Hereditary Cataracts (also called Juvenile Cataracts)
Hereditary Cataract in Staffordshire Bull Terriers has been recognised as an inherited condition since the late 1970’s. Affected dogs develop cataracts in both eyes at an early age. The condition is not congenital, so the lenses are normal at birth but cataracts appear at a few weeks to months in age, progressing to total cataract (and resulting blindness) by 2 to 3 years of age.
The mutation, or change to the structure of the gene, 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
The mutation responsible for the disease has recently been identified at the Animal Health Trust. Using the information from this research, we have developed a DNA test for the disease. 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. Under most circumstances, there will be a much greater number of carriers than affected animals in a population. It is important to eliminate such carriers from a breeding population since they represent a hidden reservoir of the disease that can produce affected dogs at any time.
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.
Results of mating with different diagnoses of the disees - Clear x Clear: All puppies clear by birth. – Clear x Carrier: 50% of the puppies are clear, 50% are carriers. – Clear x Affected: All puppies are carriers, but non are affected - Carrier x Carrier: 25% clear, 50% carrier, 25% affected.. – Carrier x Affectet: 50% carrier, 50% affected – Affected x Affected: All puppies will be affected.
PHPV – Persistent Hyperplastic Primary Vitreous This is another inherited cataract, although it is a much more complex condition and it is much more difficult to tell how it is inherited in pups. During a puppy’s development, certain blood vessels are needed to help develop the eye. In ‘normal’ pups, these blood vessels disappear after they are no longer needed (usually within a few weeks during development) in puppies with PHPV the vessels don’t disappear and can cause eye problems.
PHPV is congenital (this means that it will be present at birth) and can be detected in puppies as young as 6 weeks old. The condition is not progressive – this means that once it’s detected and manifests itself it will not get worse as the puppy grows older.
Dogs can be affected very mildly or quite severely, and if it is present in a breeding dog, there is no way to know how badly pups will be affected in a litter. If the Dam or Sire has mild PHPV the pups could potentially have a much worse strain of the illness.
This is why it is essential to ensure that all breeding dogs are screened for the illness along with HC and L2–HGA.