Pamela A. Davol, 76 Mildred Avenue, Swansea,
The eye is actually a relay station which collects light images, transforms these images into nervous impulses and then passes them via the optic nerve to the brain where perception of sight takes place. The eye is composed of many structures necessary for its proper function.
The globe-like eyeball sits in the folds of the eyelids which protect it from foreign materials and aides in the lubrication of the eye. The inner surface of the lid is lined with a vascular tissue called the conjunctiva. The conjunctiva forms a pocket around the front of the eye and keeps foreign material from slipping around behind the eyeball. The eye, itself, is made up of three layers. The cornea, the transparent layer around the front of the eye, and the sclera, a white, dense connective tissue around the back of the eye, make up the outer layer. The middle layer of the eye includes 1) the iris, the colored portion of the eye which is composed of blood vessels and muscle fiber which controls the opening and closing of the pupil through which light passes, 2) the ciliary body, which contains muscle fibers responsible for alterations of the shape of the lens (it is through shape changing that the lens allows focusing of the eye), and 3) the choroid, a vascular layer of tissue which lines the sclera and is important in supplying blood flow and nutrients to the structures of the eye. The innermost layer is the retinal layer, which is composed of receptor cells, the rods and cones, which collect the light, transform the light into neural impulses and relay these impulses to the brain by way of the optic nerve. The lens separates the interior of the eye into two unequal areas. The large area behind the lens, the posterior compartment, is filled with a transparent gelatinous mass called the vitreous humor. The area in front of the lens is divided into a posterior chamber, the space between the iris and the lens, and the anterior chamber, the space between the iris and the cornea. These two chambers are filled with a transparent watery fluid called the aqueous humor. Light passes first through the cornea and the aqueous humor, the pupil controls the amount of light allowed to pass through to the lens, the lens focuses the light images which pass through the vitreous humor before reaching the receptor cells of the retina. For a clear interpretation of the image by the brain, all of these structures must be healthy and transparent. Often these structures may be affected by either environmental or hereditary factors which may interrupt or interfere with their proper function and thus lead to partial or complete vision impairment. A discussion of hereditary disorders affecting the eye follows here.
Eye disorders inherited in the Labrador Retriever can be divided into two groups:
1) those affecting the internal eye directly resulting in impaired vision or blindness, or
2) those affecting the eyelids which can lead to secondary complications involving the eye.
The portion of the internal eye called the retina is a thin, delicate transparent membrane made up of six layers, one of which is composed of receptor cells. It is the function of these receptors to collect light and relay information to the central nervous system (brain) by way of the optic nerve. When layers of the retina separate from the underlying choroid membrane either partially or completely, the condition is termed retinal dysplasia. As a result of separation, the layers of the retina usually form undulating folds creating opaque areas through which light cannot easily pass. Eventually, within a short period of time, the retinal membranes will atrophy because blood and nutrients from the choroid layer will be unable to reach the detached retinal layers, and the normal retinal pattern will be lost.
There are two forms of retinal dysplasia found in Labrador retrievers; one which is found predominantly in dogs of European descent and the other found in dogs with predominantly American field trial bloodlines. The European form is inherited as an autosomal recessive gene and only effects vision. The American form is inherited as an incompletely dominant trait with recessive effects on the skeleton resulting in abnormalities of limb development (short-limbed dwarfism). Dogs which receive two recessive genes for this defect (one from each parent) will exhibit retinal detachment which will result in blindness; however, dogs receiving only one recessive gene for this defect (one parent contributes the recessive gene for this defect and one parent contributes the gene for normal retinal development) will develop retinal folds of a non-progressive nature and, therefore, may have normal to slightly impaired vision.
PRA is the most common disorder affecting the retina of the dog and is a result of the reduction of retinal blood vessels and atrophy of the receptor cells of the retina. There are two types of PRA: generalized-PRA and centralized-PRA. In generalized-PRA, there is overall retinal function loss. Both eyes are affected, though one may be at a more advanced stage than the other. The condition is progressive, as indicated, though the rate of progression varies from breed to breed and individual to individual; however, the end result in all cases is blindness. Dogs afflicted with g-PRA often can only recognize objects immediately in front of them as there is early loss of peripheral vision. Centralized-PRA also affects both eyes and is progressive, however, dogs may retain peripheral vision for several years, but there is an early loss of central vision. This second form of PRA is the most common in the Labrador Retriever.
The lens is the part of the eye which functions to bring objects to correct focus on the retina. A cataract is any opacity occurring in the lens affecting its transparency. In most cases the formation of cataracts is associated with abnormal water and calcium content within the lens substance. Causes for these abnormal chemical levels range from environmental, to metabolic, to hereditary, and can occur during the development of the lens (developmental cataracts) or be a result of degenerative changes after development has occurred (degenerative cataracts). Severity of the disorder ranges from non-progressive and slight, in which there may be no interference with vision, to progressive, in which there is a slow and gradual loss of transparency.
When the upper and/or lower eyelids roll inward so that the edge of the lid, or eyelashes produce irritation to the eye, the condition is termed entropion. Often excessive folds of loose skin or the presence of shortened eyelids may be the cause of the disorder.
Ectropion is the opposite of entropion and occurs when the lower eyelid rolls away from the eye exposing the conjunctiva. Dust and other foreign objects cause irritation and inflammation which may lead to infection.
Eyes should be examined yearly by a veterinary opthamologist, a specialist in the structure and function of the eye and recognition of eye disorders. Opthalmoscopic examination and electroretinography (ERG) are methods utilized to view internal structures of the eye to determine if any abnormalities exist.
