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 doesn’t have the gene will never have it; or carries one gene and because its’ 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 ‘carrier state’.