Inheritance of Cataracts in Dogs
By Leos Kral
Foreword
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.
Dominant Inheritance?
The
question mark in the sub-heading above indicates that the data presented in the
cited articles is not conclusive.
Golden
Retriever
The
study summarized here [1] 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 [2] 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 [3] it is mentioned that the mode of inheritance is thought to be
dominant. The phenotypic expression of cataracts in this breed is very
variable.
Recessive
Inheritance
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 [4] 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 [5] 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 [6] 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.
Standard
Poodle
Two
studies, one carried out in the
Miniature
Schnauzer
I
have not yet obtained the original article but in a review article by K. C.
Barnett [3] it is mentioned that the mode of inheritance has been shown to be autosomal recessive by test breeding. These are posterior
cortical cataracts.
Afghan
Hound
I
have not yet obtained the original article but in a review article by K. C.
Barnett [3] 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:
- normal x normal --->
produces cataracts in some but not all offspring
- affected x affected --->
produces all affected
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:
Contact
Information:
Dr.
Leos Kral
Department of
E-Mail: lkral@westga.edu
Fax: (770) 836-6633
Phone: (770) 836-4546
References
- Rubin LF (1974) Cataract in
Golden Retrievers. J Am Vet Med Assoc 165: 457-458.
- Gelatt KN,
- Barnett KC (1978) Hereditary cataracts in the dog. J. Small Anim Prct. 19:
109-120
- Koch SA (1972) Cataracts in
interrelated old English Sheepdogs. J Am Vet Med Assoc 160:
299-301.
- Narfstrom K (1981)
Cataract in the West Highland white terrier. J Small Anim Pract 22:
467-471.
- Barnett KC (1980) Hereditary cataract in the Welsh Springer spaniel. J
Small Anim Pract. 21:
621-625.
- Rubin LF,
- Barnett KC, Startup FG (1985) Hereditary
cataract in the standard poodle. Vet Rec 117:
15-16.
Copyright 1998, 1999 Leos Kral. Australian Shepherd Health and Genetics Information
Resource and Health Registry. All rights reserved.
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