Hereditary Cataracts in Leonbergers.
So what is a cataract when the term is applied to the eye? Well you must think of the eye as a camera, a biological camera if you like, and just like a camera the eye contains a film or digital memory stick known as the retina. The retina turns light into a nerve message which is transmitted to the brain and it is in the brain that the message is turned into what we call sight. The retina lines the back of the eye and for it to be exposed to light the front wall of the eye is transparent. This transparent tissue is known as the cornea and once the light has passed through the cornea it then passes through another transparent structure, the lens. Just as the lens in a camera helps focus the light onto the film/memory stick, this lens helps focus light onto the retina.
A cataract is any opacity of the lens – it can be of any size from pinhead to total lens involvement. It may affect one or both eyes and there are many causes of cataract development. For example an internal inflammation of the eye may result in cataract formation and it is the ophthalmologist’s job to define the cause. Sadly there are currently some 21 breeds of dog in the UK in which the cataract seen may be inherited, usually as a recessive trait, and one of those breeds is the Leonberger.
Now, I’ve said there can be many causes for cataract formation, so how do we know your breed has this disease? I’ve also said that it is the job of the ophthalmologist to define the cause on the basis of his clinical examination – so what is he looking for? Well, primarily it’s the pattern of the cataract inside the lens which dictates the diagnosis, but the ophthalmologist also checks the rest of the eye for any other feature which may indicate any other possible cause. The age at which the cataract makes it’s appearance is also significant and in the Leonberger the disease is seen mainly in the young adult. However, it may occur as early as 6 months of age or as late as 8 years of age. It is not sex-linked but currently there is insufficient data to be certain of the mode of inheritance. However, it is very similar to the cataract seen in the Labrador and this we believe to be a recessive trait.
So let’s have a look at your breed. The cataract is seen primarily as a posterior polar, subcapsular opacity, usually in both eyes. That means that occasionally only one lens may be affected. It is also possible that the appearance of the cataract may differ between the two eyes of the same dog in terms of extent and density, but this is the exception rather than the rule. Usually the cataract is confined to the back half of the lens, but occasionally the whole lens can become involved. So it’s the characteristic appearance that makes diagnosis relatively straightforward. What the ophthalmologist sees is a typically triangular shaped whitish/greyish patch at the back of the lens (i.e. posterior), in a central position (i.e. polar) and within the substance of the lens, not on its external surface or capsule (i.e. subcapsular). However, the mildest example can be a small accumulation of whitish specks flecks in the same region of the lens. Sometimes the classical triangular patch is more round or stellate in shape, and there can be opaque extensions into the lens material around this patch. Actual small fluid filled spaces (vacuoles) may also be seen i.e. round the patch.
These cataracts are seen quite readily once the pupil has been dilated (the pupil is the hole in the iris behind which the lens is found). Any cataract is spotted using an ophthalmoscope and its detail is further described using an instrument called a slit lamp. Often a cataract may be seen with the naked eye as it stands out against the background of the coloured part of the retina. The use of the “drops” (a drug which is short acting and harmless) is an essential part of the examination procedure, the drug taking 20 minutes to dilate the pupil fully. Already there are DNA based tests for inherited cataract in other breeds and I expect a test will appear for your breed in the not too distant future. In the meantime it is most essential that you continue testing your stock through the BVA/KC/ISDS scheme – this scheme is based on annual clinical examination and from the affected results you can work out where the carriers are in the pedigree. A one-off DNA test will be really useful when it arrives, but for the moment you must rely on the ophthalmologist and his ophthalmoscope.
Professor Peter G C Bedford
BVetMed,PhD,FRCVS,DVOphthal,DipECVO,ILTM.
Emeritus Professor of Veterinary Ophthalmology
Royal Veterinary College,London University.
A cataract is any opacity of the lens – it can be of any size from pinhead to total lens involvement. It may affect one or both eyes and there are many causes of cataract development. For example an internal inflammation of the eye may result in cataract formation and it is the ophthalmologist’s job to define the cause. Sadly there are currently some 21 breeds of dog in the UK in which the cataract seen may be inherited, usually as a recessive trait, and one of those breeds is the Leonberger.
Now, I’ve said there can be many causes for cataract formation, so how do we know your breed has this disease? I’ve also said that it is the job of the ophthalmologist to define the cause on the basis of his clinical examination – so what is he looking for? Well, primarily it’s the pattern of the cataract inside the lens which dictates the diagnosis, but the ophthalmologist also checks the rest of the eye for any other feature which may indicate any other possible cause. The age at which the cataract makes it’s appearance is also significant and in the Leonberger the disease is seen mainly in the young adult. However, it may occur as early as 6 months of age or as late as 8 years of age. It is not sex-linked but currently there is insufficient data to be certain of the mode of inheritance. However, it is very similar to the cataract seen in the Labrador and this we believe to be a recessive trait.
So let’s have a look at your breed. The cataract is seen primarily as a posterior polar, subcapsular opacity, usually in both eyes. That means that occasionally only one lens may be affected. It is also possible that the appearance of the cataract may differ between the two eyes of the same dog in terms of extent and density, but this is the exception rather than the rule. Usually the cataract is confined to the back half of the lens, but occasionally the whole lens can become involved. So it’s the characteristic appearance that makes diagnosis relatively straightforward. What the ophthalmologist sees is a typically triangular shaped whitish/greyish patch at the back of the lens (i.e. posterior), in a central position (i.e. polar) and within the substance of the lens, not on its external surface or capsule (i.e. subcapsular). However, the mildest example can be a small accumulation of whitish specks flecks in the same region of the lens. Sometimes the classical triangular patch is more round or stellate in shape, and there can be opaque extensions into the lens material around this patch. Actual small fluid filled spaces (vacuoles) may also be seen i.e. round the patch.
These cataracts are seen quite readily once the pupil has been dilated (the pupil is the hole in the iris behind which the lens is found). Any cataract is spotted using an ophthalmoscope and its detail is further described using an instrument called a slit lamp. Often a cataract may be seen with the naked eye as it stands out against the background of the coloured part of the retina. The use of the “drops” (a drug which is short acting and harmless) is an essential part of the examination procedure, the drug taking 20 minutes to dilate the pupil fully. Already there are DNA based tests for inherited cataract in other breeds and I expect a test will appear for your breed in the not too distant future. In the meantime it is most essential that you continue testing your stock through the BVA/KC/ISDS scheme – this scheme is based on annual clinical examination and from the affected results you can work out where the carriers are in the pedigree. A one-off DNA test will be really useful when it arrives, but for the moment you must rely on the ophthalmologist and his ophthalmoscope.
Professor Peter G C Bedford
BVetMed,PhD,FRCVS,DVOphthal,DipECVO,ILTM.
Emeritus Professor of Veterinary Ophthalmology
Royal Veterinary College,London University.
