Cataracts represent opacities of the ocular lens that impair vision and have the potential to result in blindness. Common causes of cataracts in dogs include inherited defects, metabolic disease (especially diabetes mellitus), anterior uveitis, senility, and nutritional imbalances. In a retrospective study1 covering a period of 40 years, 59 breeds of dogs had a greater prevalence of cataracts than that determined for randomly bred dogs, which indicated a genetic contribution. It has been proposed that an even greater number of breeds may be affected by cataracts, with estimates as high as 97 breeds.2,3
Inherited cataracts develop in dogs at various ages, although generally in dogs younger than what might be considered senile or geriatric or at risk for age-related cataracts. In some breeds, dogs develop cataracts shortly after birth, whereas dogs of other breeds may not develop them until between 3 and 6 years of age.4 Breed-related cataracts are considered to be genetically controlled, but the mode of inheritance has not been clearly defined in many of the breeds because of limited data on populations in which cataracts segregate.5,6 It is presumed that the Bichon Frise, Boston Terrier, Cocker Spaniel, Miniature Schnauzer, and Staffordshire Bull Terrier breeds have autosomal recessive modes of inheritance for cataracts.6-8
The current treatment modality for cataracts in dogs is phacoemulsification. Although use of this technique has relatively high success rates, it can cause blindness or painful sequelae.9,10 Thus, for breeds in which a known genetic contribution exists, many investigators have sought to identify the gene or genes involved in expression of cataracts.8,11,12 A single base-pair insertion in the canine HSF4 gene was identified as responsible for cataract formation in Boston Terriers and Staffordshire Bull Terriers, whereas a single base-pair deletion in that same gene was correlated with cataract formation in Australian Shepherds.8 The mode of inheritance for the 2 terrier breeds is considered autosomal recessive, but it is presumed that Australian Shepherds have a dominant mode of inheritance. Variation in the mode of inheritance among breeds, even when the same gene is altered, is a critical component to localization of the genetic defect responsible for cataracts. Because the mode of inheritance is incompletely defined for many breeds, the study reported here involved a pedigree analysis of cataracts in Jack Russell Terriers (now referred to as Parson Russell Terriers in the American Kennel Club registry).
Jack Russell Terriers are also prone to lens luxation, which may influence a particular dog's potential risk for complications associated with cataract treatment.13 Displacement of a lens may be secondary to trauma or primary as a result of deterioration of the zonular fibers.14 Similar to the situation for cataracts, lens luxation can be treated specifically by surgical removal of the displaced lens or lenses.15 Preventing the condition by genetically selecting against animals predisposed to developing the condition would be greatly preferable. Heritability of primary lens luxation has been determined for some breeds of dogs, with the suggestion that the disorder is a simple autosomal recessive condition on the basis of studies14,16,17 in which males and females were equally affected with a relatively narrow age of onset. In other studies,18,19 it has been suggested that lens luxation has a dominant mode of inheritance. Although primary lens luxation is found in Jack Russell Terriers, as well as other terrier breeds,20,21 the mode of inheritance has not been determined.
Breeders and owners have identified cataracts and primary lens luxation as conditions that represent a major impact on the health of Jack Russell Terriers. Therefore, objectives of the study reported here were to characterize the heritability and mode of inheritance of cataracts and primary lens luxation in Jack Russell Terriers. An additional objective was to determine whether the mutation associated with cataracts in Boston Terriers and Staffordshire Bull Terriers also was found in Jack Russell Terriers.
Canine Eye Registration Foundation
Genetic correlation between 2 traits
Environmental correlation between 2 traits
SOLAR, version 4.0.2, Southwest Foundation for Biomedical Research, San Antonio, Tex. Available at: www.sfbr.org/solar/. Accessed Mar 12, 2007.
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