Inheritance of Cataracts in Dogs
By Leos Kral
When I performed the literature search for articles dealing with research on the inheritance of cataracts in dogs, I was surprised by how little is actually known. There are only about a dozen articles which directly address the mode of inheritance and in many cases there is just not enough data on which a definitive statement can be based. I was also surprised that there are no recent studies that address this issue. Most of the papers were published in the 70's and only two were published in the early 80's. This is especially surprising given the tenuous nature of what is actually known.
This article is divided into three parts. The first part summarizes reports by breed in which the inheritance is thought to be dominant. The second part summarizes reports by breed in which the inheritance is thought to be recessive. The third part presents what I have been able to gather so far regarding the inheritance of cataracts in the Australian Shepherd.
Please note that while certain types of cataracts will be described below for different breeds, it does not mean that those specific types of cataracts are necessarily the only ones that affect that breed. It simply means that those were the types of cataracts studied.
The question mark in the sub-heading above indicates that the data presented in the cited articles is not conclusive.
The study summarized here  involves the inheritance of posterior cortical cataracts. While in most dogs these are partial cataracts that involve the Y sutures, in some dogs they are complete. Interestingly, this variation can be seen in the same family of dogs. For example, in one pedigree, the mating of two dogs with partial cataracts produced offspring most of whom had partial cataracts and one offspring who had complete cataracts. Given the data, the most likely conclusion is that inheritance is autosomal dominant. However, the dataset of only 3 one-generation pedigrees is fairly small. Based on one of the pedigrees the author suggests that, perhaps, heterozygotes have partial cataracts and homozygotes have complete cataracts. Unfortunately, this is only a hypothesis that would need to be tested, not a proven fact that should be acted upon.
The authors of this study  explored the inheritance of cataracts which were variable in appearance but mainly affecting the posterior cortical region. About a third of the dogs had posterior cataracts, about a third had opacities toward the equatorial regions, and about a third had a combination of these types. The authors presented a multigenerational pedigree of cataract inheritance and concluded that it did not identify the mode of inheritance but guessed that it may be dominant with incomplete penetrance. While the pedigree shows inheritance of cataracts over several consecutive generations (a likely scenario if the trait is autosomal dominant provided cataract allele frequency is low and dogs from different lines are bred), the phenotype of most siblings and parents was not determined and thus very little confidence is placed in the conclusion.
American Cocker Spaniel
I have not yet obtained the original article but in a review article by K. C. Barnett  it is mentioned that the mode of inheritance is thought to be dominant. The phenotypic expression of cataracts in this breed is very variable.
While I would not bet a substantial amount of money, the data in many of the articles cited below is a bit more convincing that the data in the articles cited above.
Old English Sheepdog
The author  presents a fairly extensive multigenerational pedigree. While there are certain types of matings one would want to see to test the hypothesis, the data presented seem to indicate a recessive mode of inheritance. Some of the cataracts observed were congenital and some were adult onset. Some were cortical, some were nuclear and some were a combination of both types. It would be necessary to follow up on some of these types individually to determine if these are due to the same gene or different genes. The fact that retinal detachment was also seen in the pedigree is troubling because it would have to be determined if that is an unrelated trait or if it perhaps influences some types of cataract.
West Highland White Terrier
The cataracts cited in this Swedish study  were posterior cortical cataracts most of which involved the Y sutures and some of which were complete. While the multigeneration pedigree is not as complete as one would like, it has many characteristics of autosomal recessive inheritance. However, a dominant mode of inheritance can not be absolutely ruled out if the trait is not completely penetrant. Interestingly, individuals who have only partial cataracts can produce offspring with complete cataracts.
Welsh Springer Spaniel
The cataracts described in this breed  have very early onset (possibly congenital), are progressive, and completely obliterate vision by 18 months. The three generation pedigree presented is consistent with an autosomal recessive mode of inheritance. Unfortunately, the presumed homozygous affected to homozygous affected mating produced only one surviving offspring with cataracts so the conclusion is not ironclad.
studies, one carried out in the
I have not yet obtained the original article but in a review article by K. C. Barnett  it is mentioned that the mode of inheritance has been shown to be autosomal recessive by test breeding. These are posterior cortical cataracts.
