Early-Onset Hereditary Cataract in Boston Terriers
Early onset hereditary cataract (EHC), sometimes also referred to as juvenile hereditary cataract, is a condition that is known to affect the French Bulldog. In the UK the condition is listed on schedule B of the BVA/KC/ISDS Eye Scheme, meaning it is considered to be an inherited problem in the French Bulldog and is officially ‘under investigation’. Although there are no published scientific reports describing the clinical characteristics of EHC in this breed, anecdotal evidence indicates the cataracts usually develop within the first year of life, are bilateral, symmetrical and progressive.
The mutation responsible for HC in several breeds has recently been identified at the Animal Health Trust. Using the information from this research, we have developed a DNA test for HC in Staffordshire bull terriers and Boston terriers and are now also able to extend the test to the French Bulldog. This test not only diagnoses dogs affected with the disease but can also detect those dogs which are carriers, displaying no symptoms of the disease but able to produce affected pups.
The mutation probably occurred spontaneously in a single dog but once in the population has been inherited from generation to generation like any other gene. The disorder shows an autosomal recessive mode of inheritance: two copies of the defective gene (one inherited from each parent) have to be present for a dog to be affected by the disease. Individuals with one copy of the defective gene and one copy of the normal gene - called carriers - show no symptoms but can pass the defective gene onto their offspring. When two apparently healthy carriers are crossed, 25% (on average) of the offspring will be affected by the disease, 25% will be clear and the remaining 50% will themselves be carriers
Under most circumstances, there will be a much greater number of carriers than affected animals in a population. However breeders may wish to use carriers in their breeding programmes to keep desirable traits within their lines. It is perfectly acceptable to cross carriers with clear dogs, no affected pups will then be produced and, on average, 50% of the litter will be clear and 50% carriers. However, it is important that any dogs from carrier x clear matings that will subsequently be used to breed from are DNA tested to distinguish those pups which are clear from those which are carriers.
The test is available now and information on submitting samples is given below. Breeders will be sent results identifying their dog as belonging to one of three categories:
CLEAR: the dog has 2 copies of the normal gene and will neither develop Hereditary Cataract, nor pass a copy of the Hereditary Cataract gene to any of its offspring.
CARRIER: the dog has one copy of the normal gene and one copy of the mutant gene that causes Hereditary Cataract. It will not develop Hereditary Cataract but will pass on the Hereditary Cataract gene to 50% (on average) of its offspring.
AFFECTED: the dog has two copies of the Hereditary Cataract mutation and is affected with Hereditary Cataract. It will develop Hereditary Cataract at some stage during its lifetime, assuming it lives to an appropriate age.
Many breeders of French Bulldogs have generously contributed samples to the research leading to our identification of the mutation in this breed and we would like to thank them for their cooperation. Without these samples we could not have introduced this test.