Information for Dog Breeders and Owners
Eye Certification for Purebred Dogs
Animal Eye Services provides eye certification for pure breed dogs. A thorough examination of the eye is performed, usually after drops to dilate the pupil have been applied. A report is provided outlining any abnormalities found and their significance with respect to effects on vision and their heritability.
Why have eye certification?
Eye certification is important for breeders as it helps to prevent the breeding of purebred dogs which have known genetic eye diseases. Some of these diseases can be both blinding and/or painful to the dog.
Eye certification has been offered for some time throughout the world for breeders of purebred dogs. There are various national eye certification schemes in existence in USA (Canine Eye registry Foundation) in the UK, Europe and Scandinavia.
Dr Bernays is a panellist for the Australian Canine Eye Scheme (ACES), a certification scheme modelled on schemes in Europe and North America. ACES was started in Australia to standardise the recognition and reporting of hereditary eye diseases in purebred dogs. To learn more about ACES, visit the ACES information page on the Australian Veterinary Association website.
Procedure for eye certification
Please turn up approximately 20 minutes before your appointment so that drops to dilate the pupil can be given. Pupil dilation is important so that the structures in the back of the eye can be examined carefully.
Please make sure your dog has a microchip inserted for positive identification and bring your kennel club registration papers when presenting for eye certification.
Genetic vs Congenital Disease
What is a genetic disease?
Genetic disease (= “heritable”, “inherited”) is a disease whose underlying basis is due to action of a gene or several genes acting to produce the defect. Genes in general are the biological code for the production of different types of proteins. Some proteins such as collagen are structural proteins while others are enzymes which facilitate various chemical reactions in the body. Very few eye diseases which occur in dogs have been identified as definitely having a genetic cause.
In general most of the retinal degenerations seen in dogs are now known as having a genetic cause. The defective gene codes for an abnormal variant of an enzyme involved in the complicated cascade which converts the energy of light into electrical impulses. The end result of the defective gene is that the light receptor cell dies and the retina degenerates.
Many cataracts are also suspected to have an underlying genetic basis but this is not yet proven. There are a number of well known genetic cataracts in laboratory animals such as rats where the outcome of breeding specific individuals together is highly predictable. Our approach is to assume a cataract to be hereditary unless another specific cause is identified eg trauma, unrelated intraocular inflammation or specific metabolic diseases such as diabetes.
Other diseases such as entropion are more difficult to describe as being truly genetic (i.e. the action of specified genes) but the abnormality usually results from the breeding together of animals with poor conformation where this poor conformation predisposes to the eye disease. Eye certification can still identify poor conformational types which when bred together might result in the abnormality. The heritability of this poor conformation may be very complex involving the action of many different genes often acting in different ways to produce the final result. Some of these diseases where we are unable to put the title “genetic” to them are still described as having a “breed predisposition” i.e. the disease is seen at higher incidence in the breed than the general population at large.
What is a congenital disease?
Congenital disease means that the problem is present from birth. Congenital diseases may or may not be genetic. Where the disease is not genetic it may be the result of an insult to the embryo or foetus which occurred at a very specific time during in utero development. Had the insult occurred a few days either side of this time then the defect might not have occurred.
Phenotype vs Genotype
Phenotype refers to the outward appearance of the structure. Phenotype may be the result of either genes or effects of the environment.
Genotype refers to the actual genes acting on that structure.
