Inherited Canine Eye Disorders

Inherited canine eye disorders comprise a large number of ocular pathologies and their occurrence depends on genetic inheritance by the offspring from their parents. General knowledge about hereditary eye diseases has increased significantly during the last twenty years, and today they are among the best described and best characterized group of all inherited canine diseases.

Introduction to Inherited Canine Eye disorders

Eye problems can be divided into congenital and hereditary, trauma, inflammatory, immune-mediated, tumors and others. They are fairly common and their symptoms can vary in severity, ranging from mild irritation or conjunctivitis to blindness or even eye loss. Sometimes development of an eye disorder is associated with a separate health condition, such as diabetes mellitus, hypocalcemia, hyperadrenocorticism and hypothyroidism. Some dog breeds have predispositions to certain diseases, based on their physical traits. For example, flat-faced dog breeds, such as Pugs, Shih Tzus or Bulldogs have a higher risk of developing corneal exposure or corneal dystrophies which can lead to vision loss. In the same manner, dogs with long hair on their head, as Poodles or Maltese, more often suffer from infections and irritations more frequently than other  breeds.

Other than by the environment, eye diseases can be caused by genetic heritage. They are present from birth, but their symptoms may develop later in life. Causes of congenital disorders, except genetic, can be spontaneous malformations, uterine conditions, such as infections and inflammations during pregnancy, toxicity or nutritional deficiencies during pregnancy.

Currently, over 30 genetic mutations have been identified that are associated with various forms of  inherited canine eye disorders. The reason that so many eye disorders have been identified is primarily because the eye is an easily accessible organ, which can be examined without invasive techniques. This enables fairly simple and easy detection of various abnormalities of the eye and its surrounding tissue. Other than that, inherited canine eye disorders share many similarities with genetic eye conditions in human patients. This drew attention to affected dogs as animal modela for human diseases and expanded research of the genetic and molecular background of this group of disorders in dogs.

Dog Eye Anatomy

To understand inherited canine eye disorders it is important to know the basic structure of the eye, and the functions of its various tissues which are affected most often.

Anatomy of the canine eye

When entering the eye, light first passes through the cornea, a see-through eye structure. The amount of light that enters is regulated by the lens through changes in its shape. In the back of the eye is the retina, a thin tissue located near the optic nerve. The light which is focused by the lens falls onto the retina, which converts it into neural signals and sends them to the brain through the optic nerve. The light is processed onto retina by light-sensitive cells, called photoreceptors. There are two main types of photoreceptors, rods and cones. Rods have an important role for vision in dim light and also night vision, while cones are responsible for color vision and function best in relatively bright light. In the back of the retina is the choroid, an extremely vascular layer which provides the retina with the nourishment and oxygen it needs. Abnormalities in any of these structures lead to vision disturbances and anomalies.

Inherited Canine Eye Disorders

Achromatopsia

Achromatopsia, also known as canine day blindness, is a disorder characterized by hemeralopa, a condition of blindness in full sunlight. Bright light troubles the affected dog’s vision and causes images to appear blurry, which decreases as the light dims. The disorder was firstly identified in a human patient, but today is known to affect also Labrador Retriever and German Shepherd Dog. It is caused by abnormalities of the retina, more specifically of the cones, the photoreceptors responsible for vision in bright light. Achromatopsia in dogs manifests itself in cone photoreceptor dysfunction, severely reduced vision capability, pendular nystagmus, rod monochromacy, photophobia, reduced or complete loss of color vision- causing color blindness. An identical mutation in human achromatopsia patients has been identified in affected Labrador Retrievers and German Shepherd Dogs. It is inherited in an autosomal recessive pattern.

