Understanding the ACVO eye sceening program
by C.A. Sharp
First published in the Australian Shepherd Journal, Sep-Oct 2002, Rev. May 2013
Every serious breeder of Australian Shepherds soon learns that inherited eye disease is a major concern. We learn that eyes need to be checked not once, but many times throughout a dog’s life. We know something about what diseases we need to watch out for. But sometimes the process is confusing. How do the vets decide what passes and what doesn’t? How come if it’s OK in another breed it isn’t in Aussies or vice versa?
The American College of Veterinary Ophthalmologists (ACVO) sets the criteria for examination. ACVO certifies veterinary ophthalmologists in North America. A vet who is not board certified should not be relied upon for screening exams or specialized treatment of eye disease. ACVO has a genetics committee which keeps screening criteria up-to-date with current knowledge of canine inherited eye disease.
The ACVO maintains a list of diseases known or suspected to be inherited in dogs. Though we commonly refer to getting “eye certification,” ACVO states that their diplomates (vets who have passed the board exam) do not “certify” or “register” dogs, but give breeding advice. Registration of the findings of cooperating ACVO vets and the issuance of certificates in North America is the domain of the Canine Eye Research Foundation (CERF) or the Orthopedic Foundation for Animals (OFA).
ACVO’s breeding advice is based on either or both of two criteria: Severity of the condition and strong evidence that it is inherited. If a dog has a condition that meets these criteria, ACVO vets will advise the owner not to breed it. Something that may be inherited but doesn’t cause significant reduction in the dog’s vision may be classed as “Breeder Option,” meaning the breeder is put on notice that this condition is or may be inherited but it is her call as to whether to breed the dog or not. Intermediate or diffuse cataracts would be an example of a “No Breeding” condition while punctate cataracts fall under “Breeder Option.” (This specific situation will be discussed in greater detail later.)
The following defects will result in “No Breeding” advice in any breed of dog:
- Cataracts (intermediate or diffuse, with some exceptions)
- Lens luxation/subluxation
- Optic nerve coloboma
- Retinal detachment
- Retinal dysplasia (geographic or detached)
- Persistent hyperplastic primary vitreous
- Progressive Retinal Atrophy (PRA)
Some but not all of these are a concern for Australian Shepherd breeders. We certainly have cataracts. Lens luxation or subluxation is not uncommon in homozygous (double) merles, but very rare otherwise. Optic nerve coloboma is one of the CEA defects, as is retinal detachment. Retinal dysplasia and persistent hyperplastic primary vitreous are very rare in the breed and most breeders will never encounter them or know of an Aussie breeder who did. Glaucoma is also quite rare, but it can be secondary to some autoimmune problems which are not. The possibility of autoimmune disease should be investigated in any Aussie that is diagnosed with glaucoma. The Progressive Rod Cone Degeneration (prcd) form of PRA is present in Aussies.
ACVO’s genetics committee has developed a list of inherited disorders specifically for Australian Shepherds, as well as for many other breeds. Several dozen additional breeds do not have such lists due to lack of data, in many cases because the breed is rare. In order to get on a breed’s specific list the disease must meet several criteria:
- Published research indicating inheritance
- A minimum of 1% of the breed examined annually is affected
AND at least 5 animals were reported as affected in a 5-year period,
OR 50 affected dogs in any 5-year period regardless of total number examined
- Breed club request (subject to ACVO review and agreement)
- “Overwhelming” consensus among ACVO genetics committee members
1% of the dogs examined in a breed may sound like an awfully small number, but if the disease is caused by a recessive gene almost a fifth of the dogs in the breed may be carriers. If inheritance is complex, even more dogs might carry genes that could produce the disease if they happen to be mated with a dog that can “fill in the blanks” to produce affected pups.
Before a disease is listed by ACVO, they determine whether the disorder is or is probably inherited, though the mode of inheritance may remain unknown. ACVO considers not only published research, but whether the frequency in a breed is on the increase or is greater than seen in other breeds as well as whether it is following family lines. If the appearance or location of a defect is characteristic for a breed or if it looks identical to something known to be inherited in another breed, this indicates that it is inherited. A disorder that tends to occur at a particular age and progress in a particular manner is also likely to be inherited.
There are eight defects that occur in Aussies which have met ACVO’s criteria for breed specific listing as of May 2013:
- Microphthalmia with multiple ocular defects (homozygous merle)
- Distichiasis *
- Corneal dystrophy *
- Iris coloboma
- Iris hypoplasia *
- Iris-to-iris *
- All others
- Persistent hyaloid artery *
- Regional dysplasia – folds *
- Generalized retinal atrophy (PRA)
- Choroidal hypoplasia and/or coloboma and/or retinal detachment (CEA)
- Coloboma/staphloma without retinal detachment (may be CEA or homozygous merle eye)
* Breeder option
Except as noted above, ACVO recommends against breeding dogs with these defects. The exceptions usually (but not always) cause little effect on vision. All of the disorders qualify for specific breed listing on the basis of the frequency with which they are reported in the breed.
So why might a dog that has something wrong with its eyes pass its ACVO exam? The defect may be something that is not on either the all-breed or specific breed list. If something is very rarely seen in Aussies or in dogs as a whole, the examiner may note it just for reference, but it will pass. Some conditions, like PPM, may be present in small puppies but absent when the puppy is older. These will pass in a baby’s exam but the examiner will recommend a recheck and it may not pass if found later.
Cataracts are a particularly thorny problem in Aussies. Knowledgeable people know they occur frequently and can be difficult to breed away from, so how come some dogs with cataracts still pass their eye exams?
