Dog General Progressive Retinal Atrophy (PRA)
PRA was first described in the Gordon Setter in Sweden in 1911. The first breed to be involved in the United Kingdom was the Irish Setter. Subsequently there have followed reports of the disease in many other breeds of dog. The precise mode of inheritance has been proven to be a recessive gene in probably fourteen breeds, but the disease condition has been reported in a total of ninety or so breeds. Some of the known breeds which can develop Generalised PRA are:
Cardigan Welsh Corgi
English Cocker Spaniel
Toy & Miniature Poodles
It has become possible to distinguish two essential subdivisions of the PRA complex, namely developmental and degenerative. According to whether the photo-receptor disease (i.e. elements in the retina which convert the light energy into electrical energy and give the so called vision in the brain) commences before or after maturation of the retina. As a generalisation it has been noted that in the developmental diseases eg Collie, onset of night blindness tends to occur before the occurrence of obvious changes in the retina when viewed by an ophthalmoscope, while in the degenerative condition eg Labrador, a degree of retinal change is usually present at the time of night blindness onset.
In the Irish Setter and the Collie there is an arrested development of the photo-receptors in the early post natal period, so that clinical and behavioural signs of blindness may be obvious by six months of age or earlier. With both these breeds research has shown there to be an obvious bio-chemical deficiency in the function of the retina.
The age at which PRA symptoms start in various breeds is:
3 months – 2 years: *Miniature Long-Haired Dachsund, Cardigan *Welsh Corgi, Cairn Terrier, *Irish Setter, Rough Collie, Smooth Collie
1 – 1 1/2 years: Norwegian Elkhound.
1 – 2 1/2 years: *English Springer Spaniel, Tibetian Terrier, Tibetian Spaniel, Miniature Schnauzer
3 – 5 years: * English Cocker Spaniel,*Labrador Retriever.
4 – 6 years: *Toy Poodle, *Miniature Poodle.
( * – seen by the author in New Zealand.)
Some of the more specific clinical features which we find with Generalised PRA are:
1. The condition is bilateral and symmetrical, i.e. both eyes are affected equally although there may be a small lag interval between the two.
2. You may notice that the dog in the early stages of blindness has good day vision but deficient night vision. In order to be able to display night blindness the dog of course has to be walked at night preferably without a lead so that it can display its visual deficiency. This may be seen as bumping into objects such as trees or even walking over the edge of swimming pools and falling into the water and trouble negotiating stairs (this is often reported by owners). Ultimately there is a point at which the dog is totally blind both night and day.
3. Far vision is better than near vision with a reduced ability to see stationary objects. Also there develops a degree of tunnel vision – the dog sees things immediately in front of it but not peripherally.
4. Dilated pupils are frequently observed by the owner. This is due to the reduced nerve impulse to the brain because the retina is dying, and as a consequence the feed-back to the iris is less so that the iris, instead of constricting in bright light, tends to be more relaxed and thus we get a dilated pupil. Also it will be noted that the eyes glow and look more yellow or green than previously. This is due to the dilated nature of the pupils and the increased reflectivity of the retina-it reflects light more than absorbing it because the retina is becoming thinner.
5. PRA is an inherited condition, which in those breeds examined (some 14 now) is the classic recessive type which everyone who did biology in school will remember as the first part of Mendelism. Very simply, a dog having both genes for PRA will develop the disease; if it doesnâ€™t have the gene will never have it; or carries one gene and because itsâ€™ dominant associated gene prevents the recessive genes expression, the dog will appear outwardly completely normal, remaining fully sighted all its life but it is a true example of the â€˜carrier stateâ€™.
Any carrier has a statistical chance of passing on the same carrier status to 50% of his or her progeny who are themselves out of the â€˜clearâ€™ mate. These carriers remain hidden within the breed population until one day two carriers are bred together. Then, statistically 25% of the resultant progeny may inherit the gene to be affected by PRA and may go blind in early, or later, adulthood. Some of the littermates will carry the gene hidden in their genetic make-up, to possibly hand it on to the next generation. So here are the results of five possible matings with a recessive mode of inheritance:
1. Both parents affected
all progeny affected.
2. One parent affected – one carrier parent
50% progeny affected – 50% progeny carriers.
3. One parent affected – one parent free
all progeny carriers.
4. Both parents carriers
50% progeny carriers – 25% progeny free – 25% progeny affected.
5. One parent free – one carrier parent
50% progeny carriers – 50% progeny free.
