Progressive retinal atrophy (PRA) is a group of genetic diseases seen in certain breeds of

dog The dog (''Canis familiaris'' or ''Canis lupus familiaris'') is a domesticated descendant of the wolf. Also called the domestic dog, it is derived from the extinct Pleistocene wolf, and the modern wolf is the dog's nearest living relative. Do ...

s and, more rarely, cats. Similar to

retinitis pigmentosa Retinitis pigmentosa (RP) is a genetic disorder of the eyes that causes loss of vision. Symptoms include trouble seeing at night and decreasing peripheral vision (side and upper or lower visual field). As peripheral vision worsens, people may ...

in humans, it is characterized by the bilateral degeneration of the

retina The retina (from la, rete "net") is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. The optics of the eye create a focused two-dimensional image of the visual world on the retina, which then ...

, causing progressive vision loss culminating in blindness. The condition in nearly all breeds is inherited as an

autosomal recessive In genetics, dominance is the phenomenon of one variant (allele) of a gene on a chromosome masking or overriding the effect of a different variant of the same gene on the other copy of the chromosome. The first variant is termed dominant and t ...

trait, with the exception of the Siberian Husky (inherited as an X chromosome linked trait) and the

Bullmastiff The Bullmastiff is a British breed of dog of mastiff type and large size, with a solid build and a short muzzle. It was developed as a guard dog in the nineteenth century by cross-breeding the English Mastiff with the now-extinct Old English Bu ...

(inherited as an

autosomal dominant In genetics, dominance is the phenomenon of one variant (allele) of a gene on a chromosome masking or overriding the effect of a different variant of the same gene on the other copy of the chromosome. The first variant is termed dominant and t ...

trait). There is no treatment.

Types of PRA

In general, PRAs are characterised by initial loss of rod photoreceptor cell function followed by that of the cones and for this reason night blindness is the first significant clinical sign for most dogs affected with PRA. As other retinal disorders, PRA can be divided into either dysplastic disease, where the

cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Locations * Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery w ...

s develop abnormally, and degenerative, where the cells develop normally but then degenerate during the dog's lifetime. Generalized PRA is the most common type and causes

atrophy Atrophy is the partial or complete wasting away of a part of the body. Causes of atrophy include mutations (which can destroy the gene to build up the organ), poor nourishment, poor circulation, loss of hormonal support, loss of nerve supply t ...

of all the neural retinal structures. Central progressive retinal atrophy (CPRA) is a different disease from PRA involving the

retinal pigment epithelium The pigmented layer of retina or retinal pigment epithelium (RPE) is the pigmented cell layer just outside the neurosensory retina that nourishes retinal visual cells, and is firmly attached to the underlying choroid and overlying retinal visual ce ...

(RPE), and is also known as retinal pigment epithelial dystrophy (RPED).

Generalized PRA

Commonly affected breeds: *

Akita is a Japanese name and may refer to: Places * 8182 Akita, a main-belt asteroid * Akita Castle, a Nara period fortified settlement in Akita, Japan * Akita Domain, also known as Kubota Domain, feudal domain in Edo period Japan * Akita, Kumamoto, ...

- Symptoms at one to three years old and blindness at three to five years old. Selective breeding has greatly reduced the incidence of this disease in this breed. *Miniature longhaired

Dachshund The dachshund ( or ; German: "badger dog"), also known as the wiener dog, badger dog, and sausage dog, is a short-legged, long-bodied, hound-type dog breed. The dog may be smooth-haired, wire-haired, or long-haired, and comes in a variety of c ...

- Symptoms at six months old. * Papillon - Slowly progressive with blindness at seven to eight years old. * Tibetan Spaniel - Symptoms at three to five years old. *

Tibetan Terrier The Tibetan Terrier is a medium-sized breed of dog that originated in Tibet.American Kennel Club (2013). '' Get to know the Tibetan Terrier.'' Retrieved from http://www.akc.org/breeds/tibetan_terrier/index.cfm Despite its name, it is not a membe ...

- PRA3/RCD4 disease of middle age dogs. http://www.ttca-online.org/html/Petersen-Jones_PRA_article.pdf * Samoyed - Symptoms by three to five years old.

Rod-cone dysplasia

This type of PRA has an early onset of severe vision loss. It is caused by a defect in the gene for cGMP-

phosphodiesterase A phosphodiesterase (PDE) is an enzyme that breaks a phosphodiester bond. Usually, ''phosphodiesterase'' refers to cyclic nucleotide phosphodiesterases, which have great clinical significance and are described below. However, there are many oth ...

