INTRODUCTION

The spectrum of inherited ocular disease in today's pedigree dog population is a relatively complex one with many breeds and all parts of the eye being involved. In the United Kingdom forty four of the one hundred and sixty seven breeds currently Kennel Club registered are involved in ten primary ocular disease conditions, not including the adnexal problems. Unfortunately accurate incidence figures for some of these disease situations are rarely available due to inadequate breeder subscription to both official and breed club examination schemes. Another forty-two breeds are potentially involved where the incidence of an ocular defect of possible inherited nature is greater than random selection would allow. The overall picture varies on a worldwide basis, to some extent reflecting the variable levels of awareness and concern within individual countries. The avoidance of genetically determined disease should be of high priority in the production of sound dogs of good temperament

Inherited disease may be congenitally present or develop clinically at any age, but the dye is cast at the time of organogenesis. A distinction has to be drawn between the terms "congenital" and "inherited". The former refers to defect which is present clinically at the time of birth and the latter to any defect which is determined genetically. Thus inherited defects may be present congenitally whilst others do not express themselves clinically until early adolescence or even later in life. So it is with cataract, and whilst it is true that cataract surgery has become almost routine in some countries through the introduction of phacoemulsification, the incidence of this disease can be reduced by effective breeding programmes. These rely upon early and accurate diagnosis, and sadly there are complications to both these considerations.

CONGENITAL HEREDITARY CATARACT

Cataract is defined as any opacity of the lens and/or its capsule. Congenital inherited cataract involves the central embryonic and foetal nuclear portion of the lens, whilst those lens fibres which make up the cortex usually remain transparent. Thus congenital nuclear cataract is often described as stationary, with the effect on the dog's sight being dictated by the extent of the opacity. Such patients may be managed by using long acting mydriatic drugs, but when cortical involvement occurs lens removal may prove necessary. The condition occurs as a recessive trait in the Miniature Schnauzer and there are clear indications that it may become established in other breeds like the Old English Sheepdog, Golden Retriever and West Highland White unless breeders respond adequately to early findings in these breeds. Congenital cataract may also accompany microphthalmos in breeds like the Cocker Spaniel, Cavalier King Charles Spaniel, Rottweiler, Rough Collie, Doberman, Old English Sheepdog, Standard Poodle and West Highland White Terrier as part of a multiocular defect. Congenital cataract can involve the lens capsule as a secondary feature of Persistent Hyperplastic Primary Vitreous (PHPV) or Persistent Pupillary Membrane (PPM). Remnant pupillary membrane may render the anterior capsule opaque at the points of attachment whilst varying degrees of posterior capsular opacity and posterior cortical cataract may accompany PHPV.

Diagnosis of congenital cataract is possible from the age of five or six weeks as part of a routine litter screening programme. PPM is best detected before a mydriatic is used, but the use of a mydriatic is essential for the diagnosis of nuclear cataract or PHPV associated cataract.

DEVELOPMENTAL HEREDITARY CATARACT

Opacity of the lens is relatively common in the canine species and several causes ranging from dietary amino acid deficiency in young dogs to senility may be described. Inherited developmental cataract may appear at any age after birth but mainly affects young to middle aged subjects. It is the pattern of opacity coupled with the age of the dog which dictate the diagnosis of hereditary cataract (HC). Currently in the United Kingdom eighteen breeds are affected with HC, and its existence is suspected in another sixteen breeds for which survey work is currently being completed.

The breeds affected and the ages beyond which it is considered that HC is unlikely to occur are listed below:

 Belgian Shepherd Dog 9 years

 Boston Terrier--two forms: early 3 years late 8 years

 Cavalier King Charles Spaniel 7 years

 German Shepherd Dog 3 years

 Irish Red and White Setter 9 years

 Large Munsterlander 9 years

 Leonburger 9 years

 Miniature Schnauzer 3 years

 Norwegian Buhund 5 years

 Old English Sheepdog 3 years

 Poodle (Standard) 18 months

 Retriever (Chesapeake Bay) 3 years

 Retriever (Golden) 9 years

 Retriever (Labrador) 9 years

 Siberian Husky 5 years

 Spaniel (American Cocker) 6 years

 Spaniel (Welsh Springer) 3 years

 Staffordshire Bull Terrier 18 months

The lens enlarges throughout life due to the constant production of new fibres which are laid down around the embryonic and foetal nuclear material. These new fibres make up the lens cortex and it is their abnormal formation due to factors as yet undetermined which is responsible for cataract development. Both dominant and recessive inheritance traits are involved in H.C. and there is considerable variation in the presenting clinical picture and associated prognosis. For example, a 5% incidence of posterior polar cataract in the Labrador and Golden Retriever breeds has to be tempered with the fact that only 5% of these dogs go on to develop a generalised cortical cataract which necessitates surgery. Whilst the young Boston Terrier or Miniature Schnauzer always develop total cortical opacitation bilaterally, the Siberian Husky or the Norwegian Buhund develop cataract which seldom involves anterior cortical material. The American Cocker Spaniel may present with bilateral blindness or there may be a unilateral involvement in which the cataract is restricted to a very small part of the lens. It is these characteristics which are so very important in the differential diagnosis of cataract type. Trauma, prior uveitis, nutritional and age-related changes represent the most important differentials, but pattern and the nature of change are such that diagnostic confusion is only occasionally encountered. The late age at which HC can occur in a breed like the Golden Retriever might beg an age-related change as the cause, but the involvement of only the posterior polar cortex is not an age change. Often the HC of the American Cocker Spaniel is accompanied by lens induced uveitis and here lies the bones of a possible chicken and egg situation. Established incidence helps with this possible dilemma, but occasionally opacities are seen which fit no known pattern of inheritance.

Of course cataract surgery has become almost routine due to the introduction of phacoemulsification, so potential restoration of sight is always a possibility. However in general cataract surgery is only attempted when the eye is rendered functionally blind or there is clear evidence that the developing opacity will involve all or most of the lens cortex. Fortunately surgery can offer the individual a chance of useful vision but the control of H.C. needs to be approached as a breed problem.

CONCLUSION

Breed incidence, age spectrum and pattern of cataract formation are the essential features of a diagnosis of hereditary cataract. Despite possible surgical solution breeding programmes should be exercised to reduce blindness caused by hereditary cataract.