The exact cause of a cataract observed in an individual dog is often difficult to determine. Congenital cataracts may or may not be inherited. Cataracts may be primary or secondary. Primary cataracts are the type observed when no other ocular abnormality is present. Secondary cataracts are those that are associated with or accompany another eye disorder, such as generalized Progressive Retinal Atrophy (PRA). Genetic cataracts in dogs may be either inherited in a dominant or recessive mode, or both. Genetic factors are the most important cause of cataracts in the dog. The age of onset, initial appearance, and rate of a cataract's progression are often predictable for a given breed. The specific pathogenesis (mechanism) of inheritable cataracts in dogs has not been defined, but likely results from a variety of different defects in lens cellular metabolism.

Dominant Cataracts: Dominant means that only one of the two genes of a pair must be affected in order for the individual to demonstrate the characteristic controlled by that gene. In a true dominant, only one parent must carry in order for the offspring to be affected. Unfortunately, it is possible to have a dominant with incomplete penetration; i.e., transmission of the trait to offspring will result in presence of the gene but not its expression. It is also possible to be incompletely dominant: the phenotype differs between heterozygote and homozygote with respect to the gene in question.
One type of cataract due to a dominant gene is a posterior polar cataract appearing as an inverted Y or triangle, or irregular circle of opacity, at the pole, and not associated with the hyaloid artery. These cataracts show little tendency to progress until senile changes occur. Other types of cataracts include those occurring in the cortical region of the lens, cortical and polar region, and total. These are usually, but not always, bilateral and symmetrical. Cortical cataracts progress and dogs show defective vision at 1-3 years of age. Congenital cataracts often reveal a posterior capsular deformity. The capsular defect in this condition appeared as a homogenous (hyaline) staining material lacking in continuity.

Breeds in which cataracts have been reported suggestive of a dominant type of inheritance include Golden Retrievers, Labrador Retrievers, Pointers, German Shepherds, and Beagles.

Recessive Cataracts: Recessive mode of inheritance requires both genes to be present in order for a characteristic to be expressed in an individual; i.e., both parents must carry the gene in order for the offspring to be affected. The recessive type cataracts are usually bilateral, symmetrical, and progressive. This type is observed earlier, sometimes as early as a few months of age, and progresses until total and mature at 2-3 years of age. Marked variation in age of onset and mode of progression of cataracts is present, especially in different breeds, and some heritable cataracts don't become apparent until middle age. A cataract appears dense with flecks in the pupillary region, often roughly in the form of the suture lines. Cortical and peripheral opacities are also seen and the most dense part is in the nuclear region of the lens.

Autosomal recessive and Polygenic:
In veterinary ophthalmology, the two most common modes of inheritance of cataracts are autosomal recessive and polygenic. For autosomal recessive traits to be expressed, both parents must pass an affected gene to the offspring, while polygenic traits are those in which many genetic factors interact to determine the expression or lack of expression of a trait. Those animals that have the ability to pass on autosomal recessive traits, although they do not express the traits themselves, are carrier animals. The most frequent type of cataracts in dogs apparently results from a simple autosomal recessive. This type usually occurs at a younger age than does the dominant type and progresses until the dog is completely blind, usually between 2 and 5 years of age.

The incidence or frequency of occurrence of cataracts varies. It is also possible that both dominant and recessive types of inheritance could occur within the same breed. To understand how various ocular disorders are inherited and how their frequency can be reduced requires knowledge of both the age of onset and the mode of inheritance of the disease.

Congenital vs. Non-Congenital Cataracts: Two categories represent the ages at which heritable eye diseases first become apparent: congenital problems (problems that are present at birth) and disorders that occur later in life. Not all congenital eye diseases are heritable; congenital diseases, therefore, do not necessarily indicate defective genes in a line of dogs. Two characteristics typical of many heritable ocular disorders are late onset and the presence of carriers. Many heritable eye problems are not apparent until the animal is middle-aged, between 5 and 9 years old (in Newfs, late occurring cataracts seem to appear between 18 months and 3 years). Consequently, prospective pet owners are unable to detect a problem when obtaining a puppy. For the same reason, it is difficult for dog breeders to choose breed stock that is free from potentially heritable ocular disease.

