Inherited Eye Disease: What Screening Actually Catches

Inherited Eye Disease: What Screening Actually Catches

Most inherited eye disease is painless. That is the problem. A dog does not tell you its retina is dying, and a cat with a slowly clouding cornea simply stops jumping onto things. By the time an owner notices, the question is usually how much is left rather than what can be saved.

This is a guide to what the tests actually cover, which is narrower than most owners assume.

Progressive Retinal Atrophy Is a Family, Not a Disease

The retina is the light-sensitive lining at the back of the eye. It contains rods, which work in dim light, and cones, which handle daylight and color. In progressive retinal atrophy (PRA) these cells die off, gradually and painlessly, until the animal is blind. Rods usually go first, which is why night blindness is the first sign.

The critical thing to understand is that PRA is a group of diseases that happen to look alike. Different breeds carry different broken genes producing the same end result, and over 20 separate PRA-causing mutations have been identified in dogs.

So a PRA-clear result names one variant. The Golden Retriever has three separate PRA genes, each needing its own test. A prcd-clear Labrador has not been screened for anything else.

The cat side, where the numbers are unusually clean

The rdAc variant in the CEP290 gene is the best-characterized feline PRA. Affected cats have reduced retinal function by 8 months, changes a vet can see at 1 to 2 years, and visual impairment by 5 to 6 years.

Its distribution carries a lesson. In the survey of 846 cats across 41 breeds, wedge-faced Siamese ran an allele frequency of 0.265, with the Colorpoint Shorthair, Oriental Shorthair and Balinese higher still. Apple-head Siamese came in at zero out of 31 cats. The risk sits in the show-type conformation, not the breed name, which the authors attribute to a founder effect inside show lines.

The same survey found rdAc at zero in the Maine Coon, Persian, Ragdoll, Scottish Fold, Sphynx and American Shorthair. Commercial labs list several of those as testable breeds. The published result is zero.

The Dachshund case, and why it matters to three other breeds

The Dachshund form, cord1, is caused by a 44 base pair insertion in RPGRIP1. It is cone-led, meaning daylight vision goes before night vision, the reverse of classical PRA. Onset in pet dogs is wildly variable: of 48 cases, half appeared before 4 years, half between 4 and 10, and one at 15.

Here is the part that matters more broadly. 16% of visually normal control dogs were homozygous for the insertion, and 20% of PRA cases were not. The OFA's own guidance is that over 25 breeds carry this variant, including the Beagle, Rottweiler and French Bulldog, but that outside Miniature Longhaired Dachshunds and English Springer Spaniels the risk of actually developing clinical PRA is low.

If you own one of those three breeds and a panel came back "at risk" for RPGRIP1, that is a risk marker, not a diagnosis.

Collie Eye Anomaly: The One Case Where DNA Beats the Exam

Collie eye anomaly (CEA) is a birth defect rather than a degeneration. The choroid, the blood-vessel layer behind the retina, fails to develop properly before birth. Everything is present on day one and does not worsen.

Severity runs in three tiers. Choroidal hypoplasia, a pale patch where the underdeveloped choroid lets the white sclera show through, is present in every affected dog. Coloboma, a pit or hole in or beside the optic disc, occurs in 10% to 20%. Retinal detachment occurs in 2% to 5% and is the outcome that actually costs vision. Most affected dogs see perfectly well.

In one Italian population, 29.64% of 334 Border Collies were carriers. Figures for the Rough Collie are all over the place, ranging from 12.5% carriers in one small sample to 55.5% affected in another, so treat the Border Collie number as the solid one and assume the mutation runs high across the Collie group generally.

The "go-normal" problem

Choroidal hypoplasia is a pale patch. As a puppy's retina matures it lays down pigment, and that pigment can cover a mild lesion over. A mildly affected dog examined too late looks identical to a clear one. Ophthalmologists therefore examine litters at 6 to 8 weeks.

The consequence is that a mildly affected dog that went normal will pass an adult eye exam and be bred as though clear, while still passing on two-copy risk to its puppies. The DNA test works at any age and cannot be masked. This is the clearest case anywhere in eye screening where the genetic test genuinely beats looking.

Cataracts, and the Thing That Is Not a Cataract

A cataract is any opacity in the lens. Inherited cataracts are the commonest cause in dogs, with Merck listing 20 predisposed breeds including the Labrador and Golden Retriever.

Nuclear sclerosis is not a cataract. It is the normal age-related increase in lens density that gives an older dog's pupil a hazy blue-grey cast, and it does not cause blindness. It looks close enough to a cataract that an owner cannot reliably tell, and completely different through an ophthalmoscope. The practical distinction is behavioral: a dog with nuclear sclerosis still navigates normally.

