Genetic Health

Can You See? Inherited Eye Disease

By February 17, 2015July 5th, 2017No Comments

Inherited Eye Disease in Aussies

by C.A. Sharp

(ed. note: C.A.Sharp is the president of the Australian Shepherd Genetics Institute, an organization ‘dedicated to the increase and diffusion of knowledge of genetics in the Australian Shepherd, and the inherited diseases from which it sometimes suffers.’ She is a science writer and an internationally recognized lay expert on canine genetics and hereditary diseases.)

Eye defects are the most common inherited problem in Australian Shepherds. Even discounting the problems that result from merle-to-merle breeding, they are still the most likely genetic diseases a breeder will encounter. Conformation lines are more heavily affected, but working lines experience them also. What are the problems that occur in Aussies and how common are they? How did this happen to our breed and what can breeders do about it?

Aussies belong to a family of herding breeds which also includes Border Collies, Rough and Smooth Collies, Shetland Sheepdogs, Bearded Collies, the English Shepherd and a smattering of other breeds either unknown or extremely rare in North America. It is likely that these breeds share some degree of common ancestry. Not surprisingly, they also share several eye diseases. Cataracts, Collie Eye Anomaly (CEA), and Progressive Retinal Atrophy (PRA), to name the most devastating, occur in most or all of these breeds. Prior to the 1970’s, Australian Shepherd breeders were not much concerned with eye disease. Most breeders were ranchers or farmers. If matings were planned, as opposed to just happening, the criteria were performance-based: Will breeding this bitch to that dog result in a tougher style for working rank range cattle? Will the puppies this pair produces have their same firm but “paws and mouths off” attitude toward sheep? Will Bessie bred to Joe’s retriever produce a herder that can help out in hunting season?

Little time was spent by most breeders on in-depth study of pedigrees. Indeed, pedigree information might be sparse or lacking altogether. This breeding technique, while lacking in several respects, had some advantages. It tended toward greater genetic diversity within individual dogs and therefore lessened the likelihood that bad genes for things like eye disease would match up. And if eye disease did become apparent, affected dogs would be culled because they could not earn their keep.

Several pioneer breeders of the modern Australian Shepherd began line-breeding and in-breeding their dogs in the 1960s. By doing so, they managed to develop distinct type and style over the space of a few generations. This resulted in a much more uniform breed in regard to looks, character and working style. The problem is that breeding related dogs together concentrates not only the positive traits but hidden negatives as well. It can also result in the loss of useful genes without breeders realizing they have done so. The use of popular sires can intensify both the positive and negative effects of line-breeding, because over time most dogs will wind up being to some degree related. If that degree is high, it can be very difficult to find a line clear of a particular disease.

Current Status
Today we find ourselves faced with multiple, independently inherited, eye diseases. Most common is probably merle ocular dysgenesis—the complex of defects observed in homozygous merles. Of the non-merle related diseases, cataracts and iris coloboma are most common, followed by CEA (Collie Eye Anomaly). Persistent pupillary membrane and distachiasis occur, though they are not common. And PRA is rare, if it is present at all. Hyaloid arteries, though not recognized as a hereditary problem, may be one.

Merle Ocular Dysgenesis
Dogs affected with this condition will exhibit some combination of the following: Microphthalmia, eccentric pupils, coloboma or other irregularities of the iris, lens luxation, cataract, retinal dysplasia or detachment, persistent pupillary membrane, equatorial staphyloma and lack of a tapetum. Since the condition is produced solely by breeding merle-to-merle, it can be avoided by not doing so. When two merles are bred together, breeders commonly cull puppies displaying amounts of white markings deemed “excessive” by the breed standards.

While this will eliminate most affected pups, not every homozygous merle will have “too much white.” The actual status of such dogs may not be known until, and unless, their eyes are examined. The presence of cryptic or “phantom” merles, though rare, can further cloud the picture. These dogs do not appear merle at first glance. Some will appear non-merle on even close inspection because their areas of merling are obscured by white markings or were on the tail, which was docked. Everyone who has such a dog and uses it for breeding must keep in mind that it is a merle and, if bred to another merle, will produce puppies with merle ocular dysgenesis.