The key to prevention of eye disorders lies in the attempt to control for both environmental and hereditary factors. Proper nutrition is essential to healthy eye development. From a hereditary standpoint, screening breeding stock for the presence of eye disorders and eliminating affected individuals from the gene pool is a major contribution toward eradicating eye disorders within a breed.
It is recommended that dogs which are destined to be bred first have their eyes examined by a member of the American College of Veterinary Opthamology (ACVO). Puppies may be screened as early as 6 weeks of age for the presence of congenital eye disorders, but since many of these disorders do not present until adulthood, it is recommended that dogs be examined yearly. A dog found to be free of eye disorders will receive an application for number certification with the Canine Eye Registration Foundation (CERF). CERF has developed a network in co-operation with the ACVO and breeders to collect and distribute information on occurrence of eye disorders within the various breeds of dogs in an effort to prevent further spread of hereditary eye disorders.
In 1970, Barnett and colleagues reported that the occurrence of retinal dysplasia in Labradors was attributed to a recessive gene; in 1972, the same group reported that cataracts in the Lab are transmitted through a dominant gene. The occurrence of gPRA has been attributed to a recessive gene in other breeds, however, the form of gPRA affecting the Lab, although also suspected of being transmitted through a recessive gene, differs in both its clinical manifestation and in that its transmittance may be sex-linked (based on recent research at Cornell). Transmittance of cPRA appears to be dominant with variable penetrance. Modes of inheritance of entropion and ectropion are still unknown. Often factors other than heredity, such as trauma, nutrition, metabolism, etc., play a role in the development of eye disorders and, thus, may complicate interpretation of genetic predisposition.
No. The screening process for eye disorders can only determine "phenotype" (genetic expression) but cannot determine "genotype" (actual genetic make-up). Because disorders like RD and gPRA are results of recessive genes, it is possible that a dog may carry the recessive gene without expressing it. In such a case, the recessive gene fo the disorder is present but "masked" by the dominant gene for normal eye structure and function. Such a dog would be called "a carrier" of the disorder. To be affected by the disorder, the dog must receive a recessive gene from both parents. For these reasons, eye disorders attributed to recessive genes are often difficult to completely eradicate from the gene pool because "a carrier" may be bred many times before it is realized that the dog does indeed carry the disorder as shown by afflicted offspring. In order for the disorder to affect offspring, the mate must also be a carrier. Over the years, a carrier will produce many more carriers. The more carriers in the gene pool, the greater the risk of producing offspring which will be afflicted by the disorder. In a litter born of screened parents, there is a 0-50% risk (0-5 out of 10 puppies) of the litter being carriers of a recessive disorder such as RD; there is a 0-25% risk (0-2 out of 10 puppies) of the litter will actually develop an eye disorder such as RD. For these reasons it is recommended that dogs suspected of being carriers through expression of the disorder in offspring no longer be allowed to breed. Dominant genes are more easily eradicated from the gene pool because individuals carrying the gene will always express it. However, one complication lies in the fact that some dominant disorders are not observed until later in life, and an individual may be bred prior to discovery of the disorder. Once a hereditary eye disorder has been diagnosed, eliminating the affected individual from the breeding program and alerting owners of offspring to the potential risk may help in preventing future generations of developing hereditary eye disorders.
Unfortunately, as indicated above, Labrador Retrievers can be afflicted with a number of eye disorders which may lead to complications, including blindness, as the dog matures. For this reason, Wing-N-Wave has its breeding stock examined annually by a member of the American College of Veterinary Opthamologists (ACVO) as a means to minimize risk for hereditary eye disorders in future generations. Additionally, certain eye disorders, such as Retinal Dysplasia (RD) and cataracts, are congenital in nature and may be detected at a very early age, particularly in puppies who are severely afflicted. For this reason, Wing-N-Wave has all puppies examined by an ACVO member at 7 weeks of age. Please be aware, however, that an examination at this age has its limitations because the retina is not fully developed until the puppy is approximately 4 months of age. Therefore, we urge puppy owners to have their puppy's eyes examined annually.Because of the hereditary nature of disorders such as RD and PRA (progressive retinal atrophy), and because PRA may not present until a dog is 4-6 years of age, Wing-N-Wave offers a lifetime warranty against RD and PRA.
During the eye examination, many times puppies are found to have a condition termed "Persistent Pupillary Membranes (PPMs)." The following is an explanation of this condition by Dr. D.A. Wilkie in Kirk's Current Veterinary Therapy XI :
During embryogenesis, the anterior chamber is filled with vascular tissue, the pupillary membrane. This mesodermal tissue normally undergoes degeneration at the end of gestation and in the immediate postpartum period. Incomplete atrophy leaves remnants termed persistent pupillary membranes (PPMs). Persistent pupillary membranes arise from the collarette zone of the iris and may attach to the inner cornea, to the anterior lens, or to the iris. Endothelial opacities or focal cataracts are occassionally found in association with attachment of PPMs. Most PPMs do not interfere with vision and are considered to be incidental findings in many breeds. Exceptions include the Basenji and Corgi, in which the defect is suspected to be inherited; in its severe form, impairment of vision can result, usually by corneal opacification. No treatment is indicated, and the condition is nonprogressive.
In the Labrador Retriever, this condition has not been associated with the formation of opacities or cataracts, does not interfere with vision, is considered an "incidental finding," and does not prohibit number certification with the Canine Eye Registration Foundation (CERF). In many cases, PPMs completely dissolve by the time the puppy reaches one year of age.
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