Canine Multifocal Retinopathy

Canine multifocal retinopathy (CMR) is an inherited renal disease affecting several dog breeds. It is caused by mutations in the BEST1 gene, also known as VMD2 gene (Vitelliform Macular Dystrophy 2), which is encodes a bestrophin protein. This protein participates in the correct forming of the pigment epithelium in the retina. Mutations in the VMD2 gene result in pigment epithelium atrophy. Identical mutations have been identified in human patients, where they cause different retinal disorders, commonly known as bestrophinopathies.
CMR can be divided into three separate forms, CMR1, CMR2 and CMR3. They differ among each other in genetic mutation, but share the same phenotype. Phenotypically, CMR is characterized by lesions of variable size which form on the retina, changing the appearance of the eye. Sometimes these lesions result in minor retinal folding. The symptoms usually appear when the puppy is only a few months old, with little or no progression of symptoms after one year of age.
CMR1 generally affects Mastiff-related breeds: Dogue de Bordeaux, English Mastiff and Italian Corso. Research shows that the appearance of CMR1 in Mastiff Breeds is independent of each dog’s geographic origin, indicating that the causative mutation appeared before the development of this specific dog breed group. Other than Mastiff dogs, CMR1 is also known to affect the Australian Shepherd, Bulldog, Pyrenean Mountain Dog, Dogo Canario and French Bulldog.
CMR2 only affects Coton de Tulears, a dog breed from Madagascar which was bred under strict control of the royal house and isolated for many generations before the breed’s worldwide distribution during the past 35 years.
CMR3 is a recently recognized separate form of CMR, which is known to affect Lapponian dog breeds: Swedish Lapphund, Finnish Lapphund and Lapponian Herder.

CMR is inherited in an autosomal recessive pattern.

Congenital Stationary Night Blindness (CSNB)

Canine congenital stationary night blindness (CSNB) is a retinal disease affecting the Briard dog breed. It is a juvenile-onset disorder with first symptoms occurring around six months of age. As it causes degeneration of the retinal rod photoreceptors, the first symptom is night blindness, while day vision remains intact. Congenital stationary night blindness is a slow progressive disorder, which can lead to complete blindness. Although night blindness develops at very young age, changes cannot be detected ophtamologically until two to three years of age, when the disease has already progressed. Canine congenital stationary night blindness is inherited in an autosomal recessive pattern and dog heterozygous for causative mutation develops no symptoms. A high carrier rate was discovered in two separately conducted researches. In the Briard population from the United States the carrier rate was 10.2%, and in Briard populations from the Czech Republic, Poland, Hungary and Slovakia the carrier frequency was 11%.

Collie Eye Anomaly (CEA)

Collie eye anomaly (CEA) is an inherited eye disorder caused by abnormalities in the choroid, which is tissue that supplies nourishment and oxygen to the retina. Due to changes in the choroid, collie eye anomaly is also known as choroidal hypoplasia. It is known to affect Rough Collies, Smooth Collies, Border Collies, Australian Shepherds and Shetland Sheepdogs. Symptoms of CEA can vary in their severity among affected dogs. A typical indication is a choroid thinning, which develops at around five to eight weeks of age. This change is not detectable by the naked eye, which makes inspection of puppies’ eyes by an ophthalmologist crucial. The disorder may also cause defects of the sclera, characterized by colobomatous lesions and present as pits within the optic nerve head. Tortuous retinal vessels and multiple retinal folds can be present as well. In severely affected dogs, especially those with colobomas, retinal detachments occur, which leads to blindness. Severely affected dogs also develop occasional intraocular hemorrhage.

Collie eye anomaly is inherited in an autosomal recessive pattern. Researchers have discovered extremely high carrier rates; approximately 70 to 97% of Rough and Smooth Collies in the United States and Great Britain population are carriers, while 68% of Swedish Rough Collies are affected.

Dry Eye Curly Coat Syndrome (CCS)

Dry eye curly coat syndrome (CCS) is a skin and eye disorder unique to Cavalier King Charles Spaniels. Scientifically the disorder is known as congenital keratoconjuctivitis sicca or ichtyosiform dermatosis. The condition causes inflammation of the cornea and conjunctiva due to an inability to produce tears. An inadequate amount of tears in the eye causes an inability to eliminate foreign objects such as hairs or dust, causing infections and painful ulceration, as well as blindness. Affected dogs can be recognized when they are still puppies. The coat is unusually curly for Cavalier King Charles Spaniels, teeth and toenails may seem deformed and mouth and eyes will seem dry and lacking moisture. As the affected puppy ages, it develops a detoration of the skin which causes seborrhea.