, which leads to retinal levels of

cyclic guanosine monophosphate Cyclic guanosine monophosphate (cGMP) is a cyclic nucleotide derived from guanosine triphosphate (GTP). cGMP acts as a second messenger much like cyclic AMP. Its most likely mechanism of action is activation of intracellular protein kinases in re ...

ten times normal.

Rod-cone dysplasia type 1

Irish Setter The Irish Setter ( ga, sotar rua, literally "red setter") is a setter, a breed of gundog, and family dog. The term ''Irish Setter'' is commonly used to encompass the show-bred dog recognised by the American Kennel Club as well as the field-bred ...

- Rod cell response is nearly absent. Night blindness by six to eight weeks old, often blind by one year old. *

Sloughi The Sloughi , or Arabian Greyhound (Arabic: سلوقي), is an ancient breed of domesticated dog, specifically a member of the sighthound family. It originates from North Africa and is found in Algeria, Tunisia, Morocco and Libya. History ...

- 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 Collies form a distinctive type of herding dogs, including many related landraces and standardized breeds. The type originated in Scotland and Northern England. Collies are medium-sized, fairly lightly-built dogs, with pointed snouts. Many ...

- 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 The Cardigan Welsh Corgi (; Welsh for "dwarf dog") is one of two separate dog breeds known as Welsh Corgis that originated in Wales; the other is the Pembroke Welsh Corgi. It is one of the oldest breeds of the British Isles. Cardigan Welsh Corg ...

Rod dysplasia

Norwegian Elkhound The Norwegian Elkhound is one of the Northern Spitz-type breeds of dog and is the National Dog of Norway. The Elkhound has served as a hunter, guardian, herder, and defender. It is known for its courage in tracking and hunting elk and othe ...

- 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 breed.

Early retinal degeneration

*Norwegian Elkhound - Night blindness by six weeks old, blind by twelve to eighteen months old.

Photoreceptor dysplasia

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 The Miniature Schnauzer is a breed of small dog of the Schnauzer type that originated in Germany in the mid-to-late 19th century. Miniature Schnauzers may have been developed from the smallest specimens of the Standard Schnauzer, or crosses betw ...

- Slowly progressive, not seen until two to five years old. *

Belgian Shepherd Dog The Belgian Shepherd (also known as the Belgian Sheepdog, Belgian Malinois, or the Chien de Berger Belge) is a breed of medium-sized herding dog from Belgium. While predominantly considered a single breed, it is bred in four distinct varieti ...

- Complete blindness by eight weeks old.

Cone degeneration

Alaskan Malamute The Alaskan Malamute () is a large breed of dog that was originally bred for its strength and endurance to haul heavy freight as a sled dog and hound. It is similar to other arctic breeds such as the husky, the spitz, the Greenland Dog, Canad ...

- Temporary loss of vision in daylight (

hemeralopia Hemeralopia (from Greek ''ημέρα'' hemera, "day", and ''αλαός'' alaos, "blindness") is the inability to see clearly in bright light and is the exact opposite of nyctalopia (night blindness), the inability to see clearly in low light. Heme ...

) at eight to ten weeks old. There is a purely rod cell retina by four years old.

Cone-rod dystrophy

Glen of Imaal Terrier The Glen of Imaal Terrier ( gle, Brocaire Uí Mháil) is a breed of dog of the terrier category and one of four Irish terrier breeds. It is sometimes called the Irish Glen of Imaal Terrier or the Wicklow Terrier, and the name of the breed is ...

- CRD3 results in gradual blindness with onset around 4 years of age (often detectable as retinal thinning as early as 3 years of age). Caused by a mutation in gene ADAM9, the disease is analogous to CRD9 in humans. A genetic test is now available from Optigen, LLC, that will identify whether a dog is affected, a carrier (heterozygous), or clear.

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 A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In most chromosomes the very long thin DNA fibers are coated with packaging proteins; in eukaryotic cells the most important of these proteins are ...

. *

Poodle , nickname = , stock = , country = Germany or France (see history) , height = , maleheight = , femaleheight = , weight = , maleweight = , femaleweight = , coat ...

