Portuguese Water Dog & PRA (prcd): What the DNA Test Covers — and the Second PRA Gene It Doesn’t

The short answer: prcd-PRA (progressive rod-cone degeneration) is a late-onset, inherited form of progressive retinal atrophy caused by a single recessive variant in the PRCD gene, so a dog needs two copies to go blind and carriers with one copy stay perfectly healthy. Affected Portuguese Water Dogs are born sighted and usually begin losing night vision around 3–5 years of age before progressing to total, painless blindness in both eyes. The prcd DNA test is a genetic risk and carrier screen, not a clinical diagnosis — it detects only the PRCD c.5G>A variant, and clinical PRA is confirmed by a veterinary ophthalmologist. The most important honesty point for this breed: a “clear” prcd result does NOT mean your Portuguese Water Dog is free of PRA, because the breed carries a second, entirely separate early-onset PRA caused by a CCDC66 frameshift (Murgiano 2020). Realistically, PWD owners and breeders should consider BOTH the prcd test and the CCDC66 test. There is no cure, but blind and night-blind dogs adapt remarkably well with supportive environmental care.

What prcd-PRA is, and what the PRCD gene does

SamMy cousin’s Portuguese Water Dog started bumping into things at night around age four — is that this prcd thing?

Elena MarshThat pattern fits prcd well: Zangerl 2006 showed the PRCD c.5G>A variant causes rods to fail first, which is exactly why low-light or night vision goes before daylight vision.

Progressive retinal atrophy (PRA) is a group of inherited diseases in which the light-sensing photoreceptors of the retina gradually die. The prcd form — short for progressive rod-cone degeneration — is caused by a single base change, c.5G>A, p.(Cys2Tyr), in the PRCD gene (OMIA:001298-9615). It is inherited in an autosomal recessive pattern.

Zangerl and colleagues identified this variant in 2006 and made a striking discovery: the identical mutation causes a form of retinitis pigmentosa in humans, making the dog a natural model for the human disease. In dogs the disease is late-onset — puppies are born with normal, healthy retinas, and vision loss typically begins around 3–5 years of age. Because rod photoreceptors degenerate first, the earliest sign is usually night blindness or disorientation in dim light, progressing over months to years toward total blindness. The process is bilateral and non-painful.

Historically, prcd has been one of the more common inherited eye variants in the Portuguese Water Dog — indeed the PWD was among the first breeds for which the prcd test was offered. Lab population summaries have reported a mutant-allele frequency around 0.33, but this should be read as a lab-reported/estimated figure from population screening, not a fixed, peer-reviewed constant.

The PWD-specific twist: two different PRA genes (prcd and CCDC66)

SamMy dog’s prcd test came back clear — that means his eyes are safe, right?

Elena MarshNot quite — Murgiano 2020 described a second, separate early-onset PRA in Portuguese Water Dogs caused by a CCDC66 frameshift, so prcd-clear only rules out that one gene.

This is the single most important thing for a Portuguese Water Dog owner to understand. The breed does not have just one PRA — it has (at least) two genetically distinct forms. The classic one is late-onset prcd. But in 2020, Murgiano and colleagues reported a second, separate early-onset PRA caused by a frameshift variant in the CCDC66 gene.

These two diseases are caused by different genes, follow different timelines, and are detected by different DNA tests. A dog can be genetically clear of prcd and still carry — or be affected by — the CCDC66 form. That is why a clear prcd certificate, on its own, does not prove a PWD is PRA-free. Realistically, thorough PWD screening means considering both the prcd test and the CCDC66 test.

The practical takeaway: interpret a prcd “clear” result narrowly. It is genuinely good news for the PRCD gene, but it is silent on CCDC66 and on any PRA-causing variant not yet mapped in the breed.

Risk screen vs clinical diagnosis: the ophthalmologist’s role

SamIf DNA tests exist, do I even still need an eye specialist to check my dog?

Elena MarshYes — the DNA test tells you genotype, but only a veterinary ophthalmologist confirms actual disease using a fundus exam and, where needed, an ERG.

A prcd DNA test is a genetic risk and carrier screen, not a clinical diagnosis. It answers a narrow question — how many copies of the PRCD c.5G>A variant does this dog carry — and nothing more. It cannot tell you whether the retina is currently healthy, nor whether disease from another cause is already underway.

Clinical PRA is confirmed by a veterinary ophthalmologist through a dilated fundus (retinal) examination and, when needed, an electroretinogram (ERG) that measures the electrical response of the photoreceptors. Because prcd is late-onset, a young dog that is genetically affected (two copies) can still have a completely normal-looking retina on exam — the genotype precedes the visible disease by years. The two tools are complementary: DNA reveals genetic status early and permanently, while the eye exam reveals the actual state of the retina at a point in time.