CCS is inherited in an autosomal recessive pattern. Due to severe discomfort the disease’s symptoms cause and the fact that no preventive treatment has yet been developed, affected dogs are usually euthanized.

Hereditary Cataract (HC)

A canine hereditary cataract is a clouding of the lens in the dog’s eye, causing a decrease in vision. It is a leading cause of blindness and is one of the most common intraocular diseases in dogs. Cataracts can be divided into two groups, primary and secondary. While primary cataracts appear due to genetic heritage and can be passed on to puppies from parents, secondary cataracts form due to other eye defects, such as progressive retinal atrophy, retinal dysplasia and glaucoma. Primary hereditary cataracts are known to affect 97 different breeds, and some breeds show more than one form of cataract. It is bilateral and symmetrical in the two eyes, and shows a progressive nature until development of full blindness. The owner of the affected dog may observe cloudy or bluish-gray looking eyes. The symptoms appear at around a few weeks to a few months, until progression to full blindness by two to three years of age. Other than genetics, development of cataracts is also influenced by external factors, such as levels of exposure to UV light, or anti-oxidant intake.

Among Staffordshire Bull Terrier, Boston Terriers and French Bulldogs, hereditary cataracts are inherited in an autosomal-recessive pattern.

A dog with hereditary cataract

Primary Lens Luxation (PLL)

Primary lens luxation (PLL) is a disease from group of inherited canine eye disorders characterized by weakened zonular fibers, which are ligaments in the eye that hold the lens in its normal position behind the iris and pupil. In the case of a partial breakdown of the zonular ligaments, lens becomes partially dislocated, condition known as lens subluxation, while a complete breakdown of the zonular ligaments results in a lens fully dislocated from its normal position, a condition recognized as lens luxation. While lens luxation can also occur as a consequence of trauma or tumor, PLL is a form of this disorder that arises spontaneously and is not caused by a previous disease or injury.

PLL is inherited in an autosomal recessive pattern. Certain dog breeds are more prone to the condition, such as terriers and terrier mixed breeds. The genetic mutation causing the disease appears to be extremely widespread among dog breeds, with researches showing up to a 50% carrier rate among terrier dog breeds and a 26% carrier rate among the Chinese Crested dog population.

Yorkshire Terrier with lens luxation

Oculoskeletal Dysplasia (OSD)

Oculoskeletal dyslasia (OSD) affected dogs exhibit short-limbed dwarfism as well as severe ocular defects, such as vitreous dysplasia, retinal detachment and cataracts. Usually, retinal dysplasia is known to affect several dog breeds but without any serious consequences, while in the Labrador Retriever as well as in Samoyeds, retinal folds are associated with OSD, a sever disorder.

OSD is inherited as an autosomal recessive trait. Heterozygous dogs show milder ocular deformities (retinal folds), while skeletal development remains normal.

Progressive Retinal Atrophy (PRA)

Progressive retinal atrophy, or PRA, is a common term used for a group of inherited canine eye disorders that affect the retina. Forms of PRA have been diagnosed in more than 100 dog breeds and they can vary in their age of onset and disease severity. Until now, researchers have discovered at least 22 different causative mutations in 19 genes that are associated with various forms of PRA in over 50 breeds. In the final stages of the disease, complete vision loss occurs and the dog remains fully blind. You can read more about PRA symptoms, types, genetics and diagnostics here.

PRA affected Labrador Retriever

Primary Open Angle Glaucoma (POAG)

Primary open angle glaucoma (POAG) is an eye condition causing a build-up of pressure in the eye. The disorder affectings Beagles. The intraocular pressure causes an irreversible damage of the optic nerves, luxation of the lens and narrowing of the iridocorneal angle. This results in red and dilated pupils as well as painful appearance of the dog’s eyes. POAG is progressive and leads to vision loss.