Diabetic cataracts occur in over 85% of diabetic dogs regardless of how well the diabetes is controlled, because excess glucose is converted to sorbitol inside the lens, which draws in water and clouds the fibers. They can appear within weeks. Cats largely do not get them; feline cataracts are most often secondary to uveitis.

The Siberian Husky is the breed where cataracts genuinely dominate the eye picture. In an ACVO members' report covering 1,345 examined Huskies: 107 dogs with inheritable cataracts (8%), 44 with corneal dystrophy (3%), and 4 with PRA (under 1%). Onset can be as early as 3 months. Note this is examined show and breeding stock, not a random sample.

There is a genetic marker, in CPT1A, associated with the Husky cataract. Its discoverers concluded that its weak specificity makes it unsuitable for preventive DNA testing. For this breed the annual eye exam remains the only tool.

Golden Retriever Pigmentary Uveitis

This one is breed-specific, common in older dogs, has no DNA test, and is invisible to owners until it has already done damage. If you own an older Golden it is the single most important thing on this page.

The uvea is the pigmented middle layer of the eye. In GRPU it becomes chronically inflamed, cysts form, and pigment sheds onto the front of the lens. That debris blocks the eye's drainage channels, pressure rises, and the eye goes blind from secondary glaucoma.

Estimates of how common it is in older Goldens range from about one in ten to nearly one in four, depending on the study. Roughly 50% of affected eyes lose vision within a year of diagnosis, and in one cohort 7 of the 9 eyes that lost vision went on to develop glaucoma. Average age of onset is about 8.5 years, which is to say after most dogs have already been bred.

The diagnostic sign is radial pigment on the front surface of the lens, which is considered definitive on its own. Uveal cysts, by contrast, appear in only 13% to 42% of affected eyes, so cyst-spotting is not a screen.

What to do: an annual dilated exam by a board-certified veterinary ophthalmologist, starting at age 2, and every 4 to 6 months with pressure checks once diagnosed. Early detection and treatment has been reported to cut the blindness rate from roughly 45% to about 17%.

Cherry Eye

Dogs have a third eyelid in the inner corner of the eye, with a tear gland tucked behind it, anchored by a strip of fibrous tissue. When that anchor gives way the gland flips out and sits on the eye as a pink lump.

Two things worth pulling out of that table. Brachycephalic dogs carry adjusted odds of 6.93 against mesocephalic dogs, so this is a conformation problem. And cherry eye is routinely listed as a Beagle problem on breed sites when in the largest dataset available it did not reach statistical significance in Beagles at all, while Labradors are actively protected.

It is overwhelmingly a young dog's condition, with a median age at first diagnosis of 0.63 years and about 75% of cases in dogs under one.

Do not let anyone remove the gland. It produces somewhere between about a third and a half of the watery portion of the tear film, and removing it substantially raises the risk of keratoconjunctivitis sicca, which is painful, permanent and can impair vision in its own right. English Bulldogs, the breed most affected by cherry eye, already carry 12.24 times the odds of dry eye. Repositioning is the correct surgery, with re-prolapse in about 5% to 20% of cases.

The Cornea

The cornea is the clear front window of the eye. It has no blood vessels, because they would block the light, and it is kept alive and clear by the tear film spread across it with every blink.

Corneal dystrophy is a genetically determined deposit of fatty or crystalline material within the cornea. It is bilateral, symmetrical, non-painful and does not involve blood vessels. The Husky is the most studied breed, at 3% of those 1,345 examined dogs. Treatment is usually unnecessary.

Corneal ulceration is the opposite in every respect: one eye, painful, urgent. French Bulldogs run 1.6% against 0.7% in other dogs, an adjusted odds ratio of 4.38. Across all brachycephalic types the odds against crossbreds are 11.18, and the mechanism is three problems stacking:

1. A shallow orbit and a large eyelid opening, leaving more cornea exposed.
2. Lagophthalmos, an inability to fully close the lids, so the eye is not properly wiped and re-wetted on each blink.
3. Reduced corneal sensitivity, from fewer corneal nerve endings.

That third one deserves emphasis. A flat-faced dog neither protects its eye reflexively nor complains much when it is damaged. In a Frenchie or a Bulldog, the absence of obvious squinting is not reassurance.

Pannus, or chronic superficial keratitis, is an immune-mediated disease in which a pink, blood-vessel-filled sheet of tissue creeps in from the edge of the eye across the cornea, laying down pigment as it goes. It starts at the outer lower edge, which is why owners often describe it as brown appearing in the corner of the eye. German Shepherds carrying a particular MHC class II haplotype are 2.7 times more likely to develop it, and homozygous carriers 8 times more likely. Ultraviolet light intensifies it, which is why it is worse at altitude and in sunny climates. It is controllable with lifelong topical treatment and not curable.