Commercial prcd screening is widely available, including UC Davis VGL’s “PRA-prcd” test, Embark, Wisdom Panel, Paw Print Genetics, and Laboklin.

Recessive genetics: clear, carrier, affected — and breeding

SamI was told my dog is a “carrier” — does that mean he’s going to go blind?

Elena MarshNo — with a recessive variant like prcd, a carrier has just one copy and stays healthy; only dogs with two copies become affected, as Zangerl 2006 established.

Because prcd is autosomal recessive, each dog falls into one of three genotypes. A clear dog has zero copies. A carrier has one copy and is healthy — a carrier will never go blind from prcd. Only an affected dog, with two copies, is at risk of developing the disease.

For breeding, the recessive pattern gives a clear safety rule: as long as at least one parent of every mating is genetically clear, no puppy can be born affected, even if the other parent is a carrier. This lets breeders keep valuable carriers in the gene pool without producing blind dogs — important in a breed where the variant has been historically common. Remember, though, that this logic protects only against prcd; the same litter should also be evaluated for CCDC66 (Murgiano 2020) to be genuinely PRA-conscious.

Living with night blindness: no cure, but strong supportive care

SamIf my dog does develop this, is there any treatment that stops it?

Elena MarshThere’s no cure for prcd, but night-blind and blind dogs adapt very well when their environment is kept consistent and predictable.

There is currently no cure and no proven treatment that halts prcd. Antioxidant supplements are sometimes discussed, but their efficacy against prcd is not established — they should not be presented as a fix. What genuinely helps is supportive, environmental management.

Dogs rely heavily on smell, hearing, and memory, and most cope with gradual vision loss far better than their owners expect. Practical measures include keeping furniture in consistent positions, adding night lighting for the early night-blind stage, using clear verbal cues to warn of steps and turns, and hazard-proofing pools, stairs, and sharp corners. Because the condition is painless and progresses gradually, most affected dogs continue to live full, happy lives with these adaptations.

The best plan pairs early DNA knowledge with regular ophthalmologist checks, so that changes are anticipated and the home is adapted before, rather than after, vision is lost.

Frequently asked questions (FAQ)

Q. If my Portuguese Water Dog is prcd-clear, is it safe from PRA?
Not entirely. A clear prcd result only covers the PRCD c.5G>A variant; this breed also has a separate CCDC66 early-onset PRA (Murgiano 2020), so a thorough screen should include both the prcd test and the CCDC66 test, plus regular veterinary ophthalmologist exams.

Q. My dog is a prcd carrier — will he go blind?
No. prcd is autosomal recessive, so a carrier has only one copy and stays healthy; carriers never go blind from prcd (Zangerl 2006). Only dogs with two copies (affected) are at risk of developing the disease.

Q. Does a normal eye exam mean my young dog is in the clear?
Not necessarily. prcd is late-onset, so a genetically affected dog can have a normal-looking retina for years before signs appear. A DNA test reveals genetic status early, while a fundus exam or ERG shows the retina’s current state — the two are complementary.

Q. Is there any cure or supplement that stops prcd?
No. There is no cure, and antioxidants have not been proven to halt prcd. Management is supportive: consistent furniture, night lighting, verbal cues, and hazard-proofing help night-blind and blind dogs live comfortably.

References

• Zangerl B, et al. 2006. Identical mutation in a novel retinal gene causes progressive rod-cone degeneration in dogs and retinitis pigmentosa in humans. Genomics. https://pubmed.ncbi.nlm.nih.gov/16938425/
• OMIA:001298-9615. Progressive rod-cone degeneration (PRCD) in Canis lupus familiaris. https://omia.org/OMIA001298/9615/
• Murgiano L, et al. 2020. A frameshift variant in CCDC66 is associated with a novel early-onset progressive retinal atrophy in the Portuguese Water Dog. Scientific Reports. https://www.nature.com/articles/s41598-020-77980-5
• UC Davis Veterinary Genetics Laboratory. PRA-prcd (progressive rod-cone degeneration) test. https://vgl.ucdavis.edu/test/pra-prcd

About the author

Elena Marsh

Editor & writer (not a veterinarian)

A writer with a molecular-biology background and a lifelong dog and cat owner. Not a veterinarian — she translates peer-reviewed genetics research and primary data into plain language, always as information rather than diagnosis.