Inherited retinal diseases are among the leading causes for incurable vision loss in the human and canine populations. In dogs, most of these conditions are classified as progressive retinal atrophy (PRA). The retina is a layer of nervous tissue that covers the back of the eyeball where the sensation of vision occurs. The whole eye is just a container for this tissue that supplies the eye with the necessary nutrition and focuses light on the retina. When light enters the eye, it passes through the lens and is refracted, focusing an image onto the retina. Through complex connections of retinal nerves, the information is passed to the visual centers of the brain where it is interpreted. Dogs are born with an undeveloped retina that reaches maturity at 3–6 weeks postnatally.
The structure of the mature canine retina is broadly similar to that of man. The outermost layer, comprising the light-sensitive rod and cone photoreceptors, abuts and is nourished by the retinal pigment epithelium (RPE). Each photoreceptor has outer (OS) and inner (IS) segments. Phototransduction is initiated on the stacked disks of the OS while the biosynthetic functions are concentrated in the IS. The connecting cilium, bridging the OS and the IS, has an important role. Of the photoreceptor cells, cones are responsible for day and color vision, whereas rods confer night or dim light vision. In both humans and dogs, rods and cones are unevenly distributed across the retina and the number of rods by far exceeds the number of cones.
Hereditary diseases of the eye can be present at birth or occurring later in life. Although of these diseases can be treated, it is much better to control these diseases and, ultimately, eradicate them by breeding from sound dogs. With this in mind, eye certification programs for the control of hereditary eye diseases are recommended. Breeding animals are examined by a veterinary eye specialist and certified free from hereditary eye diseases. This practice has lead to the reduction of hereditary eye diseases in some breeds, examples being cataracts in the Golden Retriever and Afghan Hound and collie eye anomaly in the Shetland Sheepdog. However, because most hereditary eye diseases are due to recessive genes, the carrier state cannot be diagnosed by ophthalmoscopic and slit lamp examination.
Progressive Retinal Atrophy (PRA) is a collective term comprising a group of hereditary degenerative lesions of the retina. In generalized progressive retinal atrophy retina loses its function. The condition always affects both eyes. The first sign noticed by the owner is night blindness or poor vision in subdued light, which progresses over months or years to total blindness. There is no treatment that will either halt or reverse the retinal degeneration. The central progressive retinal atrophy, also called RPE dystrophy, is characterized by accumulation of pigment in the layer of pigmented lining of the retina, which results in day blindness and terminates in total blindness. The rate of vision loss is much slower than with generalized PRA, and not all dogs become totally blind. Breeds affected include the Miniature and Toy Poodle, Lhasa Apso, Tibetan Terrier, Tibetan Spaniel, English and American Cocker Spaniels, Labrador and Golden Retriever, and Irish Setter amongst others.
The Lhasa Apso breed shows a late-onset form of PRA, although the age of onset is variable, differing by several years among affected dogs. Even though the number of affected animals is low, the proportion of carriers in some countries is estimated to be as high as 25 percent. Given the absence of a genetic test, carriers may only be identified when one of the parents develops the disease, or when one or more of the descendants show signs of being affected. In any case, the identification of the carrier is too late, since it may have many half-siblings that may also be carriers, and the carrier, as well as its full and half siblings has been mated, usually more than once. The fact that signs of the disease appear several years after the dog has reached its sexual maturity and has been mated, causes the mutated gene to be passed on to the next generation and to be maintained in the population. In Irish Wolfhounds, PRA is of the early-onset type. The fact that the disease manifests itself very early allows owners to exclude affected animals from the breeding population. However, there is no way of identifying carriers so, when these carriers are mated, this ensures that the mutated allele remains in the population.
If the unknown carrier is a popular dog, the mutated allele may even increase in frequency. In addition to excluding affected animals from the breeding population, their siblings, and in general all suspected carriers, may also be excluded. However, this may mean removing a significant number of individuals having highly valuable characteristics, and some of them would not be carriers. The only way of identifying carriers is through genetic testing and, for this to be possible, it is necessary to identify the gene associated with the disease. A number of studies have been done to uncover the mutated gene underlying PRA in this breed. Irish Wolfhounds do not have the PDEB or PDEA gene mutations that cause PRA in Irish setters and Cardigan Welsh corgis, respectively.
New form of Progressive Retinal Atrophy in Swedish vallhund dogs
Beginning in the late 1990s, a new retinal disease in Swedish vallhund dogs was recognized. The retinal abnormalities seen by indirect ophthalmoscopy were different from any known forms of canine inherited retinopathy described to date, which appears to be breed specific. (Finnish Kennel Club: http://jalostus.kennelliitto.fi/frmEtusivu.aspx; Swedish Kennel Club: http://kennet.skk.se/avelsdata/). With the gradual increase of the breed's international popularity, this Swedish vallhund retinopathy was also recognized in other parts of the world, including North America.2 Three distinct stages of the disease based on the severity of the clinical signs have been identified. Dogs in Stage 1 exhibit no clinical signs of vision loss. The stage 2 is mainly diagnosed at 6.2±3.1 years but changes were seen as early as 1.1 and as late as 12.6 years of age. While the majority of dogs with these degenerative changes did not appear to have vision problems initially, some owners reported that their dogs exhibited mild to moderate signs of night-blindness as the areas of retinal thinning expanded. Stage 3-changes were observed as early as 9.2 years and as late as 15.4 years. These dogs suffered from loss of night-vision and severely impaired day-vision; some dogs were assessed as completely blind by their owners.
More about PRA
- Genetic and phenotypic variations of inherited retinal diseases in dogs: the power of within- and across-breed studies
- Novel Form of Progressive Retinal Atrophy in Swedish Vallhund Dogs