Collies like other breeds of dogs have a number of inherited eye defects. Some of these are quite severe while others are relatively minor. The important ones we think should be considered are Collie eye anomaly (CEA) and progressive retinal atrophy (PRA).
Collie eye anomaly was first reported in 1953. The original report included a description of pale area in the retina due to a deficiency of blood vessels along with a bulging of the back of the eye and retinal detachments. Since the first reports a number of studies have been completed and tens of thousands of Collie dogs have been examined. Based on these studies and examinations, specific breeding recommendations have been proposed. CEA is the incomplete development of the eye that is present as early as the 28th day of development. The defect involves the sclera (white outer wall of the eye) and the choroid (blood vessel layer in back of the eye). Additionally, the retina (portion of the eye that turns light into electricity), the retinal blood vessels and the optic nerve are also involved. Clinically the severity of CEA is variable--ranging from no apparent vision defect to total blindness. It is found in rough and smooth Collies and all color coats are involved. Most Collies (80 to 90%) with CEA do not demonstrate vision problems. CEA is a simple recessive defect. This means that a gene from both mother and father (homozygous state) must be present for CEA to develop. Carrier animals (who have the gene from only mother or father) and normal animals cannot be separated based on an ophthalmic or eye examination. A number of researchers have attempted to separate the various aspects of the disease but were unable to do so. When the disease was first described, such a large percentage of the population was affected that a number of grading systems were devised to make classifications of individuals easier. It was the feeling of some people at the time that the severity of the disease might be lessened by breeding individuals with minor problems to each other. Certainly this idea has had its place in history of breeding better Collies. Unfortunately dogs with minor afflictions can and do produce severely afflicted offspring. Likewise blind parents can produce less afflicted offspring. An individual with a mildest problem is just as bad as a totally blind dog for the purpose of genetic selection. Because the grading system remains firmly entrenched within the Collie breeding community, a discussion of the grades and categories is appropriate:
| Grade: | Findings: |
| Grade 1 | torturous retinal vessels, extremely small areas of choroidal hypoplasia |
| Grade 2 | torturous retinal vessels, substantial areas of choroidal hypoplasia |
| Grade 3 | tortuous retinal vessels, substantial areas of choroidal hypoplasia (blood vessel loss) with pits (colobomas) or areas of out pouching (ectasia) in the posterior segment |
| Grade 4 | all the above defects with a retinal detachment |
| Grade 5 | all the above defects with a retinal hemorrhage |
It is possible for one eye to have a different grade than the other but both eyes in almost all cases are affected. "Go normal" is a term used to describe an affected individual, Grade 1 or Grade 2, in which the area of choroidal hypoplasia fills in so it appears normal during later examinations. These animals act genetically like the affected individuals that they are. They can set a breeding program back years. Because the lesion is present at birth, puppy eyes can be checked as early as 5 to 6 weeks of age. For the ease of the examiner and to facilitate a more accurate exam, evaluation at 6 to 8 weeks is recommended.
Progressive retinal atrophy (PRA) is a collective term used to describe a variety of inherited retinal diseases in dogs. Within this category is a syndrome known as rod-cone dysplasia or PRA in the Collie. A dysplasia indicates an abnormal developmental process where tissue, in these cases the rod and the cones, never form normally. Clinically affected animals exhibit night blindness as early as 12 weeks of age and progress to total blindness by one year of age. All affected individuals become totally blind. Changes can be seen in appearance of the retina by 6 months. Late stages are characterized by a reduction of retinal blood vessels, increased reflection from inside the eye, pigment changes in the retina and a pale optic disc. An electroretinogram (ERG) is the recorded electrical changes in the eye over a time that results from a light being shined in the eye. ERG changes are evident as early as 1-to-12 weeks of age. The test involves placing a contact lens on the cornea to which a wire has been attached. Two other wires have been attached to the skin around the eye. These wires are then connected to amplifiers and an oscilloscope. A dim light is then flashed into the eye at various intervals using a variety of filters. It is possible to separate the rod and the cone function and differentiate affected puppies from normal animals before the onset of clinical signs. Rod-cone dysplasia (PRA) is inherited as a recessive gene. Therefore a homozygous individual (one who received a gene from both mother and father) is affected while the normal animals and carriers (who received a gene from either mother or father) appear normal and show no changes in vision or electroretinographic changes. A test is been devised to detect carriers short of test breeding. Unfortunately, the test will not be available for quite a few years. Test breeding involves breeding of suspected patients to known afflicted individuals. Because the disease is expressed in the homozygous form only the production of affected offspring indicates that the suspected individual is a carrier. Statistically the more normal individuals produced as a result of test breeding the greater the assurance the suspect is a normal dog or a non-carrier. At least 6 puppies must be produced from the suspected individual to have a 95% level of confidence that the suspect is normal and a non-carrier.
MERLE COLLIES
The fundus of the Merle Collie presents a special challenge in differentiating affected individuals from non-affected in terms of Collie eye anomaly. The Merle fundus is often lightly pigmented and exhibits a normal amount of choroidal hypoplasia (loss of blood vessels). Some of these individuals cannot be designated as normal based on a clinical examination. Test breeding or an ERG may be required.
Have any questions on this subject? Contact
Dennis Hacker via E-mail:
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