The American College of Veterinary Ophthalmologists (ACVO) classifies cataracts by where they form in the lens. The lens has three layers of tissue: The nucleus at its core, an outer layer called the cortex, and a skin referred to as the capsule. The front and back are termed anterior and posterior, respectively. The outer rim of the lens is called the equator. Each lens also has anterior and posterior suture lines where the tissue came together during development.
Since not every cataract is inherited, ACVO classifies those that are likely due to environmental causes—generally injury or other diseases—as “significance unknown.” Small cataracts are referred to as “punctate.” These may or may not grow and those that do not aren’t considered significant. Therefore, a dog with punctate cataracts will pass an exam, but if the cataracts advanced to either the intermediate or diffuse type at a subsequent exam it will not.
Cataracts may be in one eye (unilateral) or both (bilateral) and the later are considered the most likely to be inherited. Cataracts may start in one eye before the other, so a unilateral cataract may be discovered to be bilateral on a subsequent exam. Of the types of bilateral cataracts that will not pass CERF exams, posterior cataracts are anywhere from 4 to 10 times more likely to be found in Aussies than any other bilateral cataract. The HSF4 mutation identified by AHT is significantly associated with bilateral posterior cataracts.
A review of Canine Eye Research Foundation statistics for the breed from 1999 – 2006 indicated that approximately .78%, or roughly one Aussie in 130, has this type of cataract. None of the other types of bilateral cataract (anterior, equatorial cortical, capsular, or nuclear) occur even half as frequently. The second most frequent type is bilateral nuclear cataracts, which occur in only in 1 of 550 dogs. The other types all occur in less than 1 in 1000 dogs.