Mutation causing POAG is inherited in an autosomal recessive pattern, but the condition is curable when detected early enough. Unfortunately, due to its progressive nature, it is often not detected until the blindness onset.

Diagnosis and Treatment

Eye health and inherited canine eye disorders can be tested through various diagnostic tools, depending on the source of the disorder. Internal eye pressure can be measured with a tonometer, and thus glaucoma can be detected. Abnormalities within the eye structures are observed using an ophthalmoscope. In inherited canine eye disorders diagnostics ultrasound can also be applied. Unfortunately, many defects within the eye cannot be detected until the final stages of the disease’s progress.

An ophthalmic dog eye examination

Most inherited canine eye disorders are passed on in an autosomal recessive pattern. Therefore, dogs can be clear, carrier or affected regarding the causative mutation. Carriers do not develop the disease’s symptoms and can pass the defected genes on to their offspring for many generations, until two carriers are mated which results in affected puppies. DNA testing is the only way to detect carriers or affected puppies with late onset forms of the inherited canine eye disorders, and prevent development of the inherited canine eye disorders in future puppies.

References:

Tanaka N, Dutrow EV, Miyadera K, Delemotte L, MacDermaid CM, Reinstein SL, et al. (2015). Canine CNGA3 Gene Mutations Provide Novel Insights into Human Achromatopsia-Associated Channelopathies and Treatment. PloS ONE 10(9): e0138943. doi:10.1371/journal.pone.0138943

Forman, O.P., Penderis, J., Hartley, C., Hayward, L.J., Ricketts, S.L., and Mellersh, C.S. (2012). Parallel mapping and simultaneous sequencing reveals deletions in BCAN and FAM83H associated with discrete inherited disorders in a domestic dog breed. PLoS Genet. 8, e1002462.

Andres Veske, S.E.G.N. (1999). Retinal Dystrophy of Swedish Briard/Briard–Beagle Dogs Is Due to a 4-bp Deletion inRPE65. Genomics 57, 57–61.

Guziewicz, K.E., Slavik, J., Lindauer, S.J.P., Aguirre, G.D., and Zangerl, B. (2011). Molecular Consequences of BEST1 Gene Mutations in Canine Multifocal Retinopathy Predict Functional Implications for Human Bestrophinopathies. Invest Ophthalmol Vis Sci 52, 4497–4505.

Lowe, J.K., Kukekova, A.V., Kirkness, E.F., Langlois, M.C., Aguirre, G.D., Acland, G.M., and Ostrander, E.A. (2003). Linkage mapping of the primary disease locus for collie eye anomaly☆. Genomics 82, 86–95.

Mellersh, C.S., Pettitt, L., Forman, O.P., Vaudin, M., and Barnett, K.C. (2006). Identification of mutations in HSF4 in dogs of three different breeds with hereditary cataracts. Veterinary Ophthalmology 9, 369–378.

Farias, F.H.G., Johnson, G.S., Taylor, J.F., Giuliano, E., Katz, M.L., Sanders, D.N., Schnabel, R.D., McKay, S.D., Khan, S., Gharahkhani, P., et al. (2010). An ADAMTS17 Splice Donor Site Mutation in Dogs with Primary Lens Luxation. IOVS51, 4716–4721. Inherited canine eye disorders.

Goldstein, O., Guyon, R., Kukekova, A., Kuznetsova, T.N., Pearce-Kelling, S.E., Johnson, J., Aguirre, G.D., and Acland, G.M. (2010). COL9A2 and COL9A3 mutations in canine autosomal recessive oculoskeletal dysplasia. Mamm. Genome 21, 398–408.

Kuchtey J. (2011.): Mapping of the disease locus and identification of ADAMTS10 as a candidate gene in a canine model of primary open angle glaucoma. PLoS Genet. 2011;7:e1001306.

Miyadera, K. (2012): Genetic and phenotypic variations of inherited retinal diseases in dogs: the power of within- and across-breed studies. Mamm Genome, 23: 40-61.

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