Issue Description A group of genetic diseases seen in certain
breeds of dogs and, more rarely, cats. It is characterized by the
bilateral degeneration of the retina, causing progressive vision loss
culminating in blindness. The condition in nearly all breeds is
inherited as an autosomal recessive trait, with the exception of the
Siberian Husky (inherited as an X chromosome linked trait) and the
Bullmastiff (inherited as an autosomal dominant trait). Other Names PRA
Diagnosis Progressive vision loss in any dog in the
absence of glaucoma or cataracts can be an indication of PRA. It
usually starts with decreased vision at night, or nyctalopia. Other
symptoms include dilated pupils and decreased pupillary light reflex.
Fundoscopy to examine the retina will show shrinking of the blood
vessels, decreased pigmentation of the nontapetal fundus, increased
reflection from the tapetum due to thinning of the retina, and later
in the disease a darkened, atrophied optic disc. Secondary cataract
formation in the posterior portion of the lens can occur late in the
disease. In these cases diagnosis of PRA may require
electroretinography (ERG). For many breeds there are specific genetic
tests of blood or buccal mucosa for PRA.
Treatment There is no treatment.
Types of PRA Generalized PRA is the most common type and
causes atrophy of all the neural retinal structures. Central
progressive retinal atrophy (CPRA) is a different disease from PRA
involving the retinal pigment epithelium (RPE), and is also known as
retinal pigment epithelial dystrophy (RPED). Generalized PRA can be
divided into either dysplastic disease, where the cells develop
abnormally, and degenerative, where the cells develop normally but
then undergo a damaging change. PRA can be further divided into
affecting either rod or cone cells. Rod cells detect shape and motion,
and function in dim light. Cone cells detect color and definition, and
function in bright light.
Commonly affected breeds:
Akita - Symptoms at one to three years old and blindness at three
to five years old.
Miniature longhaired Dachshund - Symptoms at six months old.
Papillon - Slowly progressive with blindness at seven to eight
Tibetan Spaniel - Symptoms at three to five years old.
Tibetan Terrier - Symptoms at less than one year old, often blind
by two years old, and cataract formation by four years old.
Samoyed - Symptoms by three to five years old.
This type of PRA has an early onset of severe vision loss. It is
caused by a defect in the gene for cGMP-phosphodiesterase, which leads
to retinal levels of cyclic guanosine monophosphate ten times normal. Rod-cone Dysplasia Type 1
Irish Setter - Rod cell response is nearly absent. Night blindness
by six to eight weeks old, often blind by one year old.
Sloughi - A DNA test can identify whether Sloughis have the
mutated recessive gene. This has enabled breeders to breed away from
PRA, and the disease is now rare in the breed.
Rod-cone Dysplasia Type 2
Collie - Rod cell response is nearly absent. Night blindness by
six weeks old, blind by one to two years old.
Rod-cone Dysplasia Type 3
Cardigan Welsh Corgi
Norwegian Elkhound - Characterized by dysplasia of the rod cell
unit and subsequent degeneration of the cone cell unit. Rod cell
response is nearly absent. Night blindness by six months old, blind by
three to five years old. Rod dysplasia has now been bred out of this
Early Retinal Degeneration
Norwegian Elkhound - Night blindness by six weeks old, blind by
twelve to eighteen months old.
This is caused by an abnormal development of both rod and cone cells.
Dogs are initially night blind and then progress to day blindness.
Miniature Schnauzer - Slowly progressive, not seen until two to
five years old.
Belgian Shepherd Dog - Complete blindness by eight weeks old.
Alaskan Malamute - Temporary loss of vision in daylight
(hemeralopia) at eight to ten weeks old. There is a purely rod cell
retina by four years old.
Progressive Rod-cone Degeneration (PRCD)
This is a disease with normal rod and cone cell development but late
onset degeneration of the rod cells that progresses to the cone cells.
It is inherited as an autosomal recessive trait and has been linked to
the ninth canine chromosome.
Poodle - Night blindness by three to five years old, blind by five
to seven years old.
English Cocker Spaniel - Occurs late in life, usually at four to
eight years old.
American Cocker Spaniel - Night blindness by three to five years
old, blind one to two years later.
Labrador Retriever - Night blindness by four to six years old,
blind at six to eight years old.
Portuguese Water Dog
Chesapeake Bay Retriever
Australian Cattle Dog
Nova Scotia Duck Tolling Retriever
This condition is linked to the X chromosome.
Siberian Husky - Night blindness by two to four years old.
Samoyed - More severe disease than the Husky.
Bullmastiff - Inherited as an autosomal dominant trait due to a
mutation in the gene for rhodopsin.
Abyssinian - Two forms exist. One is inherited as an autosomal
dominant trait and has an early age onset. The other is inherited as
an autosomal recessive trait and has a middle age onset.
Early onset PRA has also been reported in the domestic shorthaired
cat and Persian. The Siamese also likely has a hereditary form of PRA.
Despite belief among breeders to the contrary, there is apparently no
link between coat color in Persians and the development of PRA.
Central Progressive Retinal Atrophy (CPRA)
CPRA is also known as retinal pigment epithelial dystrophy (RPED). The
cause of this condition is the loss of the retinal pigment
epithelium's ability to effectively process the photoreceptor outer
segment (POS) and subsequent accumulation of POS material in the RPE
and loss of function. The loss of function of the RPE leads to
photoreceptor degeneration. Vitamin E deficiency may play a role in
the development of CPRA. It is characterized by accumulation of
pigment spots in the retina surrounded by retinal atrophy and a
mottled appearance of the pigmented nontapetal fundus. The pigmented
spots eventually coalesce and fade as the atrophy of the retina
increases. It is an inherited condition (in the Labrador Retriever it
is inherited as an autosomal dominant trait with variable penetrance).
CPRA occurs in older dogs. Peripheral vision is retained for a long
time. Vision is better in low light and better for moving or distant
objects. Not all affected dogs go blind. Secondary cataracts are
Commonly Affected Breeds
English Cocker Spaniel
English Springer Spaniel
Chesapeake Bay Retriever
Cavalier King Charles Spaniel
Briard - has an especially high frequency.
It can also, but very rarely, be found in the Papillon.
Hereditary Retinal Dysplasia
There is another retinal disease in Briards known as hereditary
retinal dysplasia. These dogs are night blind from birth, and day
vision varies. Puppies affected often have nystagmus. It is also known
as lipid retinopathy.