Progressive retinal atrophy, usually shortened to PRA, is not one disease but a family of inherited conditions in which the light-sensing cells of the retina slowly die. The dog is born with normal sight, and the loss comes later, often so gradually that owners only notice when the dog starts bumping into things at dusk. There is no treatment that stops it. What makes PRA a good example of how canine genetics works in practice is that many of its forms have a known cause in a single gene, so a cheek swab can tell a breeder long before any symptom appears whether a dog is clear, a carrier or at risk.
Key facts
- PRA is a group of inherited retinal degenerations that cause progressive loss of vision, ending in blindness.
- The most widespread form, prcd-PRA, is caused by a variant in the PRCD gene and is recessive; OMIA lists the variant in 25 breeds.
- Night blindness is usually the first sign; most dogs with prcd-PRA lose their sight completely within one to two years of the first signs.
- A DNA test only covers the variant it looks for. A clear result for one form does not rule out other forms, so breeding dogs still need eye examinations.
How PRA affects the eye
The retina contains two kinds of photoreceptor cells. Rods handle dim light and movement; cones handle daylight and color vision. In most forms of PRA the rods fail first, which is why the first sign is poor vision in dim light: a dog that hesitates on stairs in the evening, will not go into a dark room or misjudges a ball thrown at dusk. As the disease moves on to the cones, daytime vision also fades. In the late stages a veterinary ophthalmologist sees a shiny, overly reflective back of the eye and thinned retinal blood vessels, and the pupils may stay wide open in bright light.
PRA is not painful in itself. Many dogs cope remarkably well because the loss is slow and they rely heavily on smell and hearing, especially in a home where the furniture stays in place. Some forms are linked to cataracts later in the disease.
The main genetic forms
Researchers have identified a long list of PRA-causing variants in dogs, and each tends to be concentrated in particular breeds. A few examples show the range.
prcd-PRA (progressive rod-cone degeneration)
This is the form most owners meet. It was traced to a single-letter change in the second codon of the PRCD gene by Zangerl and colleagues in 2006; the same change also causes a form of retinitis pigmentosa in people. According to the OMIA entry for prcd-PRA, the variant has been found in 25 breeds, including the Labrador Retriever, English and American Cocker Spaniel, Golden Retriever, Australian Shepherd, Chihuahua, Yorkshire Terrier, Toy and Miniature Poodle and Portuguese Water Dog. Onset is typically in adolescence or early adulthood, but it varies a lot between breeds.
rcd1 (rod-cone dysplasia 1)
The first PRA mutation ever identified in dogs was found in Irish Setters, in the PDE6B gene, which codes for part of an enzyme the photoreceptors need to turn light into a nerve signal. Here the cells never develop properly and vision fails in young dogs. Irish Setters are known to carry more than one genetically distinct form of PRA, which is a good reminder that one test does not cover a whole breed.
cord1 (cone-rod dystrophy 1)
In cord1 the cones are affected first, so daylight vision suffers early. The disorder was mapped in a research colony of Miniature Longhaired Dachshunds, where a 44-base-pair insertion in the RPGRIP1 gene segregated with the disease (Mellersh et al., 2006). In pet dogs outside that colony the age of onset has turned out to vary considerably, and some dogs with two copies of the variant show very mild signs, which suggests other genes modify the effect. Genoscoper's cord1-PRA test page and the Dachshund PRA page describe how results were reported for this form.
Other breed-specific forms include rcd3 in the Cardigan Welsh Corgi and the PRA variants that were tested in English Springer Spaniels and Papillons and Phalènes.
How PRA is inherited
Most known forms of PRA are autosomal recessive. A dog has two copies of each gene, one from each parent, and needs two copies of the disease variant to be affected. That gives three possible results:
- Clear (normal/normal): no copy of the tested variant. The dog will not develop that form of PRA and cannot pass it on.
- Carrier (normal/affected): one copy. The dog does not develop the disease but passes the variant to about half of its puppies on average.