The most common independently inherited problem in Aussies, cataracts are also the most difficult for a breeder to deal with. Lens opacities can be caused by a number of things, but hereditary cataracts will always be bilateral, though one eye may develop them six months to a year before the other. Some remain small but others will progress until the dog has lost all functional vision. Most cataracts seen in Aussies are posterior polar, meaning they start in the middle of the back side of the lens. They may first be noted as early as a year, or as late as 7 or 8 years. Animals with “late onset” cataracts have produced affected “early onset” offspring.

Most, though not all, inherited cataracts in Aussies appear to be dominant with incomplete penetrance. The mode of inheritance of the other types is not yet known. Due to the extremely variable age of onset, regular annual eye exams of all breeding stock are critical. If an animal is bred even once, it should be checked yearly until it is at least 9 years old. If cataracts occur, affected animals should no longer be bred. Owners of the affected dog’s parents, siblings and offspring should be notified. These near relatives should be bred only to dogs which are not closely related and which come from families free of cataracts. If they should produce cataract-affected offspring, they should be pulled from breeding.

Iris Coloboma
Less devastating but still common, is the iris coloboma. Affected dogs are missing part of the iris. In many dogs the effect on their vision is minimal, however a large coloboma can force a dog to squint in bright light because the iris is incapable of contracting to reduce the amount of light entering the eye. This can cause minor discomfort as well as temporarily reducing the range of vision while squinting. When having eye exams performed, it is important that the irises be examined before dilation. Some small colobomas may not be apparent when the eye is dilated and thus missed.

The mode of inheritance for iris coloboma is unknown. Until fairly recently, virtually all iris colobomas in Aussies were seen in merle dogs. In the past few years more and more iris colobomas are being reported in non-merles. The reason for this is not known, but it is possible that the gene for the defect is located on the same chromosome as the merle gene. Such genes are said to be “linked.” Linked genes tend to be inherited together, though the process of recombination will mix them up. However, if two genes are located close to each other, they will only rarely become separated. This may have happened with the genes for merle and iris coloboma, resulting in the recent increase in non-merle Aussies with the defect. Affected animals should not be bred. Unaffected individuals which produce it repeatedly, particularly with multiple mates, should be pulled from breeding.

Collie Eye Anomaly
This disease was once at least as common as cataract, if not more so. It seems to have reduced in recent years, probably because the mode of inheritance has been identified. Aussies affected with CEA will exhibit choroidal hypoplasia, optic disc coloboma, and retinal dysplasia/detachment. The particular defect observed may differ from eye to eye, but both eyes will be affected. Due to the “go normal” phenomenon, in which the developing tapetum pigmentation obscures areas of choroidal hypoplasia, it is vital that all puppies be checked at a young age. Certainly no later than 8-10 weeks, preferably sooner. Most affected dogs have functional vision, but some are blind in one or both eyes.

CEA is a simple recessive disorder. All affected animals have two genes for CEA, therefore both of their parents are carriers. Affected animals should not be bred, except when used in test-matings to clear suspected carriers. Carriers should not be used at public stud nor should their puppies be sold for breeding. However, a high-quality carrier animal’s positive traits can be retained via the use of test-mating. The carrier should be bred to a mate known not to carry CEA. The resulting offspring should then be test-mated to a CEA affected animal to determine their status. Individuals which produce six or more unaffected offspring from such a mating can be assumed to be clear of CEA. Affected animals used for test-matings should not be merle to avoid any possibility of merle-related defects clouding the results.

Persistent Pupillary Membrane
The pupillary membrane covers the pupil prior to birth. It is supposed to be gone by the time a puppy opens its eyes. Sometimes, however, it persists. If it resolves within a few weeks, there is probably no reason to worry. However if it remains, it can affect vision. PPM can occur in one or both eyes. PPMs occur in three types: iris-to-iris, iris-to-lens and iris-to-cornea. The first rarely causes any visual problem. However, attachment to either the lens or the cornea can result in opacities at the point of attachment. Those opacities can be blinding. The mode of inheritance for PPM is not known, so the best course of action is not to breed dogs in which a PPM fails to resolve. If an unaffected animal produces it repeatedly, particularly with multiple mates, it should no longer be bred.