I have not yet obtained the original article but in a review article by K. C. Barnett  it is mentioned that the mode of inheritance has been shown to be autosomal recessive by test breeding. These begin as equatorial cortical cataracts and progress to occlude both the anterior and posterior cortex.
Inheritance of Cataracts in the Australian Shepherd
There are no reported studies in the scientific literature of Australian Shepherd cataracts. CERF records over the last 7 years show that each year an average of 135 Aussies are diagnosed with some form of cataracts. That is about 5% to 6% (or 1 in every 20) of all Aussies that are examined. The most common form of cataracts is the posterior cortical cataract. Specifically, 52% of diagnosed cataracts are posterior cataracts, 26% are anterior cataracts, 14% are nuclear cataracts, and 9% are equatorial cataracts.
Over the past several months I have been examining pedigrees and CERF forms of affected Aussies. Since most of the pedigrees just show me the ancestry of affected individuals, it is impossible to use them to deduce the mode of inheritance (see Importance of Complete Pedigrees). However, almost all involve individuals with posterior cortical cataracts - the most common type in Aussies. Interestingly, these are quite variable in phenotypic expression. Some are partial and triangular (Y suture involvement), some are just punctate, and a small fraction completely obliterate the lens. For the most part these cataracts are bilateral, though one eye may be affected before the other. Some seem to be congenital, most appear by the age of 2 years and a significant percentage can appear as late as 4 or 5 years of age.
From the review of the literature in the previous part of this article, it is obvious that different breeds of dogs appear to have different types of cataracts. It is impossible to infer from other breeds how cataracts are inherited in the Australian Shepherd. Even if one were to look at similar types of cataracts, the answer is still not obvious. For example, both Golden Retrievers and West Highland White Terriers are affected by types of cataracts similar to the ones that affect the Aussie (posterior cortex with Y suture involvement that can also become completely opaque). Yet one appears to be inherited as a dominant trait and the other as a recessive trait.
From examination of the Aussie pedigrees available to me, it is impossible to determine the mode of inheritance. If I had to guess, I would say that posterior cortical cataracts in the Australian Shepherd are inherited as an autosomal recessive trait. But note: at this stage it is just a GUESS. I have almost no confidence in it. The only reason that I willing to go at least a millimeter out on a limb on this is that affected individuals usually have one or more of a small number of dogs on both the dam's and the sire's side of the pedigree. That is not proof, however, that could just be coincidence.
While I could establish a breeding colony to determine the mode of inheritance of cataracts in the Australian Shepherd, I would prefer not to have to do so. First, it would take 5 to 20 years to make the determination. Second, I admit to being squeamish about breeding individuals that I know would have health defects. But I do not believe that establishing a breeding colony and waiting all those years is necessary if you, the breeders, will help. If you are aware of cataracts in your lines, please get in touch with me. With your help we can obtain complete pedigrees which should allow us to determine the mode of inheritance of cataracts. Specifically, to test the hypothesis that posterior cortex cataracts are inherited as an autosomal recessive trait, I am looking for the following types of breedings and outcomes:
Please, if you produced such a litter, get in touch with me. If you carried out one of these breedings but obtained different phenotypes in the litters, get in touch with me. After all, this may not be a recessive trait and you may be able to prove it. Note that all information will be held in the strictest confidence.
Cataracts are a fairly significant health problem in the Australian Shepherd. Due to the fact that genetically affected individuals may test normal during early breeding age and not show any hint of cataracts until after they have been bred one or more times, the high incidence in the breed is likely to continue. If the trait is recessive, this problem is only exacerbated. Only the development of a DNA test to detect individuals carrying the cataract gene will allow us to decrease the incidence of cataracts. But such a test is not likely to be developed nor properly utilized in breeding programs if the mode of inheritance of cataracts in the Australian Shepherd is not determined first.
If you want to help, here is how you can contact me:
Fax: (770) 836-6633
Phone: (770) 836-4546
Copyright 1998, 1999 Leos Kral. Australian Shepherd Health and Genetics Information Resource and Health Registry. All rights reserved.