If you own a Collie, Sheltie, Australian Shepherd or Border Collie, you have probably seen two acronyms turn up on the same DNA panel: CEA (Collie Eye Anomaly) and PRA (Progressive Retinal Atrophy). They sit next to each other on the test results, they both affect the back of the eye, and they are routinely mixed up — even in casual conversation between breeders. But they are completely different diseases, caused by different genes, with different timelines and very different consequences for your dog’s vision.

Getting them straight matters. A dog can be clear of one and affected by the other. Confusing the two leads to misplaced panic over a harmless condition, or false reassurance about a serious one. Here is how to tell them apart.

The one-sentence difference

CEA is a congenital, structural problem: the back of the eye doesn’t form properly before birth, and in most dogs it stays exactly the same for life. PRA is a late-onset, degenerative problem: the eye is built normally, then the light-sensing cells slowly die off, and the dog goes blind.

That single distinction — malformation that stays put versus normal tissue that breaks down over time — drives almost everything else.

Different genes, different tests

CEA in herding breeds is caused by a single, well-defined mutation: a roughly 7.8 kilobase deletion in the NHEJ1 gene on chromosome 37. It is inherited as a simple autosomal recessive, so a dog needs two copies to be affected. A DNA test for CEA is highly reliable because it looks for that one specific deletion. If you want the biology, our explainer on the NHEJ1 gene mutation covers exactly what goes wrong during development.

PRA is not one disease but a family of disorders. Different breeds carry different mutations — prcd-PRA (the PDE6B and related pathways), rcd forms, PRA-cord1, and many others. A “PRA panel” for one breed tests for a completely different mutation than the panel for another breed. This is the single biggest source of confusion: there is no universal PRA test, whereas CEA has essentially one. When a lab report says “PRA: clear,” always check which PRA variant was tested — a clear result for one form says nothing about the others.

Different timelines: when it shows up

This is the most practical way for an owner to tell them apart.

CEA is detectable in puppies. Because it is a developmental malformation, the changes are present from birth. The ideal screening window is 6 to 8 weeks of age, before retinal pigment thickens and masks the mild changes — a phenomenon we explain in detail in the article on why some affected dogs later appear clear on examination. A reputable breeder will have had the whole litter eye-examined by a veterinary ophthalmologist at this age.

PRA shows up in adulthood. Depending on the form and breed, clinical signs typically appear between 3 and 6 years of age, sometimes later. The dog is born with normal vision, passes its puppy eye exam with flying colours, and only years later starts struggling. This is why a clean puppy eye certificate tells you a great deal about CEA but almost nothing about PRA.

Different signs: what you and your vet see

The classic early flag for PRA is night blindness: the dog becomes hesitant in dim light, reluctant on dark stairs, or bumps into things at dusk while seeing fine in daylight. That is the rods dying first. CEA, by contrast, rarely produces any owner-noticeable sign, because the affected zone sits in the peripheral retina outside the main line of sight.

On examination, the two look nothing alike down the ophthalmoscope. CEA shows choroidal hypoplasia — a pale, under-pigmented patch where the blood-vessel layer never developed properly, sometimes with a coloboma (a pit or notch near the optic disc). PRA shows a thinning, increasingly reflective retina with attenuated (shrunken) blood vessels and, eventually, a pale optic disc — the picture of tissue wasting away.

How the conditions actually behave

CEA is, for most dogs, a non-event. Around three-quarters to four-fifths of affected dogs have only mild choroidal hypoplasia and keep entirely normal vision their whole lives. A minority with large colobomas face a real risk of retinal detachment, which is a genuine emergency — but that is the exception, not the rule.

PRA is the opposite. There is no mild, harmless version that stays put. Once the degeneration starts, it progresses to blindness over months to a couple of years. The good news is that dogs adapt remarkably well to gradual, painless vision loss, especially in a stable home environment. The bad news is that there is currently no treatment to stop it.

Reading your own report without panicking

When you get a DNA panel or an eye-exam certificate back, work through it like this:

1. Separate the two. CEA and PRA are independent. Affected for one says nothing about the other.
2. For CEA, the result is binary and reliable — clear, carrier, or affected. “Affected” usually means a mild, stable condition; ask your ophthalmologist whether a coloboma was seen.
3. For PRA, identify the exact variant tested and confirm it is the right one for your breed. A “clear” on the wrong panel is meaningless.
4. Match the test type to the timeline. A puppy eye exam screens for CEA structure now; it cannot rule out PRA, which needs the DNA test (and, later, adult eye exams).
5. When in doubt, get a board-certified veterinary ophthalmologist to interpret the findings rather than reading the acronyms cold.

If you are weighing up a breeding pair or vetting a litter, browsing the full set of genetics articles will help you understand how both conditions pass down — and why testing for both, not just one, is the responsible standard.

The bottom line

CEA is the condition your puppy is born with and almost always lives normally with. PRA is the condition that arrives in middle age and steals vision for good. They share a few letters and a region of the eye, and nothing else. Once you can map any report onto “congenital and stable” versus “late and progressive,” you will never confuse them again — and you will know exactly which conversation to have with your vet.