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.
Carriers can still be bred to clear
dogs. On average, 50% of such a litter
will be clear and 50% carriers; there
can be no affected produced from such a
mating. Pups which will be used for
breeding can themselves be DNA tested to
determine whether they are clear or
carrier.