Occasionally a dog or some of its litter mates have signs of a heritable eye disease, although the sire and dam appear normal. In this situation, the parents are considered carriers of the disease. All of the siblings should be considered carriers as well, unless test matings indicate otherwise. The most prudent and appropriate approach is for those presumed carriers to not be used in a breeding program.

Secondary Cataracts: A secondary cataract, for example those occurring secondary to progressive retinal atrophy, may begin as a cortical type, often posterior with typical appearance of vacuoles (clear spaces) arranged in a wedge-shaped area from the periphery of the lens. These are progressive and ultimately mature to total opacity, thus not allowing examination of the retina for presence of retinopathy (retinal disease).

Summary
Types of inheritance have been reported and described above. Dominant and recessive forms of this condition should be tested for and positive breeding animals should be eliminated.
Heritable ocular disease can be passed to offspring by a genetic carrier who does not manifest the disease or by affected dams and sires that are too young to exhibit the disease. It is best not to use any suspected carriers. This is the most effective way to eliminate a recessive trait from a population. Families of dogs that have a greater occurrence of heritable eye disease tend to have a greater number of disease carriers as well. Animals chosen for breeding, therefore, should be distantly related, or not related at all, to affected animals. Any dog with cataracts for which no underlying cause can be found is considered to have a genetic predisposition for cataract formation; these dogs should not be bred. Depending upon the proposed mechanism of inheritance, various recommendations against breeding relatives of the affected animal may be made.
Breeders should have their breeding stock checked annually by a Diplomat of the American College of Veterinary Ophthalmologists (ACVO). With early detection, affected animals can be eliminated from a breeding program, thus helping to decrease the frequency of the condition in that breed.

Anterior Subcapsular Cataract: 
Implications for the Pet Owner: The condition will not generally interfere with the function of the animal.
Implications for the Breeder: The pattern of inheritance has not been studied, primarily because of the late onset and relatively benign course of the trait. But again, it is most prudent not to breed these animals.

Peripheral Cortical Cataract: 
Implications for the Pet Owner: There is a possibility that affected dogs will develop visual deficiencies, and regular ophthalmoscopic evaluation of the progression of the cataract is indicated. The presence of progressive retinal atrophy must be excluded. Cataract surgery by a veterinary ophthalmologist is an option for those dogs with visual impairment due to cataracts. Blind dogs are generally good pets, if kept indoors and away from dangerous situations.
Implications for the Breeder: Although the genetic characteristics are not known, the concerned breeder should withdraw the affected animal from the breeding program and should make an effort to have parents, siblings and progeny of the affected dogs carefully examined. Control will be difficult given the late onset of this type of cataract.

Article by Cathy Sands.

References
Ophthalmologic Diseases; Disorders of the Lens, pp 1120-1121.
Abnormalities of the Companion Animals: Analysis of Heritability, C W Foley, J F Lasley, G D Osweiler, Iowa State University Press.
Common Congenital and Heritable Ocular Disorders of the Dog, Paul H Scherlie, Jr., DVM, and Stephanie L. Smedes, DVM, Cornell University, Kal Kan Foods, Inc., 1989.
Ocular Disorders Proven or Suspected to be Hereditary in Dogs, American College of Veterinary Ophthalmologists, 1992.
Inherited Eye Diseases in Purebred Dogs, Lionel F Rubin, DVM, University of Pennsylvania, 1989.

Special Thanks to Glenn A Severin, DVM, DACVO and Steven M. Roberts, DVM, DACVO of Colorado State University who have answered endless questions for me.   September 1996

For more information about Canine Cataracts read the article by 
Wendy Shepard Chisholm V.M.D. at: 
amchessieclub.org/aeyedis1.html
Part 2 at: amchessieclub.org/aeyedis2.html

Treatment available for Canine Cataracts:
cvm.uiuc.edu/petcolumns/showarticle.cfm?id=340