Canine Retinal Dystrophies

The term Progressive Retinal Atrophy (PRA) is usually used when describing a bilateral generalized retinal degenerative disease primarily affecting the photoreceptor cells, the rods and/or the cones.¹ PRA is a rather common spontaneous disease entity in dogs, and more than 100 dog breeds are affected by some form of hereditary retinal degeneration. Clinically PRA is characterized by visual impairment especially in the dark (scotopic conditions), but with progression of disease also daylight vision (photopic conditions) is affected. Initial clinical signs vary in the disease as well as the progression. The reason for this is that PRA is actually a group of diseases on the cellular level in which the molecular genetic and/or the biochemical characteristics vary. It has been shown that many specific mutations are causative to the defects, among which progressive rod cone degeneration or the prcd gene defect is one of the most common in dogs.² Up to now, more than 20 breeds are affected by that specific mutation, causing slowly progressive retinal degeneration and blindness in affected dogs at variable ages, but in most cases between ages 5-8 years.

Other forms of canine hereditary retinal dystrophies include cone rod dystrophy in the longhaired dachshund due to the cord1 mutation and cone rod dystrophy of wirehaired dachshunds, recently shown caused by a mutation in the nephronophthisis 4 gene. Both forms are early onset diseases.

The outermost parts of the retina include the retinal pigment epithelium (RPE). Hereditary defects can affect the RPE as well as the neuroretina. An example of the former is the RPE65 gene defect of Briard dogs. In the Great Pyrenees and Coton de Tulear dog breeds an RPE dysplasia has been characterized, caused by a mutation in the VMD gene. For the latter disease focal serous retinal and RPE detachments are formed.¹

Below is a list of some breeds affected by PRA; both the early onset and the late onset type, the mode of inheritance and the age for a tentative diagnosis by ophthalmoscopy and by electroretinography (ERG), respectively.

Feline Retinal Dystrophies

While there have been reports of PRA in cats, hereditary retinal blindness has not been considered a significant health factor in cat breeds previously. Approximately 30 years ago a recessive disease of Abyssinian cats was found in a high frequency in Sweden. Only a few years later the same breed was found to be affected by a dominant retinal dystrophy found in the United Kingdom. The mutations for both of these disorders have now been elucidated^3,4 and further studies of other cat hereditary diseases are underway.⁵ Quite recently it has become obvious, however, that there are many more hereditary retinal dystrophies affecting breed cats in the world than previously thought. The rdAc mutation has recently been shown to also affect Siamese cats in a high frequency, as well as other domestic cat breeds.⁶ In a large population genetic survey of 846 cats, the rdAc mutation was found in a total of 34% of 41 cat breeds examined and there was a significant distribution of the mutation in both USA and in Europe.

Equine Retinal Disorders

Only a few horse breeds have been shown affected by a spontaneous type of hereditary retinal disorders. The most thoroughly studied is congenital stationary night blindness (CSNB) in the Appaloosa breed of horse.^7,8 Affected horses are night blind from birth but have a normal fundus appearance and normal or near normal daylight vision. ERGs are diagnostic in young foals. The disease appears not to be progressive. Other breeds of horses have recently been shown to be affected by the disorder as well.^9,10

References

1.  Narfström K, Petersen-Jones S. Diseases of the canine ocular fundus. In: Veterinary Ophthalmology, Vol 2, 4th edition, ed: Gelatt KN, Blackwell Publishing, Oxford, UK, Pp: 944-989, 2007.

2.  Aguirre GD, Acland GM. Models, mutants and man: searching for unique phenotypes and genes in the dog model of inherited retinal degeneration. In: The dogs and its genome. Ostrander EA, Giger U, Lindblad-Toh (eds). Cold Spring Harbor Laboratory Press, Cold Spring Harbor, NY, Pp291-325, 2006.

3.  Menotti-Raymond M, David VA, Schaffer AA, et al. Mutation in CEP290 discovered for cat model of human retina degeneration J Hered 98:211-220, 2007

4.  Narfström K, Holland Deckman K, David V, et al. Mutation discovered in a feline model for early onset severe retinal blinding disease. Accepted for Association for Research in Vision and Ophthalmology, 09-A-3914-ARVO, 2009

5.  Rah H, Maggs DJ, Blankenship TN, et al. Early-onset autosomal recessive, progressive retinal atrophy in Persian cats. Invest Ophthalmol Vis Sci, 46:1742-1747, 2005.

6.  Menotti-Raymond M, David VA, Pflueger S, et al. Retinal blinding disease mutations (rdAc) identified in 16 cat breeds with high prevalence in Siamese populations. Submitted, 2009.

7.  Witzel DA, Smith EL, Wilson RD, Aguirre GD. Congenital stationary night blindness: an animal model. Invest Ophthalmol Vis Sci, 17:788-795, 1978.

8.  Sandmeyer LS, Breaux CB, Archer S, Grahn BH. Clinical and electroretingraphics of congenital stationary night blindness in the Appaloosa and the association with the leopard complex. Vet Ophthalmol 10(6): 368-375, 2007.

9.  Nunnery C, Pickett JP, Zimmermann KL. Congenital stationary night blindness in a Thoroughbred and a Paso Fino. Vet Ophthalmol 8: 415-419, 2005.

10. de Linde Henriksen M. Congenital stationary night blindness in the Danish Knabstrupper horse. The Globe; Newsletter of the International Society of Veterinary Ophthalmology, Summer issue, 2008.