- Night blindness by three to five years old, blind by five to seven years old. *

English Cocker Spaniel The English Cocker Spaniel is a breed of gun dog. It is noteworthy for producing one of the most varied numbers of pups in a litter among all dog breeds. The English Cocker Spaniel is an active, good-natured, sporting dog standing well up at ...

- Occurs late in life, usually at four to eight years old. *

American Cocker Spaniel The American Cocker Spaniel is a breed of sporting dog. It is a spaniel type dog that is closely related to the English Cocker Spaniel; the two breeds diverged during the 20th century due to differing breed standards in the US and the UK. In th ...

- 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 The Portuguese Water Dog originated from the Algarve region of Portugal. From there the breed expanded to all around Portugal's coast, where they were taught to herd fish into fishermen's nets, retrieve lost tackle or broken nets, and act as ...

Chesapeake Bay Retriever The Chesapeake Bay Retriever is a large breed of dog belonging to the retriever, gundog, and sporting breed groups. The breed was developed in the United States Chesapeake Bay area during the 19th century. Historically used by local market hunter ...

Australian Cattle Dog The Australian Cattle Dog (ACD), or simply Cattle Dog, is a breed of herding dog originally developed in Australia for droving cattle over long distances across rough terrain. This breed is a medium-sized, short-coated dog that occurs in two m ...

American Eskimo Dog The American Eskimo Dog is a breed of companion dog, originating in Germany. The American Eskimo Dog is a member of the Spitz family. It is considered an ancient dog breed due to its recent admixture with wolves. The breed's progenitors were Ger ...

Nova Scotia Duck Tolling Retriever The Nova Scotia Duck Tolling Retriever is a medium-sized gundog bred primarily for hunting. It is often referred to as a "toller". It is the smallest of the retrievers, and is often mistaken for a small Golden Retriever. Tollers are intelligent, ...

X-linked PRA

This condition is linked to the

X chromosome The X chromosome is one of the two sex-determining chromosomes (allosomes) in many organisms, including mammals (the other is the Y chromosome), and is found in both males and females. It is a part of the XY sex-determination system and XO sex-d ...

. * Siberian Husky - Night blindness by two to four years old. * Samoyed - More severe disease than the Husky.

Dominant PRA

- Inherited as an autosomal dominant trait due to a mutation in the gene for

rhodopsin Rhodopsin, also known as visual purple, is a protein encoded by the RHO gene and a G-protein-coupled receptor (GPCR). It is the opsin of the rod cells in the retina and a light-sensitive receptor protein that triggers visual phototransduction ...

Feline PRA

* 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 A domestic short-haired cat is a cat possessing a coat of short fur, not belonging to any particular recognised cat breed. In Britain they are sometimes colloquially called moggies. Domestic short-haired cats are distinct from the British Shortha ...

and

Persian Persian may refer to: * People and things from Iran, historically called ''Persia'' in the English language ** Persians, the majority ethnic group in Iran, not to be conflated with the Iranic peoples ** Persian language, an Iranian language of the ...

. 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 Vitamin E is a group of eight fat soluble compounds that include four tocopherols and four tocotrienols. Vitamin E deficiency, which is rare and usually due to an underlying problem with digesting dietary fat rather than from a diet low in vitami ...

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 Penetrance in genetics is the proportion of individuals carrying a particular variant (or allele) of a gene (the genotype) that also express an associated trait (the phenotype). In medical genetics, the penetrance of a disease-causing mutation is t ...

). 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 common.

Commonly affected breeds

* Labrador Retriever *

Golden Retriever The Golden Retriever is a Scottish breed of retriever dog of medium size. It is characterised by a gentle and affectionate nature and a striking golden coat. It is commonly kept as a pet and is among the most frequently registered breeds ...

Border Collie The Border Collie is a Scottish breed of herding dog of medium size. Widely considered to be the most intelligent dog breed, they are descended from landrace sheepdogs once found all over the British Isles, but became standardised in the Angl ...

Shetland Sheepdog The Shetland Sheepdog, often known as the Sheltie, is a breed of herding dog that originated in the Shetland Islands of Scotland. The original name was ''Shetland Collie,'' but this caused controversy amongst Rough Collie breeders of the time, s ...

English Springer Spaniel The English Springer is a dog breed, breed of gun dog in the Spaniel group traditionally used for flushing and retrieving game. It is an affectionate, excitable breed with a typical lifespan of twelve to fourteen years. They are very similar ...

