Early-onset progressive retinal atrophy (PRA) described in the Portuguese Water Dog, with progressive degeneration of photoreceptors leading to blindness. It is a form molecularly distinct from the late-onset prcd-PRA (PRCD gene) that also affects this breed. The Schapendoes presents another variant of the same CCDC66 gene causing a generalised PRA. There is no published evidence of this variant in the Spanish Water Dog.

Form validated in the Portuguese Water Dog, with onset at 2-3 years. The Schapendoes presents another CCDC66 variant (generalised PRA). There is no published evidence of this variant in the Spanish Water Dog.

- Early-onset night blindness
- Progression to day blindness and complete blindness
- Tapetal hyperreflectivity and attenuation of retinal vessels
- Pigmentary changes of the ocular fundus
- Cataracts in advanced stages
- Onset in young animals (2-3 years in the eo-PRA form)

PRA is recognised in the Portuguese Water Dog, where the prcd-PRA form (variant c.5G>A of the PRCD gene, of late presentation at 3-6 years or more) is the classic one. In 2020, a genome-wide association, linkage and homozygosity mapping study in four affected littermates identified a new early-onset form (EOPRA) in the same breed: a 1 bp insertion in CCDC66 (c.2262_c.2263insA) that produces protein truncation and cosegregates perfectly with the disease. A distinct CCDC66 variant had previously been described in the Schapendoes.

- Test breeding animals with the genetic test available for the breed (CCDC66 and PRCD)
- Do not mate two carriers: 25% risk of affected homozygotes
- A carrier can be mated with a free animal; offspring intended for breeding must be tested
- Exclude affected animals from breeding
- Annual ophthalmological examination of breeding animals and pedigree recording
- Distinguish in genetic counselling eo-PRA (CCDC66) from prcd-PRA (PRCD), due to their different molecular basis and age of onset

Differential diagnosis with other PRAs (prcd-PRA, cone-rod dystrophy) and with cataracts. Electroretinography characterises the cone-rod pattern. It is important to distinguish the early form (eo-PRA, CCDC66, onset 2-3 years) from the late prcd-PRA (PRCD, onset 3-6 years or more) of the Portuguese Water Dog, since their molecular basis and reproductive management differ. There is no treatment; management is palliative and involves adapting the animal.

1. Murgiano L, Becker D, Spector C, et al. CCDC66 frameshift variant associated with a new form of early-onset progressive retinal atrophy in Portuguese Water Dogs. Sci Rep. 2020;10(1):21162. PMID: 33273526
2. Zangerl B, Goldstein O, Philp AR, et al. Identical mutation in a novel retinal gene causes progressive rod-cone degeneration in dogs and retinitis pigmentosa in humans. Genomics. 2006;88(5):551-563. PMID: 16938425
3. Dekomien G, Vollmer C, Petrasch-Parwez E, et al. Progressive retinal atrophy in Schapendoes dogs: mutation of the newly identified CCDC66 gene. Neurogenetics. 2010;11(2):163-174. PMID: 19777273