Dogs with this defect have one or more eyelashes that grow toward the cornea, rather than away from it. The inward-growing hair can abrade the cornea. This is painful and, if not treated, can damage vision. Surgical correction is available but can be expensive. The mode of inheritance is unknown. Affected animals can be identified before they reach breeding age and should not be bred. Unaffected individuals who produce it repeatedly, especially with multiple mates, should be pulled from breeding.

Progressive Retinal Atrophy
PRA has been reported in Aussies, but only rarely. It is a recessive, though there are several genetically distinct forms. Which of these, if any, occur in Aussies is unknown. Since the disease is progressive, it may require multiple exams before diagnosis can be confirmed. All of the cases of Aussie PRA have been in active working dogs. Because of this, it is possible that dogs so diagnosed may have one of two similar trauma-induced (and therefore not genetic) conditions, Acquired Focal/Multi Focal Retinopathy or Acquired Sector Retinopathy. If an Aussie which is regularly engaged in heavy physical activity that can include blows to the head, such as working stock, is diagnosed with PRA, the owner should arrange for a second opinion from a certified veterinary ophthalmologist who is familiar not only with PRA but with the acquired retinopathies.

Hyaloid Arteries
Like PPM, these little arteries are embryonic structures that are supposed to go away. The hyaloid artery extends from the optic disc to center, back of the lens. It’s purpose is to nourish the front of the developing eye. Sometimes part or all of this vessel will persist. Sometimes a cataract will be associated with the lens attachment. These cataracts may progress until the dog is blind. Hyaloid arteries are not considered a hereditary problem, however their association with cataracts and the frequency with which they occurred among dogs which were part of the CEA study is reason for concern. Until such time as the heritability, or lack thereof, is clearly established, breeding a dog with a hyaloid-associated cataract should be discouraged. If a dog has a hyaloid, select mates for it that do not.

Damned if you do, damned if you don’t.
Unfortunately, human nature is one of the biggest promoters of the spread of hereditary problems like eye disease in purebred dogs. For some people it comes in the form of denial. Such individuals don’t bother to screen their dogs, ignore the presence of a hereditary condition, ignore the fact that an obvious condition is hereditary, or belittle its significance. Failure to acknowledge that there is a problem guarantees that the problem will continue.

Other people personalize the issue, reacting emotionally to the news as if it were a stain upon their personal worth rather than an unfortunate alignment of genes. If they cannot resolve their anger and defensive reaction, they will be unable to deal effectively with the problem.

Worst of all are those who are dishonest. If a dog develops a problem, it quietly disappears or suddenly dies of something distinctly non-hereditary. Or they breed the affected or carrier dog with full knowledge of the fact. Some will go so far as to obtain falsified “clear” eye exam documents by presenting unaffected animals of the same sex and color to an unwitting examiner.

Finally comes the poor person who tries to do right and, upon pointing out a problem, finds him or herself the target of the rage of individuals from any or all of the above categories. Verbal disparagement and threats toward the “accuser” are the usual reaction. In rare instances the situation can escalate into very real threats of legal entanglement or physical damage to self or property. And if the “whistle-blower” folds under the pressure, the dogs suffer.

Seeing your way through it.
While no breeder can guarantee that he or she will never produce a puppy affected with an eye disease, the amount of it that occurs could be reduced if breeders would act individually and cooperatively.

* Have all puppies examined by a board certified veterinary ophthalmologist prior to 8-10 weeks of age . If an animal is ever to be used for breeding, even once, have it re-examined annually until at least 9 years of age.

* Do not breed affected animals. Take appropriate care with near relatives of affected animals and cease breeding them, if necessary.

* If an animal is affected or is consistently producing a particular problem, inform the owners of its parents, siblings and offspring.

* If a dog comes from a line known to produce a particular eye disease, do not line breed on the problematic portion of its pedigree. If possible, seek unrelated mates from families known to be clear of the disease.

While it is unlikely that we will ever completely eradicate inherited eye disease, if breeders will faithfully follow the above steps there will be far less of it.

this article was first published in Double Helix Network News, Spring 1998; revised January 2007.© C.A.Sharp 2007. For reproduction guidelines and limits, go to ASHGI Legal Stuff (link will take you off-site)