The most simple way in which genes operate is on a dominant-recessive basis. Usually the dominant gene codes for the normal condition while the recessive gene is regarded as a variant which codes for the abnormal condition. The genes are found in the cell nucleus on structures called chromosomes which occur in matching pairs. Pairs of genes have a very well defined location on each of a specific pair of chromosomes. This position on the chromosome is known as a locus. In the classic situation the locus can have either a dominant or a recessive gene. Where two of the same type of gene i.e. dominant or recessive are found on matching chromosomes the animal is said to be homozygous for that gene. Where the two different variants i.e. a dominant and a recessive gene, are found at the locus on each of the chromosome pair then the animal is said to be a heterozygote. Heterozygotes are sometimes called “carriers” i.e. they carry the faulty gene on one chromosome but because there is a dominant gene on the other chromosome the normal phenotype is expressed. The abnormal phenotype is only seen when the animal is homozygous recessive for the condition. (i.e. has two recessive genes on each of the pair of chromosomes). Occasionally the defect is suspected to be due to the effect of the dominant gene rather than the recessive gene in this case only breeding homozygous recessives together will prevent the abnormal phenotype from occurring.
There has been an explosion in our understanding of the molecular basis for many genetic diseases in man in the last 10-20 years. This explosion is now starting to occur in the canine world especially in our knowledge of genetic retinal diseases in dogs. Many retinal degenerations which only become apparent in middle age in susceptible breeds can now be identified by DNA testing at the puppy stage. In many of the diseases carriers can also be identified as well so this should make things easier in future. Unfortunately many ocular abnormalities still do not have an identifiable genetic cause. Therefore eye certification (i.e. examination by a qualified veterinary ophthalmologist) is still the only way we have at present to potentially identify breeding stock with a given abnormality and make decisions about whether they be bred or not, assuming the problem might be caused by a genetic defect. The effect of the abnormality on vision is probably the most important factor which is taken into consideration when a recommendation is made as to whether the animal should be prevented from breeding.
In the rest of this section are some of the more common abnormalities of the eyes, with some general remarks about their cause and possible heritability. These abnormalities are covered in more detail in Information on Eye Diseases.
ProgressiveRetinal Atrophy (PRA)
Progressive Retinal Atrophy (PRA) is by definition a genetic condition resulting in degeneration of the retina and blindness in affected dogs. There are many different types of PRA with different modes of inheritance and different ages of onset but the most common type is seen in middle aged dogs of affected breeds and is due to a recessive gene. Classic breeds for PRA include the Miniature and Toy Poodles, Cocker Spaniels, Labrador Retrievers, Golden Retrievers and the Australian Cattle Dog. A number of different genes have been identified to date, the most common is the prcd or progressive rod-cone dysplasia gene. The first sign observed is usually night blindness with poor visual function in low light conditions progressing variably over months to years to complete blindness. There is no treatment.
Traditionally there was no way to diagnose this condition until the dog developed the condition in middle age by which time it was probably towards the end of its breeding life. This made the culling of affected breeding stock difficult with resultant continuing propogation of the gene. There has been an explosion in the development of DNA blood tests in the last 10 years meaning now that in many breeds we can identify affected individuals and carriers at an earlier age. In some breeds such as the miniature and toy poodle and the miniature schnauzer there is more than one gene suspected as a cause of the condition and there may only be a DNA test for one of the genes.
Cataracts
A cataract is an opacity or cloudiness of the lens. The lens is found inside the eye behind the iris and in front of the retina. It is ovoid in cross section and was developed in evolution as a means of bringing into focus on the retina, structures close to the eye. In younger animals the lens has the ability to change shape to accommodate so that the eye can focus on objects at near or far distance.
Many causes of cataracts have been identified. These include genetic, nutritional, metabolic, traumatic and post-inflammatory. Drug induced cataracts have also been seen in laboratory animals and are suspected to occur in other species as well. Steroid (“cortisone”) induced cataracts are seen in man but have not been suspected in dogs.
Not all cataracts are complete and in many cases only part of the lens is involved. In many of these cases the change may not be progressive over time, however if the cataract matures the eye will become blind without cataract surgery to restore vision.
Juvenile cataracts are first seen while the dog is young and are seen in a number of breeds. These cataracts usually mature early and cause blindness. This type of cataract is strongly suspected to be heritable.