Cavalier King Charles Spaniel The Cavalier King Charles Spaniel is a British breed of toy dog of spaniel type. Four colours are recognised: Blenheim (chestnut and white), tricolour (black/white/tan), black and tan, and ruby; the coat is smooth and silky. The lifespan is usual ...

Briard The Briard or is a French breed of large shepherd dog, traditionally used both for herding sheep and to defend them. It was first shown at the first Paris dog show, in 1863; the first Briard to be registered in the ''Livre des Origines Franç ...

- has an especially high frequency. It can also be found in the poodle varieties

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 Nystagmus is a condition of involuntary (or voluntary, in some cases) eye movement. Infants can be born with it but more commonly acquire it in infancy or later in life. In many cases it may result in reduced or limited vision. Due to the invol ...

. It is also known as lipid retinopathy.

Diagnosis

Progressive vision loss in any dog in the absence of

canine glaucoma Canine may refer to: Zoology and anatomy * a dog-like Canid animal in the subfamily Caninae ** ''Canis'', a genus including dogs, wolves, coyotes, and jackals ** Dog, the domestic dog * Canine tooth, in mammalian oral anatomy People with the surn ...

cataract A cataract is a cloudy area in the lens of the eye that leads to a decrease in vision. Cataracts often develop slowly and can affect one or both eyes. Symptoms may include faded colors, blurry or double vision, halos around light, trouble w ...

s can be an indication of PRA. It usually starts with decreased vision at night, or

nyctalopia Nyctalopia (; ), also called night-blindness, is a condition making it difficult or impossible to see in relatively low light. It is a symptom of several eye diseases. Night blindness may exist from birth, or be caused by injury or malnutrition ( ...

. Other symptoms include dilated

pupil The pupil is a black hole located in the center of the iris of the eye that allows light to strike the retina.Cassin, B. and Solomon, S. (1990) ''Dictionary of Eye Terminology''. Gainesville, Florida: Triad Publishing Company. It appears black ...

s and decreased

pupillary light reflex The pupillary light reflex (PLR) or photopupillary reflex is a reflex that controls the diameter of the pupil, in response to the intensity (luminance) of light that falls on the retinal ganglion cells of the retina in the back of the eye, there ...

Fundoscopy Ophthalmoscopy, also called funduscopy, is a test that allows a health professional to see inside the fundus of the eye and other structures using an ophthalmoscope (or funduscope). It is done as part of an eye examination and may be done as part ...

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 The optic disc or optic nerve head is the point of exit for ganglion cell axons leaving the eye. Because there are no rods or cones overlying the optic disc, it corresponds to a small blind spot in each eye. The ganglion cell axons form the ...

. Secondary cataract formation in the posterior portion of the

lens A lens is a transmissive optical device which focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements''), ...

can occur late in the disease. In these cases diagnosis of PRA may require

electroretinography Electroretinography measures the electrical responses of various cell types in the retina, including the photoreceptors ( rods and cones), inner retinal cells ( bipolar and amacrine cells), and the ganglion cells. Electrodes are placed on the ...

(ERG). For many breeds there are specific

genetic tests Genetic testing, also known as DNA testing, is used to identify changes in DNA sequence or chromosome structure. Genetic testing can also include measuring the results of genetic changes, such as RNA analysis as an output of gene expression, or ...

of blood or

buccal mucosa The oral mucosa is the mucous membrane lining the inside of the mouth. It comprises stratified squamous epithelium, termed "oral epithelium", and an underlying connective tissue termed ''lamina propria''. The oral cavity has sometimes been describ ...

for PRA. Absent a genetic test, animals of breeds susceptible to PRA can be cleared of the disease only by the passage of time—that is, by living past the age at which PRA symptoms are typically apparent in their breed. Breeds in which the PRA gene is recessive may still be carriers of the gene and pass it on to their offspring, however, even if they lack symptoms, and it is also possible for onset of the disease to be later than expected, making this an imperfect test at best.

Management

There is no treatment for PRA. However, most dogs that suffer from this disease adjust remarkably well. To maximize the quality of the dog's life, follow these guidelines: *Do not rearrange furniture, as the dog has memorized the layout of the environment *Use a short leash when walking a dog with blindness from advanced PRA *Erect safety barriers around pools or balconies

References

External links

Article on PRAArticle on Cataracts
{{DEFAULTSORT:Progressive Retinal Atrophy Dog diseases Disorders of choroid and retina