INHERITED EYE DISEASE IN THE SHIH TZU
For many years The Northern Counties Shih Tzu Club held an eye clinic at the November Open Show. Peter G.C. Bedford, GDBA Chair of Canine Medicine and Surgery, has written the following:-
The Shih Tzu breed has a remarkably sound eye when it comes to considering inherited diaease. It is true that there is an anatomical predisposition to a low grade pigmentary inflammation of the cornea,keratitis pigmentosa,and corneal ulceration occurs more commonly in all breeds that have a large eyelid aperture and a prominent eye.
These difficulties apart, in the UK we do not see the problems of cataract and retinal degeneration that many other breeds experience. I say “we do not see” because clinical impression is that you have a sound breed in this respect, but sadly not many dogs are examined on the official BVA scheme and real figures for reference do not exist. This is not an unusual state of affairs for the scheme is grossly under-subscribed even in those breeds where there is known inherited disease present. What tends to happen is that once a potentially inherited problem rears its ugly head there is panic within the breed and panic is not a good basis on which to move towards the control of a disease problem. Of course routine annual examination allows the breeder or the breed society to produce a useful database such that new problems do not appear out of the blue and control conditions can be easily instituted. Amongst breeders there should be a general awareness that selection for good things from the gene pool sadly also enhances the selection of undesired features. Close breeding does not actually produce the genetic defect, it merely brings it to the surface and increases its incidence.
The Northern Counties Shih Tzu Club has been running a testing session at its November show for many years and though the number of dogs seen is low there is no evidence of serious intraocular defect in your breed. Odd things are occasionally seen and one defect which does deserve mention is the condition in which the optic nerve is poorly developed. This structure carries nerve impulses from the retina to the brain and if it is defective then sight is seriously affected – or not present. Thus it is a serious defect inherited in the UK in the Miniature and Toy Poodle breeds. There is a worry about its possible prescence as an inherited defect in the Miniature Long-haired Dachshund too. Sadly the defect has been seen in one Shih Tzu – now one swallow never has made a Summer but you should be aware of the finding. It can occur in one or both eyes and it can be difficult to detect unilateral blindness simply from behaviour.
One eye test at any age, but ideally at 6 to 7 weeks before the sale of puppies will pick up this defect. Breeding from clear stock with litter screening of puppies will prevent this defect from becoming established within a breed.
Peter G.C. Bedford, GDBA Chair of Canine Medicine and Surgery
first published in the Spring 2000 Magazine
What is optic nerve hypoplasia?
Optic nerve hypoplasia is an uncommon defect in which the optic nerve fails to develop normally, leading to blindness. One or both eyes may be affected. Micropapilla refers to a smaller than normal optic disc, and is not associated with loss of sight.
How is optic nerve hypoplasia inherited?
The mode of inheritance is unclear.
What breeds are affected by optic nerve hypoplasia?
The condition occurs in the toy, miniature, and standard poodle and in the German shepherd. It occurs sporadically in other breeds as well. (See reference below for a comprehensive list.)
For many breeds and many disorders, the studies to determine the mode of inheritance or the frequency in the breed have not been carried out, or are inconclusive. We have listed breeds for which there is a consensus among those investigating in this field and among veterinary practitioners, that the condition is significant in this breed.
What does optic nerve hypoplasia mean to your dog & you?
Depending on the degree of hypoplasia (underdevelopment), an eye may be partially or completely blind. Usually if only one eye is affected, your dog will compensate for the decreased vision to the point that you are unaware of any abnormality. You may notice that the pupils of your dog’s eyes are different sizes – the pupil of the affected eye will be larger.
If both eyes are affected, your dog will have some loss of sight or will be blind.
How is optic nerve hypoplasia diagnosed?
This condition is suspected when a dog is visually impaired from birth. Based on clinical and ophthalmoscopic examination, your veterinarian will determine if optic nerve hypoplasia is the cause. This condition must be distinguished from micropapilla, a normal variation of optic disc appearance where the disc is smaller but vision is normal.
FOR THE VETERINARIAN:
With unilateral involvement, clinically you will see ipsilateral mydriasis, blindness, and absence of menace reflex and of direct pupillary light reflex, with normal consensual reflex in affected eye following stimulation of normal eye. If both eyes are affected, there will be bilateral mydriasis, no menace response, and reduced or absent pupillary light reflexes.
Ophthalmoscopic examination reveals a variable reduction in optic disc size, with normal or tortuous -appearing retinal vessels. Optic nerve hypoplasia may be difficult to differentiate from micropapilla on a routine ophthalmoscopic screening examination.
How is optic nerve hypoplasia treated?
There is no treatment. With their acute senses of smell and hearing, dogs can manage well despite reduced vision. You can help your visually impaired dog by developing regular routes for exercise, maintaining your dog’s surroundings as consistently as possible, introducing any necessary changes gradually, and being patient.
Affected dogs and close relatives should not be used for breeding.
FOR MORE INFORMATION ABOUT THIS DISORDER, PLEASE SEE YOUR VETERINARIAN.
Ackerman, L. 1999. The genetic connection. p 159-160. AAHA Press. Lakewood, Colorado. This reference contains a comprehensive list of breeds in which optic nerve hypoplasia or micropapilla have been seen. .
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