von Willebrand's Disease & Your
Breeding Program
Appeared In CMTC Newsletter "Thumbprints"
December, 1998.
Prepared by Amanda Kelly
As many Manchester breeders will know von
Willebrand's Disease (or vWD) is one of the diseases affecting Manchester
Terriers currently being targeted on a wide scale by breeders. New
procedures in detecting the presence of this disease have made selecting
suitable breeding stock very easy and virtually risk free when utilized
correctly.
Canine von Willebrand' s Disease, first
reported in 1970, is an inherited deficiency of one of the clotting
factors of the blood (von Willebran' s Factor or vWF). The disease is
quite similar to hemophilia but can appear in either males or females.
Dogs affected by vWD can have symptoms ranging from very mild to severe or
lethal. Symptoms include: prolonged bleeding from toe nails cut too short,
hemorrhage from even minor surgical procedures, hematomas, lameness,
stillbirth or early death of newborn puppies, intestinal bleeding,
nose-bleeds, etc. These symptoms can be further aggravated by stressful
situations.
Genetically, each dog carries two genes
controlling production of vWF, one inherited from each parent. Each normal
gene is capable of producing 50% of the total amount of vWF produced by a
cell, for a total of 100%. Manchester Terriers, along with Dobe rman
Pinschers and Poodles, are prone to Type 1 vWD, meaning that even diseased
genes are capable of producing a small amount of the vWF produced by a
normal gene -- typically 5-10%. Because of this, Manchesters are not prone
to as severe problems with bleeding and clotting as some oth er breeds.
This is not to say that affected Manchesters are not at risk, simply that
they are not likely to die from a bleeding toe nail or a nose bleed, or to
suffer from spontaneous bleeding as has been known to happen in Type 3
breeds for example.
Until recently the only way to test for the
presence of vWD was through the use of a protein-based test called a vWD
Factor Assay. This test measured the amount of vWF the dog was able to
produce and compared it to the 'normal amount' found in a dog presumed to
be clear of the disease. The dogs were then classified accordingly with
the higher reading being considered more desirable in a breeding program.
The problem with the Factor Assay test was that many variables could
affect the resulting readings, in cluding thyroid hormone levels,
condition of the liver, menstrual cycles, improper blood handling and
variations within the protein-based tests themselves.
Researchers at Michigan State University,
the University of Michigan and a company called VetGen have now discovered
a way to detect the presence of the gene mutation causing von
Willebrand\rquote s Disease through the use of a simple DNA test. The test
has several advantages over the old method, the most important of which is
accuracy. Because the test looks at the individuals DNA each dog needs to
be tested only once in its life. Results are not subject to change due to
any variables affecting the dog -- no matter how many times the dog is
tested the results will always be the same. Results can be obtained within
two weeks and the sample DNA is collected by simply swabbing the inside of
the dog's mouth using the soft-bristle brush provided in the test kit. The
brush can then be returned to VetGen' s labs through regular mail.
Another important feature of thi s test is
the definitive results it offers, classifying each dog as Clear, Carrier
or Affected. Because vWD is a recessive trait in Manchester Terriers both
copies of the gene present must be disease genes in order for the dog to
be "affected" by vWD (i.e. exhibit symptoms). In this case any puppies an
affected dog produces will carry at least one disease gene for vWD. When
only one copy of the disease gene is found the dog is classified as a
"carrier." Though the individual will not exhibit symptoms of the disease
its puppies will inherit the disease gene approximately 50% of the time.
Dogs possessing no copies of the disease gene are classified as "clear" of
the disease and will never pass on a disease gene. These simple
classifications will now allow the breeder to make informed decisions
concerning vWD when planning breedings.
Data from the several hundred Manchester
Terriers that have been tested show that only approximately 1% are
classified as "affected" and 10-11% as "carrier." This is very encouraging
news, eventually we hope to be able to eliminate vWD from the breed
forever, though this will have to be done with care. It is important to
note when evaluating your breeding program and your dogs' vWD results that
researchers like Patrick Venta, PhD of the College of Veterinary Medicine
at Michigan State University "do not believe that it is in the best
interest of the breed to limit the gene pool by breeding only clear to
clear." This does not negate the importance of controlling and eliminating
the disease, it simply puts it in perspective. It is, obviously, not
advisable to plan any breeding in which affected animals will be produced.
It should also be noted by those
considering testing their breeding stock that many North American kennels
now require DNA test results before breeding to or buying from a fellow
fancier.