In cats, the distinctive corneal problem is sequestrum, a brown to black patch in the middle of the cornea that appears to be unique to cats and is more frequent in Persians and Siamese.

Glaucoma

The eye continuously makes fluid and continuously drains it. Block the drain and pressure rises inside a sealed ball, crushing the optic nerve and retina. Primary glaucoma is an inherited malformation of the drainage angle. Secondary glaucoma is the drain being blocked by something else, and it is the dominant form in both cats and dogs.

Acute glaucoma is a genuine emergency. Pressures run above 40 to 60 mmHg, the pain is significant, and the vision loss is irreversible. A red, cloudy, squinting eye in an animal that is off its food is a same-day problem.

Two breed points worth knowing. The Beagle carries the best-characterized inherited glaucoma in dogs, a recessive ADAMTS10 variant causing primary open-angle glaucoma, and it has a DNA test. And in cats, where glaucoma is almost always secondary, the Siamese is the exception: primary open-angle glaucoma occurs in the breed.

You will also see primary angle-closure glaucoma described in Siberian Huskies. That rests on a published case series of three dogs. It is a real observation and not evidence of a breed-wide predisposition, and breed-health pages routinely upgrade it into one.

Flat Faces and Shallow Orbits

Selective breeding for a flat face shortens the bones of the skull but not the eyeball. The socket ends up too shallow to hold it.

In a study of 69 Persians against 10 domestic shorthairs, the mean portion of the eyeball sitting outside the bony orbit was 39.9% in Persians against 9.90% in domestic shorthairs. Roughly four times as much. The authors add an important caveat: variability was high, and the least extreme Persians overlapped with normal cats. Grade of brachycephaly drives this, not the breed name.

The same skull change kinks the nasolacrimal duct, the small channel draining tears from the inner corner of the eye into the nose. Tears that cannot drain spill over the lower lid instead. That is why the epiphora explanation you usually read, that facial folds press on the tear ducts, is wrong. The distortion is at skull level, not fold level.

Downstream, the protruding eye is prone to exposure keratitis, corneal sequestrum and anatomical entropion, and Persians are over-represented for entropion, sequestrum and retinal dysplasia in feline referral data.

What an Eye Certification Actually Covers

This is the section most owners have never read, and the OFA states it plainly:

"The Eye Certification exam consists of indirect ophthalmoscopy and slit lamp biomicroscopy. It is not a comprehensive ocular health examination, but rather an eye screening exam."

And, more usefully still:

"Gonioscopy, tonometry, Schirmer tear test, electroretinography, and ultrasonography are not routinely performed; thus, dogs with goniodysgenesis, glaucoma, keratoconjunctivitis sicca, early lens luxation/subluxation or some early cases of progressive retinal atrophy might not be detected without further testing."

Three further points that get missed:

• Certification is valid for 12 months. Annual re-examination is recommended. A certificate from three years ago says what the eyes looked like three years ago.
• It describes phenotype, not genotype. The OFA notes it is entirely possible for a clinically normal animal to be a carrier.
• Ten conditions are automatic fails in all breeds, among them cataract, glaucoma, retinal detachment and progressive retinal atrophy, each carrying an unequivocal recommendation against breeding.

There is no equivalent feline certification scheme. For cats the practical tools are the breed-relevant DNA tests plus an examination by an ACVO diplomate.

The one-line summary: a DNA test tells you what an animal carries, an eye exam tells you what an animal has, and neither on its own tells you what it will develop or pass on.

Feline Eyelid Cancer, Which Is Preventable

Prolonged ultraviolet exposure damages the DNA in skin cells, producing solar keratosis, a thickened discolored scaly patch, which becomes squamous cell carcinoma. The sites are exactly the ocular ones: eyelids, nose, ear tips and lips.

Risk tracks white and unpigmented skin, and thinly haired or hairless areas, rather than breed. It is the fourth most common feline skin cancer, roughly a third of diagnosed cats have multiple lesions, and it spreads slowly but is aggressively locally invasive, eating into surrounding tissue and bone.

Indoor cats are not exempt, since a favorite sunny windowsill delivers cumulative UV. Prevention is UV-blocking window film, keeping cats out of direct sun between roughly 10am and 2pm, and biopsying any facial scab that keeps coming back. Human sunscreen is unsafe for cats and must never be applied near the eyes.

There is a neat counterpoint here: Siamese, Himalayan and Persian cats show a lower incidence, because the facial pigment that gives a Siamese its mask also shields its eyelids.

What You Would Actually Notice

Sources & Further Reading