Early-onset progressive retinal atrophy (eo-PRA) of the Spanish Water Dog, caused by a 6-bp deletion in exon 19 of PDE6B that removes two conserved amino acids from the enzyme domain. A molecularly distinct form from late-onset prcd-PRA (PRCD), which also occurs in the breed: a negative result for one does not exclude the other. The molecular test is complementary to the ophthalmological examination.

Affected breed: Spanish Water Dog (only documented breed, OMIA:002282). Carrier frequencies: limited data.

- Early-onset vision loss (earlier than late-onset prcd-PRA)\n- Progressive photoreceptor degeneration leading to blindness\n- Fundus with PRA changes at early ages\n- No ocular pain

In 2020, a study of 18 Spanish Water Dogs (6 affected, 12 healthy) combined exclusion with a microsatellite panel and whole-genome sequencing of two affected and two healthy littermates, and identified a 6-bp deletion in exon 19 of PDE6B (c.2218-2223del) that segregated with the disease in the family. The authors designed a DNA test for breeding. It adds PDE6B to the genes already implicated in PRA (Irish Setter, Sloughi, AmStaff).

- Test breeders (PDE6B) before mating.\n- Do not cross two carriers: 25 %% risk of affected homozygotes.\n- A carrier may be crossed with a clear dog; test the offspring intended for breeding.\n- Also test prcd-PRA: both forms coexist in the breed and are independent.\n- Periodic ophthalmological examination; do not repeat matings that produce affected dogs.

Differentiate eo-PRA (PDE6B, early-onset) from prcd-PRA (PRCD, late-onset 3-6 years) by age of onset and specific test: a negative result for one does not exclude the other. Diagnosis by electroretinography and fundus examination. No curative treatment: supportive management and environmental adaptation.

1. AAVO 2020, nueva mutación en PDE6B en Perros de Agua españoles con atrofia retinal progresiva de inicio precoz (PMID 32639685)
2. OMIA:002282 PRA de inicio precoz (PDE6B)