, optic disc pallor (specialist findings).
- No pain or inflammation; behaviour adapts at home.

It is one of the most studied canine genetic diseases. In 1998, the group of Gregory Acland and Gustavo Aguirre (Baker Institute) mapped the prcd locus for the first time by linkage analysis in the founder population of prcd dogs (Acland GM et al., PNAS 1998). The linkage disequilibrium work in pure breeds by Goldstein et al. (2006) narrowed the critical region until Zangerl et al. (2006) identified the same causal mutation in the PRCD gene (a G>A substitution producing a Cys2Tyr change, c.5G>A/p.C2Y), identical in more than 20 breeds: it was the first reported case in which a single mutation explained the same retinopathy across such a broad range of breeds, which turned the prcd DNA test into a worldwide breeding standard.

- Never mate two carriers: 25% of the offspring would be affected.
- Carrier x clear = 50% healthy carriers: matings allowed to preserve valuable genes, planning replacement with clear offspring.
- Affected x clear = all offspring healthy carriers (avoid unless a well-justified project).
- Test breeding offspring even if both parents appear clear if there is a history.
- Combine DNA testing with annual ophthalmological examination (gonioscopy/ERG when applicable), because other PRAs and simultaneous ocular pathologies exist.

Differential diagnosis: other genetically distinct PRAs (rcd1, rcd3, Cord1/RPGRIP1, rcd4/C2orf71, PRAs associated with other variants), congenital retinal dysplasia and acquired toxic or metabolic PRA. Electroretinography (ERG) shows early loss of rod response before changes appear in the fundus. Management includes environmental adaptation and counselling on quality of life, which is generally good after adaptation.

1. Zangerl B, Goldstein O, Philp AR et al. 2006, mutación idéntica en un gen retiniano novel causa prcd en perros (PMID 16938425)
2. Goldstein O, Zangerl B, Pearce-Kelling S et al. 2006, mapeo por desequilibrio de ligamiento del intervalo prcd (PMID 16859891)