A cataract is any opacity in the lens of the eye or the lens capsule. The cause, size, location and severity of a cataract may vary. They can be caused by trauma, inflammation, diabetes or other metabolic disease, retinal atrophy, persistent pupillary membranes, hyaloid remnants, nutritional deficiencies, congenital abnormalities, or heredity.

Breeders are most concerned about hereditary cataracts. Any cataract that occurs prior to seven years of age and cannot reliably be ascribed to one of the above abnormalities is considered to be genetic. That means that the sire and/or dam of the litter may have carried a gene that predisposed its offspring to developing cataracts. In an attempt to decrease the prevalence of cataracts in future litters, breeders may have their breeding stock examined by an ophthalmologist, and choose not to breed sires and dams that have cataracts.

Dogs that are found to be clear of hereditary eye disease can then be listed with the Canine Eye Registration Foundation, commonly known as "CERF". Dogs can be CERF'd as young as 2 months of age; it is recommended that they be re-examined by an ophthalmologist every year they are used as breeding animals, or until the age of seven.

The location of a cataract may assist in determining its potential severity. However, the prognosis of a cataract in a particular location may vary from breed to breed. For instance, a posterior cortical cataract in a papillon is considered "mild", while a posterior cortical cataract in a pomeranian or poodle often grows in size until it impairs vision. Therefore, abnormalities found on an individual eye exam should be discussed with the ophthalmologist.

Some cataracts cause inflammation in the eye, which may lead to glaucoma or retinal detachment. Some cataracts cause blindness. In these cases, surgery under general anesthesia is often recommended. The surgery to remove cataracts is called phacoemulsification, a process where the cataract is liquefied by ultrasonic waves so that it can be removed by aspiration. A plastic lens may then be placed in the dog's eye in order to improve vision.

Cataracts were found in about 15% of the Japanese Chin examined by an ophthalmologist over the eight year period prior to the year 2000. More breeders are now screening their stock for hereditary eye disease, so we hope to see a decrease in the following decade.