Progressive
Retinal Atrophy in Miniature Long-haired Dachshunds
Progressive
retinal atrophy (PRA) is a term for retinal degenerations
occuring in many breeds of dog. Many forms of PRA exist,
each form being confined to one or a few breeds only.
The disease results in a degeneration of the light-sensitive
membrane at the back of the eye - the retina - resulting
in loss of vision, and often leading to blindness.
The
form of the disease occurring in miniature long-haired
dachshunds can be diagnosed by electroretinography at
a few months of age, although obvious clinical signs
take longer to develop. The exact course of the disease
can vary between individual dogs, and some don’t
develop symptoms until relatively late in life.
The
disease is caused by a change to a gene involved in
sight. This change, or mutation, occurred spontaneously,
but once in the population has been inherited from generation
to generation like any other gene. The mutation upsets
the delicate processes involved in vision and causes
the long-term degeneration seen. This form of PRA 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 be carriers
There
is currently no treatment for the disease. Breeding
stock are regularly checked by eye examination, although
this can only pick up affected dogs after symptoms have
developed and will never detect the symptomless carriers.
The
gene responsible has been identified at the Animal Health
Trust and we have recently identified the mutation causing
the disease. Using the information from this research,
we have developed a DNA test for the disease. This test
not only diagnoses dogs affected with this disease but
can also detect those dogs which are carriers, showing
no symptoms of the disease but producing 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 from February 14th, 2005 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 PRA, nor pass a copy of the PRA 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 PRA. It will not develop
PRA but will pass on the PRA gene to 50% (on average)
of its offspring.
AFFECTED:
the dog has two copies of the PRA mutation and is affected
with PRA. It will develop PRA 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 affecteds 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.
Samples for testing, either blood (3mls in an EDTA tube)
or cheek swabs, should be sent together with a completed
DNA Testing form and a cheque for £60-00 (inc
VAT) for each sample to Genetic Services, Animal Health
Trust, Lanwades Park, Kentford, Newmarket, Suffolk CB8
7UU. DNA testing forms can be downloaded from our
web site. DNA
testing forms and cheek swabs can also be obtained by
contacting Vikki Lett 01638 750659 ext 1223 or via e-mail.
Any
South African breeders who are interested in testing
their dogs should visit the AHT web site where details
and additional costs are set out.