A cataract is an opacity in the lens, the bit of clear, hard tissue positioned behind the opening of the pupil. The lens focuses the visual image on the retina. Anything that affects its clarity will affect vision. But cataracts can be caused by a number of things that aren’t hereditary, including eye trauma, old age and other diseases. How the cataract looks to the examiner, and where it is positioned in the lens can indicate whether it is or is not inherited. For example, years ago a commonly used puppy milk replacer lacked an important trace element. This caused opacities (small cataracts) in the eyes of dogs that ate it. These cataracts had a distinctive appearance, so once the problem was identified the vets knew it was not a hereditary problem and dogs that had it would pass.
In Aussies, our hereditary cataract has a typical location and progression, though the speed of progression and the age of onset are quite variable. To explain this, we need to take a brief digression into lens anatomy. The lens consists of a core, called the nucleus, surrounded by an outer layer called the cortex, coated with a thin layer of cells called the capsule. It is round, with convex anterior (front) and posterior (back) surfaces. The round edge may be referred to as the equator. A cataract can occur in any part of the lens.
The inherited cataracts in Aussies show up in the posterior cortex. They will be bilateral (both eyes) though the two eyes may not start to show at the same time. The cataract will progress (get bigger) over time. Progression is from punctate to intermediate to diffuse. These stages are listed as such on the exam form and refer to the size of the cataract. Not all stages may be noted if the dog is not regularly presented for exam or the cataract develops quickly.
If an Aussie has cataracts in a part of the eye that is atypical for our breed or for inherited cataracts generally, the examiner may pass it, usually with a recommendation for re-exam. If the appearance of the cataract is typical of a non-inherited cause, as in the case of the milk replacer opacities mentioned above, it will pass. Old age cataracts (nuclear sclerosis) will pass. If the cataract is only in one eye, it may pass. If the dog has only punctate cataract in one or both eyes, it will pass with recommendation for recheck. In this last case, small opacities may not be inherited but if they are bigger on a subsequent exam they should be assumed to be inherited.
Breeders and owners can find themselves frustrated by varied opinions among vets. There are a number of reasons this can occur. Depending on the problem, the eye may actually have changed especially if some time had passed between exams. If the dog has been imported or exported, there may be differences in standards or terminology from one country to another that contributed to the different findings. If what the dog has does not clearly fit into an expected appearance or pattern there may be an honest difference in expert opinion about what is going on. And, even though veterinary ophthalmologists are highly trained and skilled professionals they are human; they will occasionally make a mistake.
The diagnosis of iris coloboma can in some circumstances be prone to error. A small IC may go unnoticed if the eye is fully dilated prior to exam. ACVO has advised its diplomates to examine Australian Shepherds for IC prior to dilation. Most breeders take advantage of eye clinics for the financial savings they offer, but a clinic by necessity must streamline the process and eyes are typically dilated before the dog sees the vet. The person doing check-in may not be aware of this special provision for Aussies, so ask to have the dog looked at prior to dilation. Since this will cause a disruption of the clinic’s flow you should not object if you have to wait for your request to be accommodated.
The exam form does not list diseases; it notes the actual defect observed, the name of which may be different. Distichiasis and iris coloboma are listed as such under “Eyelids” and “Uvea” respectively. The varied listings for cataract under “Lens” were discussed previously.
PPM also appears under “Uvea” but it is subdivided into four types: Iris-to-iris, iris-to-lens, iris-to-cornea and iris sheets. PPM never causes the dog pain or discomfort and the iris-to-iris form rarely if ever causes visual problems. However iris-to-lens and iris-to-cornea may cause opacities in either the lens or the cornea which can be blinding. An iris sheet may obscure enough of the pupil to result in vision loss. Even though ACVO’s advice for PPM is “Breeder Option,” it is the author’s opinion that Aussies with PPM past the age of 6 months ought not to be bred. Ours is a numerous breed. There is no need to use a PPM affected individual when careful breeding of unaffected relatives will preserve the genes for desirable traits while reducing risk of producing further cases of PPM.
Collie Eye Anomaly is nowhere to be found on the eye exam sheet because the disease causes several defects that vary from one affected dog to another. It is these defects that will be marked on the exam sheet under “Fundus.” Choroidal hypoplasia is the most common CEA lesion. All affected dogs will have it. They may also have optic nerve coloboma or retinal detachment. The lesions may differ from one eye to the other, but both eyes will be affected. CEA is present at birth but the choroidal hypoplasia may be masked by pigment as the puppy ages (the “go normal” phenomenon.) It is important that every Aussie puppy be checked while very young, certainly no later than 10 weeks and preferably earlier, whether it is intended for breeding or not.
Reference to “homozygous merle” is also nowhere to be found on the eye exam form because it is not a specific eye defect. For the most part, people know if a dog is a homozygous merle without need to resort to an eye exam: It has two merle parents, is mostly white in color and almost always shows signs of not being able to see or hear. But a few of these dogs will have what we consider “normal” merle coloration and eye and hearing defects are not always severe. Eye examination may be what reveals the status of such dogs. Virtually all homozygous merles have abnormally small eyes and/or a plethora of other defects. The small eyes are listed under “Globe” on the eye exam form as microphthalmia. These dogs have multiple defects in both eyes, exhibiting problems listed on the form under Uvea, Lens, Vitreous and Fundus. The complex of defects will have a typical appearance, so a vet experienced in examining merle breeds will usually recognize the source of the problem.
Understanding the process of eye screening, the criteria used by the veterinary ophthalmologist in making a diagnosis and how this may be reflected on an eye exam form or by a health registry listing eye results is vital knowledge for a breeder. With it you can make better breeding decisions for yourself, your dogs and their eyes.