Most cases of cataract in dogs are of an inherited form. This disease causes cloudiness in the lens of the eye. This cloudiness may be located in the centre of the lens, or towards the front or the back of the lens. Inherited cataract is almost always bilateral (that is, in both eyes).
The disease can become apparent over a wide range of ages, ranging from when the puppy first opens its eyes to around 6-8 years of age. Cataracts that develop at or around birth are termed congenital cataract. Those that develop in dogs under 2 years of age are called juvenile cataract, while those developing in dogs between 2-6 years are termed adult onset cataract. Those that develop in dogs older than 8 years are generally not of an inherited nature, and may sometimes be due to other disease (e.g. diabetes mellitus).
Cataract may also progress (get worse) at varying rates, resulting in initial blurred vision which may often progress to complete blindness. Some cataracts may progress very rapidly. It also may not progress much at all, and this is termed static cataract. Congenital cataracts may be static in nature or they may progress. They may be inherited, or may be secondary to other inherited defects (e.g. persistent pupillary membrane or persistent hyaloid artery) or secondary to in utero infections, toxicities or other foetal trauma.
Cataract is diagnosed by eye exam once it is present in the lens, and by ruling out other causes. There are DNA tests available for the inherited form of cataract in some breeds. Most cataracts can be treated surgically, and the earlier this is performed the better the prognosis is, and the less chance there is for complications. An intraocular replacement lens is often placed, which helps improve post-surgical vision.
Breeding programs in breeds where cataract is a major concern should involve ensuring parents are clear by screening. Most areas will have a recognised registration program for inherited eye diseases, which is strongly recommended for breeders to participate in. In Australia, the Australian Veterinary Association runs the Australian Canine Eye Scheme (ACES), while breeders in the USA can certify their dogs via the Canine Eye Registration Foundation (CERF).
Cataract can also be associated with other diseases, such as diabetes mellitus, and hypocalcaemia, and also due to some toxins, including internally produced toxins (e.g. those produced due to retinal degeneration) as well as external toxins. Cataract associated with diabetes mellitus often progresses very rapidly.
Cataract should not be confused with the normal aging change of the lens of the eye called sclerosis – this is often visible as a white cloudiness in older dogs’ eyes. Often this can be confused with cataract by dog owners, but sclerosis of the lens does NOT cause loss of vision.