Inherited eye diseases in Australian Shepherds
by C.A. Sharp
First printed in Double Helix Network News, Spring 1998, Rev. May 2013
Eye diseases and defects are among the most common inherited problems in Australian Shepherds. Even discounting the eye defects that result from merle-to-merle breeding, they are still one of the most likely health issues a breeder will encounter. They are found in both conformation and working lines.
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?
Beginnings
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 which can generically be referred to as “collies.” These breeds share common ancestry with their deep roots in the British Isles. 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 1970s, 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 parents’ firm but “paws and mouths off” attitude toward sheep? Will Bessie bred to Joe’s retriever produce a herder that can double as a bird dog?
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 heterozygosity – a mix of gene versions – within individual dogs, maintained genetic diversity in the breed, 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 gene versions 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 dogs that have inherited two copies of the merle gene. Other eye diseases include, roughly in order of frequency, cataract, distichiasis, progressive rod-cone degeneration (a form of PRA), CEA, iris coloboma, persistent pupilary membrane, cone degeneration, and canine multifocal retinopathy. Hyaloid Arteries, though not recognized as a hereditary issue 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 or 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.
Cataract
Cataracts are the most common eye disease in Aussies. 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. Age of onset for hereditary cataracts varies widely from as early as 18 months into old age.
Most, though not all inherited cataracts in Aussies appear to have be dominant with incomplete penetrance. The mode of inheritance of the other types is not yet known. A mutation in a gene called HSF4 accounts for about 70% of hereditary cataracts in Aussies. Dogs with even one copy of this mutation may develop cataracts sometime during their lives, though a significant number do not. There is a DNA test available for this mutation. The cause of the remaining 30% of hereditary cataracts in the breed has not yet been identified
Due to the extremely variable age of onset, regular annual eye exams of all breeding stock are critical and HSF4 testing should be standard practice. 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. Dogs with the HSF4 mutation should be bred only to clear-tested dogs with preference given to clear offspring to carry on with.
For cataracts not due to HSF4, owners of the affected dog’s parents and 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 cataract. If they should produce cataract-affected offspring, they should be pulled from breeding.
Distichiasis
Dogs with this defect have one or more eyelashes that grow toward the cornea, rather than away from it. In most cases the hair is fine enough it does not cause a problem. Occasionally the inward-growing hair is stiff and can abrade the cornea. This is painful and, if not treated, can damage vision. Surgical correction is available but can be expensive. These abnormal lashes can develop at any time during a dog’s life.
The mode of inheritance is unknown. Minor distichia should be considered faulty and dogs that have them should be bred to mates that do not and which come from families where distichiasis is rare or absent. Seriously affected animals should be removed from breeding.
Progressive Rod Cone Degeneration (PRCD)
PRCD is a form of PRA. Though still rare it has been increasing in frequency, possibly because many breeders are not aware of it and don’t utilize the available DNA test. The disease is recessive in inheritance. Since PRCD is progressive, it may require multiple exams before diagnosis can be confirmed.
PRAs can be misdiagnosed. Any Aussie diagnosed with PRA should also have the DNA test to confirm the diagnosis. Affected dogs should not be bred. Carrier might be bred but only to mates that have tested clear and all offspring will need to be tested. Preference should be given to non-carrier offspring. Carriers should not be used at public stud.
Collie Eye Anomaly
This disease was once at least as common as cataract, if not more so. It reduced dramatically in frequency during the 1990s, probably because the mode of inheritance was known and a scientific paper established the presence of the disease in the breed. All Aussies affected with CEA will exhibit choroidal hypoplasia in both eyes. Some will have optic disc coloboma, and retinal dysplasia or detachment which may occur in either or both eyes. Most affected dogs have functional vision, but some are blind in one or both eyes.
Diagnosis of CEA presents some challenges. Due to the “masked affected” 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 weeks, preferably sooner. Delaying may result in normal eye reports on dogs that are actually affected.
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. A DNA test for this disease is available. Parents of an affected dog are carriers. Neither the parents nor the affected offspring need to have the DNA test unless there are doubts about the accuracy of the diagnosis. Siblings and other close relatives of the affected dog should be tested so their CEA status is known.
If a dog is determined to be a carrier of CEA, either through testing or because it has affected offspring, it should be bred but only to a mates that have been tested clear of the mutation. All offspring should be tested and preference given to the non-carriers. Carriers should not be used at public stud.
Iris Coloboma (IC)
Affected dogs are missing part of the iris. This condition is present at birth. In many cases the effect on 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, which could impact performance or work.
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. Almost all iris colobomas in Aussies were seen in merle dogs, however they will occasionally be found in non-merles. The reason for the high association with merles is not known. It is very possible than non-merles with normal irises might carry the genes but for some undetermined reason rarely express them but may produce them in merle offspring.
Affected animals should not be bred. Unaffected individuals which produce IC repeatedly, particularly with multiple mates, should be pulled from breeding.
Persistent Pupillary Membrane (PPM)
The pupillary membrane covers the pupil prior to birth. It is supposed to be gone by the time a puppy opens its eyes. Sometimes it persists. If the PPM resolves within a few weeks, there is 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. Large PPMs may be referred to as iris sheets. If the PPM attaches only to the iris, it rarely causes any visual problem and will pass an eye exam. 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 which have PPMs attaching to either the lens or cornea. Iris-to-iris PPM should be considered faulty, and affected dogs bred to mates that do not have PPM and do not have a family history of PPM.
Canine Multifocal Retinopathy (CMR)
CMR can be detected by 4 months of age. It causes blister-like defects in the retina and may gradually progress or go away. Diagnosis by exam can be difficult; CMR may be described as retinal dysplasia or retinal folds, both of which are reported in Aussies. In rare instances the disease can impact vision. However, most cases are noted as “breeder option” on exam reports.
CMR is due to a recessive gene mutation and a DNA test is available. Owners of Aussies who have been diagnosed with either retinal dysplasia or retinal folds should consider having their dogs tested. Relatives of diagnosed dogs or dogs which have been tested and found to have the mutation should be examined so their status is known.
Dogs whose vision has been reduced by CMR dog should not be bred. Dogs with CMR and normal vision may be bred but should only to tested-clear mates.
Cone Degeneration (CD)
In 2011 CD was identified in a Miniature American Shepherd (aka Miniature Australian Shepherd). Because of the close relationship between Minis and Aussies and the possibility it might occur in both breeds, owners of Aussies need to be aware of this disease. CD causes day blindness; affected dogs can’t see in bright light but have normal vision when the light level is low. CD can be diagnosed between 8 and 12 weeks of age.
Affected dogs should not be bred. The mutation appears to be recessive. Carriers of one copy should be bred only to mates that have tested clear for CD.
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. Its 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 in the early 1990s 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 which 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. In most cases it comes in the form of denial. People who 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 disease.
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 frequency of these diseases could be reduced if breeders would take action, both 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 (except as noted above.) Take appropriate care with near relatives of affected animals and cease breeding them if they frequently produce the disease.
– If an animal is affected or has produced a particular eye disease, 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 linebreed on the problematic portion of its pedigree. If possible, seek unrelated mates from families known to be clear of the disease.
– If DNA tests are available, make use of them.
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 to trouble our dogs.