The protein factor assay
The vWD test available in the past has been an assay of von Willebrand factor protein. It is fairly reliable in detecting affected animals, (those with two copies of the defective gene, and at risk for bleeding), because these animals usually have a very low level of the factor. But this test is very unreliable in differentiating gene carriers from clear animals. (Clear dogs are dogs with two copies of the normal gene). Further, many environmental factors influence the protein factor assay. Thus, a dog that tested in the 'normal' range one year could have a low value in the (carrier) range the next year. As a result breeders have not had much success breeding out this disease, which was frustrating to them.
The mutation-based test and its advantages
The genetic defect leading to the disease has been identified. By DNA testing the responsible mutation can be shown directly. This DNA test can be done at any age and unambiguously classify dogs for the rest of their lives into affected, carrier, and clear animals. With this test the breeder can rapidly eliminate the vWD disease gene from the breed. If a particularly valuable dog turns out to be a carrier, it can be bred to a clear animal, and non-carrier puppies saved for the next round of breeding.
The frequency of carriers was estimated to be that about 11% of Shetland Sheep Dogs are carriers of the type III vWD mutation. This is a significant health burden, and it would be good to get rid of this disease gene by using the DNA test. It should be emphasised that carriers will be clinically normal, and can be used as pets. As said above, if a carrier has excellent breed characteristics, it can be bred to a clear animal, and only clear animals among the progeny be used for further breeding.