Another type of cataract seen in young dogs is the
nuclear cataract: this type of lens is seen in the nucleus or the centre of the lens which is the earliest part of the lens to form embryonically. These cataracts usually reflect an insult during embryonic development and may not necessarily be heritable. They are often accompanied by other ocular abnormalities such as microphthalmos (undersized eye) and retinal dysplasia (abnormal structural development of the retina).
Entropion
Entropion refers to a disorder where the eyelid margin rolls in and irritates the surface of the eye. Perhaps the most common type of entropion seen involves the lateral lower eyelid or lateral canthus (where the upper and lower eyelids meet at the outer edge of the eye).
Entropion can occur because of excess skin around the eyes or where the eyelid margin is too large for the size of the eyeball. Another factor which has also been suspected is inappropriate tension in the ligament at the lateral canthus resulting in pulling in of the eyelid at the canthus. While entropion does not generally cause blindness, it results in considerable discomfort for the dog and needs to be surgically corrected often at great cost to the owner.
Distichiasis
The term distichiasis refers to abnormal hairs which arise from the eyelid margin. Normally the eyelid margin is identified by small openings of ducts leading from glands deep within the eyelid. These meibomian glands are responsible for producing lipid for the surface of the tear film and are modified sweat glands. Normally they do not contain hair producing cells, but occasionally something goes wrong in embryonic development and hairs grow from these glands. The hairs are not always but usually irritating to the surface of the eye because they stimulate pain receptors in the surface of the cornea. A common sign of distichiasis is a watery discharging eye.
In some dogs such as the Staffordshire Bull Terrier the hairs can be very stiff and the discomfort they produce can cause continuing spasm of the eyelids and eventually entropion secondary to the chronic spasm. In other breeds, such as the American Cocker Spaniel the hairs might be quite fine and will not necessarily cause significant irritation but might still stimulate excessive tearing.
Persistent Pupillary Membrane (PPM)
During the development of the eye, the front or anterior chamber of the eye is filled with a primitive embryonic tissue called the pupillary membrane. As this tissue disappears the pupil forms. Normally the pupillary membrane totally disappears but occasionally remnants of the structure remain in the eye as strands bridging the iris, or strands which travel from the iris across to the lens or cornea.
In some breeds such as the Basenji or the English Mastiff they are known to be heritable. The heritability is not known in most other breeds but the condition rarely causes vision problems unless the strands cause extensive opacification of the lens or cornea. Usually the recommendation is that the breeder can decide if he or she will or will not breed dogs with PPMs (ie breeder option).
Persistent Hyperplastic Vitreous
The vitreous is the clear jelly like mass in the posterior chamber of the eye which fills the space between the lens and the retina. The vitreous goes through three phases in its development with formation of the primary, secondary and tertiary vitreous respectively.
The primary vitreous forms as a large blood vessel which starts from the back of the eye and grows forward towards the lens. At the back of the lens this vessel, the hyaloid artery divides many times to form a network of blood vessels around the lens, known as the tunica vasculosa lentis. These blood vessels subsequently disappear before birth. Sometimes these blood vessels fail to degenerate, remaining in the eye after birth to form a large vascular plaque over the back surface of the lens. This condition is known as a
persistent hyperplastic primary vitreous.
This disease is suspected to be heritable at least in the English Staffordshire and the Doberman in which the condition was first described. It can cause vision problems but most cases are operable to restore vision. Since this is a vascular structure there is some risk however of haemorrhage within the eye during surgery.
Genetic Testing for Eye Diseases in Dogs
There has been an explosion in our understanding of the gene mutations which cause inherited eye disease in dogs over the last 10 years, principally as a result of the work of researchers at the Baker Institute for Animal Health at Cornell University in the USA and more recently the Animal Health Trust and Cambridge University in the UK.
These DNA mutations can now be tested for by Optigen, the commercial arm of the Baker Institute and the Animal Health Trust and tests provided by both these institutions can now be performed by a Melbourne based company Genetic Technologies Ltd to remove the need to send samples overseas.