From this it follows that both parents – sire and dam – and all progeny of an affected animal, must be at least carriers. A 2% incidence of the disease in the population means 24% of that population will be carriers. If the incidence is 10% then the carrier state is 45%!
No affected dog should be used for breeding or if the DNA status is known, then carriers can be bred to others of known status and offspring DNA tested. This way breeds can still be salvaged where there may be a high incidence of the carrier or affected state. The ideal, which is now available, is a blood test which can be done on all present breeding stock, to determine whether they are carrying either none, one (carrier) or both (affected) genes for PRA or any other genetic disorder (e.g. cataract). New-born pups could also be tested so that one could get an early insight into whether they may have a vision problem later in life (they could be euthanased or sterilised if they were only carriers).
This DNA test is now available in NZ with the DNA collected from an individual by a check swab or blood with the tests done in Australia.
At the moment all we can:
â€¢ Routinely examine all breeding stock and offspring for those problems that DNA testing doesnâ€™t uncover â€“eg cataract.
â€¢ Test mating is no longer needed because of the availability of the DNA test.
â€¢ A national recording scheme of all pedigree stock is essential in order that the disease be eradicated in some of the worst affected breeds.
â€¢ Cataract development may follow in the latter stages of the retinal disease. The development of cataract will of course further impede the affected animalâ€™s vision, but of course cataract surgery is not going to give any real benefit to the animal unless it is obvious that the onset of cataract produces a sudden change in vision and blindness. Surgery in selected cases can be of benefit and allow some use of the remaining retinal function but remember the retina is gradually degenerating.
â€¢ With the development of cataract it is possible that an inflammatory reaction may occur within the eye due to material leaking out of the lens into the eye. The eye reacts against this foreign protein. If this occurs the eye may show signs of redness and there will be some discomfort to your pet. Eye drops should be given for this and may involve long term treatment to control the problem. No treatment can result in continuing pain and the possibility of glaucoma developing in the eye.
â€¢ There is no known treatment for this problem as yet with total blindness the end result. The rate of progression can be different from one individual to another with the development of cataract variable as well.
There are other disease conditions which affect both cats and dogs where the final stages of the retinal disease can mimic the appearance of Generalised PRA (so called pheno-copies). It is very important from a breederâ€™s viewpoint that some distinction is made between the genetic and the non-genetic causes of vision loss in an animal. With this in mind it is most important before one hangs the label of blindness caused by a genetic disease i.e. Generalised PRA on an animal, that one consider the history, the breed, the age of onset of the disease and factors such as the appearance of the retinal disease (e.g. it is unlikely to be a genetic disease if only one eye is affected or there is an obvious difference between the two eyes).
Any disease which causes blindness in an animal is distressing probably more to the owner than to the animal. One of the advantages of Generalised PRA is that the onset of blindness is gradual thus giving the animal time to accommodate to its disability and better utilize its other senses such as hearing and smelling. Completely blind animals can have an excellent quality of life and one should not consider, unless in extreme situations, euthanasia.
New Zealand breed clubs are working to eliminate genetic disease from those breeds at risk. This involves yearly testing of pedigree animals and the interchange of information between breed members of the genetic status on dogs being used as studs (although this certainly could be improved further). The development of a national eye scheme is underway with the possible collation of all eye certificate examination results being logged into a national database.
Craig Irving â€“ Registered Specialist Veterinary Ophthalmologist
84 Pitt Street
Palmerston North Ph 06-3575887 Fax 06-3575863
DNA test providers for Inherited Canine diseases
- Genetic Science Services- Australia-www.geneticscienceservices.com
- America- www.optigen.com
- Both these sites offer a range of DNA tests for ophthalmic problems in dogs as well as other inherited diseases.
by Craig Irving
Craig Irving B.V.Sc. M.A.C.V.Sc., Cert. Vet. Ophthal.R.C.V.S. Registered Specialist Veterinary Ophthalmologist. After Craig graduated with Distinction from Massey University in 1970 he took up an internship at Melbourne University followed by a further two years in private practice in South Australia. Since 1973 Craig has operated his companion animal practice in Palmerston North, Craig is a Registered Specialist in Veterinary Ophthalmology and he conducts Eye Referral Clinics throughout New Zealand. He currently serves on the Companion Animal Society Editorial Board and the New Zealand Kennel Club Hereditary Disorders Committee. Email: firstname.lastname@example.org
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