- Affected or at risk (affected/affected): two copies. The dog is expected to develop that form of PRA at some point, although for some forms, such as cord1, the timing and severity vary.
A small number of forms are inherited differently. X-linked PRA, seen in the Siberian Husky and Samoyed, mainly affects males because the gene sits on the X chromosome. A dominant form caused by a variant in the rhodopsin gene is known in the English Mastiff and Bullmastiff.
What a DNA test can and cannot tell you
A PRA DNA test reads one specific variant. When the variant is known to cause the disease in a breed, the result is very reliable: it identifies affected and carrier dogs years before an eye exam could, which is the whole point for a breeder deciding on a mating. Two carriers can safely produce clear puppies only if one parent is clear; pairing two carriers is expected to give roughly one affected puppy in four.
The limits are just as important:
- A clear result only covers the tested form. A Labrador that is clear for prcd-PRA can still develop another, as yet unexplained, retinal disease.
- Some variants are less predictive outside the breed where they were found. Large screening studies have found PRA variants in mixed-breed dogs and in breeds where their clinical effect has not been established. Donner and colleagues, who screened over a million dogs for 250 variants in a study published in PLOS Genetics in 2023, found prcd among the variants seen across many breed backgrounds and stressed that clinical relevance has to be checked breed by breed.
- A DNA test is not an eye exam. It says nothing about the current state of the retina or about other eye conditions.
Eye examinations still matter
Breed clubs and kennel clubs therefore combine DNA testing with regular eye examinations by a certified veterinary ophthalmologist. In the UK this runs through the BVA/KC/ISDS Eye Scheme; in the United States, exams are recorded by the OFA through its eye certification registry; and in Finland the Finnish Kennel Club (Suomen Kennelliitto) records eye examination results and accepted DNA test results in its breeding database. Many breeds set a requirement for an eye examination, and sometimes a DNA result, before a litter can be registered. The guide to dog DNA test accuracy explains how to check whether a laboratory's test has been validated for your breed.
Living with a dog with PRA
A diagnosis of PRA is upsetting, but most blind dogs keep a good quality of life. Owners usually help by keeping the layout of the home stable, using baby gates at stairs, talking to the dog when approaching, keeping walks on a lead in unfamiliar places and using scented or sound toys. Your veterinarian or a veterinary ophthalmologist can advise on checking for secondary problems such as cataracts.
Frequently asked questions
At what age does PRA start in dogs?
It depends on the form. In early-onset forms such as rcd1, vision fails in puppies or young dogs. In prcd-PRA, signs usually start in adolescence or early adulthood, and in some breeds not until middle age. That is why breeders test before breeding rather than waiting to see whether a dog develops the disease.
Can a carrier dog develop PRA?
For the recessive forms, no. A carrier has one normal copy of the gene, which is enough for normal vision. It can, however, pass the variant on, so its partner should be tested clear.
Is there a cure for PRA?
No treatment currently stops or reverses the inherited forms of PRA in dogs. Gene therapy has been studied in research dogs, but it is not an available treatment.
My dog tested clear for PRA. Can it still go blind?
Yes, from other causes. A clear result rules out only the specific variant that was tested. Regular eye checks remain the way to catch other eye diseases.
Sources
- OMIA (Online Mendelian Inheritance in Animals), University of Sydney: Progressive rod-cone degeneration, PRCD-related (dog).
- 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 88:551–563.
- Mellersh CS, et al. (2006). Canine RPGRIP1 mutation establishes cone-rod dystrophy in miniature longhaired dachshunds as a homologue of human Leber congenital amaurosis. Genomics 88:293–301.
- Miyadera K, Acland GM, Aguirre GD (2012). Genetic and phenotypic variations of inherited retinal diseases in dogs: the power of within- and across-breed studies. Mammalian Genome 23:40–61.
- Donner J, et al. (2023). Genetic prevalence and clinical relevance of canine Mendelian disease variants in over one million dogs. PLOS Genetics 19(2):e1010651. doi:10.1371/journal.pgen.1010651.